aboutsummaryrefslogtreecommitdiffstats
path: root/Documentation/DocBook/videobook.tmpl
blob: 3ec6c875588ab7e8d1268e9ee974655c0cb843ca (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>

<book id="V4LGuide">
 <bookinfo>
  <title>Video4Linux Programming</title>
  
  <authorgroup>
   <author>
    <firstname>Alan</firstname>
    <surname>Cox</surname>
    <affiliation>
     <address>
      <email>alan@redhat.com</email>
     </address>
    </affiliation>
   </author>
  </authorgroup>

  <copyright>
   <year>2000</year>
   <holder>Alan Cox</holder>
  </copyright>

  <legalnotice>
   <para>
     This documentation is free software; you can redistribute
     it and/or modify it under the terms of the GNU General Public
     License as published by the Free Software Foundation; either
     version 2 of the License, or (at your option) any later
     version.
   </para>
      
   <para>
     This program is distributed in the hope that it will be
     useful, but WITHOUT ANY WARRANTY; without even the implied
     warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
     See the GNU General Public License for more details.
   </para>
      
   <para>
     You should have received a copy of the GNU General Public
     License along with this program; if not, write to the Free
     Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
     MA 02111-1307 USA
   </para>
      
   <para>
     For more details see the file COPYING in the source
     distribution of Linux.
   </para>
  </legalnotice>
 </bookinfo>

<toc></toc>

  <chapter id="intro">
      <title>Introduction</title>
  <para>
        Parts of this document first appeared in Linux Magazine under a
        ninety day exclusivity.
  </para>
  <para>
        Video4Linux is intended to provide a common programming interface
        for the many TV and capture cards now on the market, as well as
        parallel port and USB video cameras. Radio, teletext decoders and
        vertical blanking data interfaces are also provided.
  </para>
  </chapter>
  <chapter id="radio">
        <title>Radio Devices</title>
  <para>
        There are a wide variety of radio interfaces available for PC's, and these
        are generally very simple to program. The biggest problem with supporting
        such devices is normally extracting documentation from the vendor.
  </para>
  <para>
        The radio interface supports a simple set of control ioctls standardised
        across all radio and tv interfaces. It does not support read or write, which
        are used for video streams. The reason radio cards do not allow you to read
        the audio stream into an application is that without exception they provide
        a connection on to a soundcard. Soundcards can be used to read the radio
        data just fine. 
  </para>
  <sect1 id="registerradio">
  <title>Registering Radio Devices</title>
  <para>
        The Video4linux core provides an interface for registering devices. The
        first step in writing our radio card driver is to register it.
  </para>
  <programlisting>


static struct video_device my_radio
{
        "My radio",
        VID_TYPE_TUNER,
        VID_HARDWARE_MYRADIO,
        radio_open.
        radio_close,
        NULL,                /* no read */
        NULL,                 /* no write */
        NULL,                /* no poll */
        radio_ioctl,
        NULL,                /* no special init function */
        NULL                /* no private data */
};


  </programlisting>
  <para>
        This declares our video4linux device driver interface. The VID_TYPE_ value
        defines what kind of an interface we are, and defines basic capabilities.
  </para>
  <para>
        The only defined value relevant for a radio card is VID_TYPE_TUNER which
        indicates that the device can be tuned. Clearly our radio is going to have some
        way to change channel so it is tuneable.
  </para>
  <para>
        The VID_HARDWARE_ types are unique to each device. Numbers are assigned by
        <email>alan@redhat.com</email> when device drivers are going to be released. Until then you
        can pull a suitably large number out of your hat and use it. 10000 should be
        safe for a very long time even allowing for the huge number of vendors
        making new and different radio cards at the moment.
  </para>
  <para>
        We declare an open and close routine, but we do not need read or write,
        which are used to read and write video data to or from the card itself. As
        we have no read or write there is no poll function.
  </para>
  <para>
        The private initialise function is run when the device is registered. In
        this driver we've already done all the work needed. The final pointer is a
        private data pointer that can be used by the device driver to attach and
        retrieve private data structures. We set this field "priv" to NULL for
        the moment.
  </para>
  <para>
        Having the structure defined is all very well but we now need to register it
        with the kernel. 
  </para>
  <programlisting>


static int io = 0x320;

int __init myradio_init(struct video_init *v)
{
        if(!request_region(io, MY_IO_SIZE, "myradio"))
        {
                printk(KERN_ERR 
                    "myradio: port 0x%03X is in use.\n", io);
                return -EBUSY;
        }

        if(video_device_register(&amp;my_radio, VFL_TYPE_RADIO)==-1) {
                release_region(io, MY_IO_SIZE);
                return -EINVAL;
        }		
        return 0;
}

  </programlisting>
  <para>
        The first stage of the initialisation, as is normally the case, is to check 
        that the I/O space we are about to fiddle with doesn't belong to some other 
        driver. If it is we leave well alone. If the user gives the address of the 
        wrong device then we will spot this. These policies will generally avoid 
        crashing the machine.
  </para>
  <para>
        Now we ask the Video4Linux layer to register the device for us. We hand it
        our carefully designed video_device structure and also tell it which group
        of devices we want it registered with. In this case VFL_TYPE_RADIO.
  </para>
  <para>
        The types available are
  </para>
   <table frame="all"><title>Device Types</title>
   <tgroup cols="3" align="left">
   <tbody>
   <row>
        <entry>VFL_TYPE_RADIO</entry><entry>/dev/radio{n}</entry><entry>

        Radio devices are assigned in this block. As with all of these
        selections the actual number assignment is done by the video layer
        accordijng to what is free.</entry>
	</row><row>
        <entry>VFL_TYPE_GRABBER</entry><entry>/dev/video{n}</entry><entry>
        Video capture devices and also -- counter-intuitively for the name --
        hardware video playback devices such as MPEG2 cards.</entry>
	</row><row>
        <entry>VFL_TYPE_VBI</entry><entry>/dev/vbi{n}</entry><entry>
        The VBI devices capture the hidden lines on a television picture
        that carry further information like closed caption data, teletext
        (primarily in Europe) and now Intercast and the ATVEC internet
        television encodings.</entry>
	</row><row>
        <entry>VFL_TYPE_VTX</entry><entry>/dev/vtx[n}</entry><entry>
        VTX is 'Videotext' also known as 'Teletext'. This is a system for
        sending numbered, 40x25, mostly textual page images over the hidden
        lines. Unlike the /dev/vbi interfaces, this is for 'smart' decoder 
        chips. (The use of the word smart here has to be taken in context,
        the smartest teletext chips are fairly dumb pieces of technology).
	</entry>
    </row>
    </tbody>
    </tgroup>
    </table>
  <para>
        We are most definitely a radio.
  </para>
  <para>
        Finally we allocate our I/O space so that nobody treads on us and return 0
        to signify general happiness with the state of the universe.
  </para>
  </sect1>
  <sect1 id="openradio">
  <title>Opening And Closing The Radio</title>

  <para>
        The functions we declared in our video_device are mostly very simple.
        Firstly we can drop in what is basically standard code for open and close. 
  </para>
  <programlisting>


static int users = 0;

static int radio_open(stuct video_device *dev, int flags)
{
        if(users)
                return -EBUSY;
        users++;
        return 0;
}

  </programlisting>
  <para>
        At open time we need to do nothing but check if someone else is also using
        the radio card. If nobody is using it we make a note that we are using it,
        then we ensure that nobody unloads our driver on us.
  </para>
  <programlisting>


static int radio_close(struct video_device *dev)
{
        users--;
}

  </programlisting>
  <para>
        At close time we simply need to reduce the user count and allow the module
        to become unloadable.
  </para>
  <para>
        If you are sharp you will have noticed neither the open nor the close
        routines attempt to reset or change the radio settings. This is intentional.
        It allows an application to set up the radio and exit. It avoids a user
        having to leave an application running all the time just to listen to the
        radio. 
  </para>
  </sect1>
  <sect1 id="ioctlradio">
  <title>The Ioctl Interface</title>
  <para>
        This leaves the ioctl routine, without which the driver will not be
        terribly useful to anyone.
  </para>
  <programlisting>


static int radio_ioctl(struct video_device *dev, unsigned int cmd, void *arg)
{
        switch(cmd)
        {
                case VIDIOCGCAP:
                {
                        struct video_capability v;
                        v.type = VID_TYPE_TUNER;
                        v.channels = 1;
                        v.audios = 1;
                        v.maxwidth = 0;
                        v.minwidth = 0;
                        v.maxheight = 0;
                        v.minheight = 0;
                        strcpy(v.name, "My Radio");
                        if(copy_to_user(arg, &amp;v, sizeof(v)))
                                return -EFAULT;
                        return 0;
                }

  </programlisting>
  <para>
        VIDIOCGCAP is the first ioctl all video4linux devices must support. It
        allows the applications to find out what sort of a card they have found and
        to figure out what they want to do about it. The fields in the structure are
  </para>
   <table frame="all"><title>struct video_capability fields</title>
   <tgroup cols="2" align="left">
   <tbody>
   <row>
        <entry>name</entry><entry>The device text name. This is intended for the user.</entry>
	</row><row>
        <entry>channels</entry><entry>The number of different channels you can tune on
                        this card. It could even by zero for a card that has
                        no tuning capability. For our simple FM radio it is 1. 
                        An AM/FM radio would report 2.</entry>
	</row><row>
        <entry>audios</entry><entry>The number of audio inputs on this device. For our
                        radio there is only one audio input.</entry>
	</row><row>
        <entry>minwidth,minheight</entry><entry>The smallest size the card is capable of capturing
		        images in. We set these to zero. Radios do not
                        capture pictures</entry>
	</row><row>
        <entry>maxwidth,maxheight</entry><entry>The largest image size the card is capable of
                                      capturing. For our radio we report 0.
				</entry>
	</row><row>
        <entry>type</entry><entry>This reports the capabilities of the device, and
                        matches the field we filled in in the struct
                        video_device when registering.</entry>
    </row>
    </tbody>
    </tgroup>
    </table>
  <para>
        Having filled in the fields, we use copy_to_user to copy the structure into
        the users buffer. If the copy fails we return an EFAULT to the application
        so that it knows it tried to feed us garbage.
  </para>
  <para>
        The next pair of ioctl operations select which tuner is to be used and let
        the application find the tuner properties. We have only a single FM band
        tuner in our example device.
  </para>
  <programlisting>


                case VIDIOCGTUNER:
                {
                        struct video_tuner v;
                        if(copy_from_user(&amp;v, arg, sizeof(v))!=0)
                                return -EFAULT;
                        if(v.tuner)
                                return -EINVAL;
                        v.rangelow=(87*16000);
                        v.rangehigh=(108*16000);
                        v.flags = VIDEO_TUNER_LOW;
                        v.mode = VIDEO_MODE_AUTO;
                        v.signal = 0xFFFF;
                        strcpy(v.name, "FM");
                        if(copy_to_user(&amp;v, arg, sizeof(v))!=0)
                                return -EFAULT;
                        return 0;
                }

  </programlisting>
  <para>
        The VIDIOCGTUNER ioctl allows applications to query a tuner. The application
        sets the tuner field to the tuner number it wishes to query. The query does
        not change the tuner that is being used, it merely enquires about the tuner
        in question.
  </para>
  <para>
        We have exactly one tuner so after copying the user buffer to our temporary
        structure we complain if they asked for a tuner other than tuner 0. 
  </para>
  <para>
        The video_tuner structure has the following fields
  </para>
   <table frame="all"><title>struct video_tuner fields</title>
   <tgroup cols="2" align="left">
   <tbody>
   <row>
        <entry>int tuner</entry><entry>The number of the tuner in question</entry>
   </row><row>
        <entry>char name[32]</entry><entry>A text description of this tuner. "FM" will do fine.
                        This is intended for the application.</entry>
   </row><row>
        <entry>u32 flags</entry>
        <entry>Tuner capability flags</entry>
   </row>
   <row>
        <entry>u16 mode</entry><entry>The current reception mode</entry>

   </row><row>
        <entry>u16 signal</entry><entry>The signal strength scaled between 0 and 65535. If
                        a device cannot tell the signal strength it should
                        report 65535. Many simple cards contain only a 
                        signal/no signal bit. Such cards will report either
                        0 or 65535.</entry>

   </row><row>
        <entry>u32 rangelow, rangehigh</entry><entry>
                        The range of frequencies supported by the radio
                        or TV. It is scaled according to the VIDEO_TUNER_LOW
                        flag.</entry>

    </row>
    </tbody>
    </tgroup>
    </table>

   <table frame="all"><title>struct video_tuner flags</title>
   <tgroup cols="2" align="left">
   <tbody>
   <row>
	<entry>VIDEO_TUNER_PAL</entry><entry>A PAL TV tuner</entry>
	</row><row>
        <entry>VIDEO_TUNER_NTSC</entry><entry>An NTSC (US) TV tuner</entry>
	</row><row>
        <entry>VIDEO_TUNER_SECAM</entry><entry>A SECAM (French) TV tuner</entry>
	</row><row>
        <entry>VIDEO_TUNER_LOW</entry><entry>
             The tuner frequency is scaled in 1/16th of a KHz
             steps. If not it is in 1/16th of a MHz steps
	</entry>
	</row><row>
        <entry>VIDEO_TUNER_NORM</entry><entry>The tuner can set its format</entry>
	</row><row>
        <entry>VIDEO_TUNER_STEREO_ON</entry><entry>The tuner is currently receiving a stereo signal</entry>
        </row>
    </tbody>
    </tgroup>
    </table>

   <table frame="all"><title>struct video_tuner modes</title>
   <tgroup cols="2" align="left">
   <tbody>
   <row>
                <entry>VIDEO_MODE_PAL</entry><entry>PAL Format</entry>
   </row><row>
                <entry>VIDEO_MODE_NTSC</entry><entry>NTSC Format (USA)</entry>
   </row><row>
                <entry>VIDEO_MODE_SECAM</entry><entry>French Format</entry>
   </row><row>
                <entry>VIDEO_MODE_AUTO</entry><entry>A device that does not need to do
                                        TV format switching</entry>
   </row>
    </tbody>
    </tgroup>
    </table>
  <para>
        The settings for the radio card are thus fairly simple. We report that we
        are a tuner called "FM" for FM radio. In order to get the best tuning
        resolution we report VIDEO_TUNER_LOW and select tuning to 1/16th of KHz. Its
        unlikely our card can do that resolution but it is a fair bet the card can
        do better than 1/16th of a MHz. VIDEO_TUNER_LOW is appropriate to almost all
        radio usage.
  </para>
  <para>
        We report that the tuner automatically handles deciding what format it is
        receiving - true enough as it only handles FM radio. Our example card is
        also incapable of detecting stereo or signal strengths so it reports a
        strength of 0xFFFF (maximum) and no stereo detected.
  </para>
  <para>
        To finish off we set the range that can be tuned to be 87-108Mhz, the normal
        FM broadcast radio range. It is important to find out what the card is
        actually capable of tuning. It is easy enough to simply use the FM broadcast
        range. Unfortunately if you do this you will discover the FM broadcast
        ranges in the USA, Europe and Japan are all subtly different and some users
        cannot receive all the stations they wish.
  </para>
  <para>
        The application also needs to be able to set the tuner it wishes to use. In
        our case, with a single tuner this is rather simple to arrange.
  </para>
  <programlisting>

                case VIDIOCSTUNER:
                {
                        struct video_tuner v;
                        if(copy_from_user(&amp;v, arg, sizeof(v)))
                                return -EFAULT;
                        if(v.tuner != 0)
                                return -EINVAL;
                        return 0;
                }

  </programlisting>
  <para>
        We copy the user supplied structure into kernel memory so we can examine it. 
        If the user has selected a tuner other than zero we reject the request. If 
        they wanted tuner 0 then, surprisingly enough, that is the current tuner already.
  </para>
  <para>
        The next two ioctls we need to provide are to get and set the frequency of
        the radio. These both use an unsigned long argument which is the frequency.
        The scale of the frequency depends on the VIDEO_TUNER_LOW flag as I
        mentioned earlier on. Since we have VIDEO_TUNER_LOW set this will be in
        1/16ths of a KHz.
  </para>
  <programlisting>

static unsigned long current_freq;



                case VIDIOCGFREQ:
                        if(copy_to_user(arg, &amp;current_freq, 
                                sizeof(unsigned long))
                                return -EFAULT;
                        return 0;

  </programlisting>
  <para>
        Querying the frequency in our case is relatively simple. Our radio card is
        too dumb to let us query the signal strength so we remember our setting if 
        we know it. All we have to do is copy it to the user.
  </para>
  <programlisting>


                case VIDIOCSFREQ:
                {
                        u32 freq;
                        if(copy_from_user(arg, &amp;freq, 
                                sizeof(unsigned long))!=0)
                                return -EFAULT;
                        if(hardware_set_freq(freq)&lt;0)
                                return -EINVAL;
                        current_freq = freq;
                        return 0;
                }

  </programlisting>
  <para>
        Setting the frequency is a little more complex. We begin by copying the
        desired frequency into kernel space. Next we call a hardware specific routine
        to set the radio up. This might be as simple as some scaling and a few
        writes to an I/O port. For most radio cards it turns out a good deal more
        complicated and may involve programming things like a phase locked loop on
        the card. This is what documentation is for. 
  </para>
  <para>
        The final set of operations we need to provide for our radio are the 
        volume controls. Not all radio cards can even do volume control. After all
        there is a perfectly good volume control on the sound card. We will assume
        our radio card has a simple 4 step volume control.
  </para>
  <para>
        There are two ioctls with audio we need to support
  </para>
  <programlisting>

static int current_volume=0;

                case VIDIOCGAUDIO:
                {
                        struct video_audio v;
                        if(copy_from_user(&amp;v, arg, sizeof(v)))
                                return -EFAULT;
                        if(v.audio != 0)
                                return -EINVAL;
                        v.volume = 16384*current_volume;
                        v.step = 16384;
                        strcpy(v.name, "Radio");
                        v.mode = VIDEO_SOUND_MONO;
                        v.balance = 0;
                        v.base = 0;
                        v.treble = 0;
                        
                        if(copy_to_user(arg. &amp;v, sizeof(v)))
                                return -EFAULT;
                        return 0;
                }

  </programlisting>
  <para>
        Much like the tuner we start by copying the user structure into kernel
        space. Again we check if the user has asked for a valid audio input. We have
        only input 0 and we punt if they ask for another input.
  </para>
  <para>
        Then we fill in the video_audio structure. This has the following format
  </para>
   <table frame="all"><title>struct video_audio fields</title>
   <tgroup cols="2" align="left">
   <tbody>
   <row>
   <entry>audio</entry><entry>The input the user wishes to query</entry>
   </row><row>
   <entry>volume</entry><entry>The volume setting on a scale of 0-65535</entry>
   </row><row>
   <entry>base</entry><entry>The base level on a scale of 0-65535</entry>
   </row><row>
   <entry>treble</entry><entry>The treble level on a scale of 0-65535</entry>
   </row><row>
   <entry>flags</entry><entry>The features this audio device supports
   </entry>
   </row><row>
   <entry>name</entry><entry>A text name to display to the user. We picked
                        "Radio" as it explains things quite nicely.</entry>
   </row><row>
   <entry>mode</entry><entry>The current reception mode for the audio

                We report MONO because our card is too stupid to know if it is in
                mono or stereo. 
   </entry>
   </row><row>
   <entry>balance</entry><entry>The stereo balance on a scale of 0-65535, 32768 is
                        middle.</entry>
   </row><row>
   <entry>step</entry><entry>The step by which the volume control jumps. This is
                        used to help make it easy for applications to set 
                        slider behaviour.</entry>
   </row>
   </tbody>
   </tgroup>
   </table>

   <table frame="all"><title>struct video_audio flags</title>
   <tgroup cols="2" align="left">
   <tbody>
   <row>
                <entry>VIDEO_AUDIO_MUTE</entry><entry>The audio is currently muted. We
                                        could fake this in our driver but we
                                        choose not to bother.</entry>
   </row><row>
                <entry>VIDEO_AUDIO_MUTABLE</entry><entry>The input has a mute option</entry>
   </row><row>
                <entry>VIDEO_AUDIO_TREBLE</entry><entry>The  input has a treble control</entry>
   </row><row>
                <entry>VIDEO_AUDIO_BASS</entry><entry>The input has a base control</entry>
   </row>
   </tbody>
   </tgroup>
   </table>

   <table frame="all"><title>struct video_audio modes</title>
   <tgroup cols="2" align="left">
   <tbody>
   <row>
                <entry>VIDEO_SOUND_MONO</entry><entry>Mono sound</entry>
   </row><row>
                <entry>VIDEO_SOUND_STEREO</entry><entry>Stereo sound</entry>
   </row><row>
                <entry>VIDEO_SOUND_LANG1</entry><entry>Alternative language 1 (TV specific)</entry>
   </row><row>
                <entry>VIDEO_SOUND_LANG2</entry><entry>Alternative language 2 (TV specific)</entry>
   </row>
   </tbody>
   </tgroup>
   </table>
  <para>
        Having filled in the structure we copy it back to user space.
  </para>
  <para>
        The VIDIOCSAUDIO ioctl allows the user to set the audio parameters in the
        video_audio structure. The driver does its best to honour the request.
  </para>
  <programlisting>

                case VIDIOCSAUDIO:
                {
                        struct video_audio v;
                        if(copy_from_user(&amp;v, arg, sizeof(v)))
                                return -EFAULT;
                        if(v.audio)
                                return -EINVAL;
                        current_volume = v/16384;
                        hardware_set_volume(current_volume);
                        return 0;
                }

  </programlisting>
  <para>
        In our case there is very little that the user can set. The volume is
        basically the limit. Note that we could pretend to have a mute feature
        by rewriting this to 
  </para>
  <programlisting>

                case VIDIOCSAUDIO:
                {
                        struct video_audio v;
                        if(copy_from_user(&amp;v, arg, sizeof(v)))
                                return -EFAULT;
                        if(v.audio)
                                return -EINVAL;
                        current_volume = v/16384;
                        if(v.flags&amp;VIDEO_AUDIO_MUTE)
                                hardware_set_volume(0);
                        else
                                hardware_set_volume(current_volume);
                        current_muted = v.flags &amp; 
                                              VIDEO_AUDIO_MUTE;
                        return 0;
                }

  </programlisting>
  <para>
        This with the corresponding changes to the VIDIOCGAUDIO code to report the
        state of the mute flag we save and to report the card has a mute function,
        will allow applications to use a mute facility with this card. It is
        questionable whether this is a good idea however. User applications can already
        fake this themselves and kernel space is precious.
  </para>
  <para>
        We now have a working radio ioctl handler. So we just wrap up the function
  </para>
  <programlisting>


        }
        return -ENOIOCTLCMD;
}

  </programlisting>
  <para>
        and pass the Video4Linux layer back an error so that it knows we did not
        understand the request we got passed.
  </para>
  </sect1>
  <sect1 id="modradio">
  <title>Module Wrapper</title>
  <para>
        Finally we add in the usual module wrapping and the driver is done.
  </para>
  <programlisting>

#ifndef MODULE

static int io = 0x300;

#else

static int io = -1;

#endif

MODULE_AUTHOR("Alan Cox");
MODULE_DESCRIPTION("A driver for an imaginary radio card.");
module_param(io, int, 0444);
MODULE_PARM_DESC(io, "I/O address of the card.");

static int __init init(void)
{
        if(io==-1)
        {
                printk(KERN_ERR 
         "You must set an I/O address with io=0x???\n");
                return -EINVAL;
        }
        return myradio_init(NULL);
}

static void __exit cleanup(void)
{
        video_unregister_device(&amp;my_radio);
        release_region(io, MY_IO_SIZE);
}

module_init(init);
module_exit(cleanup);

  </programlisting>
  <para>
        In this example we set the IO base by default if the driver is compiled into
        the kernel: you can still set it using "my_radio.irq" if this file is called <filename>my_radio.c</filename>. For the module we require the
        user sets the parameter. We set io to a nonsense port (-1) so that we can
        tell if the user supplied an io parameter or not.
  </para>
  <para>
        We use MODULE_ defines to give an author for the card driver and a
        description. We also use them to declare that io is an integer and it is the
        address of the card, and can be read by anyone from sysfs.
  </para>
  <para>
        The clean-up routine unregisters the video_device we registered, and frees
        up the I/O space. Note that the unregister takes the actual video_device
        structure as its argument. Unlike the file operations structure which can be
        shared by all instances of a device a video_device structure as an actual
        instance of the device. If you are registering multiple radio devices you
        need to fill in one structure per device (most likely by setting up a
        template and copying it to each of the actual device structures).
  </para>
  </sect1>
  </chapter>
  <chapter>
        <title>Video Capture Devices</title>
  <sect1 id="introvid">
  <title>Video Capture Device Types</title>
  <para>
        The video capture devices share the same interfaces as radio devices. In
        order to explain the video capture interface I will use the example of a
        camera that has no tuners or audio input. This keeps the example relatively
        clean. To get both combine the two driver examples.
  </para>
  <para>
        Video capture devices divide into four categories. A little technology
        backgrounder. Full motion video even at television resolution (which is
        actually fairly low) is pretty resource-intensive. You are continually
        passing megabytes of data every second from the capture card to the display. 
        several alternative approaches have emerged because copying this through the 
        processor and the user program is a particularly bad idea .
  </para>
  <para>
        The first is to add the television image onto the video output directly.
        This is also how some 3D cards work. These basic cards can generally drop the
        video into any chosen rectangle of the display. Cards like this, which
        include most mpeg1 cards that used the feature connector,  aren't very
        friendly in a windowing environment. They don't understand windows or
        clipping. The video window is always on the top of the display.
  </para>
  <para>
        Chroma keying is a technique used by cards to get around this. It is an old
        television mixing trick where you mark all the areas you wish to replace
        with a single clear colour that isn't used in the image - TV people use an
        incredibly bright blue while computing people often use a particularly
        virulent purple. Bright blue occurs on the desktop. Anyone with virulent
        purple windows has another problem besides their TV overlay.
  </para>
  <para>
        The third approach is to copy the data from the capture card to the video
        card, but to do it directly across the PCI bus. This relieves the processor
        from doing the work but does require some smartness on the part of the video
        capture chip, as well as a suitable video card. Programming this kind of
        card and more so debugging it can be extremely tricky. There are some quite
        complicated interactions with the display and you may also have to cope with
        various chipset bugs that show up when PCI cards start talking to each
        other. 
  </para>
  <para>
        To keep our example fairly simple we will assume a card that supports
        overlaying a flat rectangular image onto the frame buffer output, and which
        can also capture stuff into processor memory.
  </para>
  </sect1>
  <sect1 id="regvid">
  <title>Registering Video Capture Devices</title>
  <para>
        This time we need to add more functions for our camera device.
  </para>
  <programlisting>
static struct video_device my_camera
{
        "My Camera",
        VID_TYPE_OVERLAY|VID_TYPE_SCALES|\
        VID_TYPE_CAPTURE|VID_TYPE_CHROMAKEY,
        VID_HARDWARE_MYCAMERA,
        camera_open.
        camera_close,
        camera_read,      /* no read */
        NULL,             /* no write */
        camera_poll,      /* no poll */
        camera_ioctl,
        NULL,             /* no special init function */
        NULL              /* no private data */
};
  </programlisting>
  <para>
        We need a read() function which is used for capturing data from
        the card, and we need a poll function so that a driver can wait for the next
        frame to be captured.
  </para>
  <para>
        We use the extra video capability flags that did not apply to the
        radio interface. The video related flags are
  </para>
   <table frame="all"><title>Capture Capabilities</title>
   <tgroup cols="2" align="left">
   <tbody>
   <row>
<entry>VID_TYPE_CAPTURE</entry><entry>We support image capture</entry>
</row><row>
<entry>VID_TYPE_TELETEXT</entry><entry>A teletext capture device (vbi{n])</entry>
</row><row>
<entry>VID_TYPE_OVERLAY</entry><entry>The image can be directly overlaid onto the
                                frame buffer</entry>
</row><row>
<entry>VID_TYPE_CHROMAKEY</entry><entry>Chromakey can be used to select which parts
                                of the image to display</entry>
</row><row>
<entry>VID_TYPE_CLIPPING</entry><entry>It is possible to give the board a list of
                                rectangles to draw around. </entry>
</row><row>
<entry>VID_TYPE_FRAMERAM</entry><entry>The video capture goes into the video memory
                                and actually changes it. Applications need
                                to know this so they can clean up after the
                                card</entry>
</row><row>
<entry>VID_TYPE_SCALES</entry><entry>The image can be scaled to various sizes,
                                rather than being a single fixed size.</entry>
</row><row>
<entry>VID_TYPE_MONOCHROME</entry><entry>The capture will be monochrome. This isn't a
                                complete answer to the question since a mono
                                camera on a colour capture card will still
                                produce mono output.</entry>
</row><row>
<entry>VID_TYPE_SUBCAPTURE</entry><entry>The card allows only part of its field of
                                view to be captured. This enables
                                applications to avoid copying all of a large
                                image into memory when only some section is
                                relevant.</entry>
    </row>
    </tbody>
    </tgroup>
    </table>
  <para>
        We set VID_TYPE_CAPTURE so that we are seen as a capture card,
        VID_TYPE_CHROMAKEY so the application knows it is time to draw in virulent
        purple, and VID_TYPE_SCALES because we can be resized.
  </para>
  <para>
        Our setup is fairly similar. This time we also want an interrupt line
        for the 'frame captured' signal. Not all cards have this so some of them
        cannot handle poll().
  </para>
  <programlisting>


static int io = 0x320;
static int irq = 11;

int __init mycamera_init(struct video_init *v)
{
        if(!request_region(io, MY_IO_SIZE, "mycamera"))
        {
                printk(KERN_ERR 
                      "mycamera: port 0x%03X is in use.\n", io);
                return -EBUSY;
        }

        if(video_device_register(&amp;my_camera, 
            VFL_TYPE_GRABBER)==-1) {
                release_region(io, MY_IO_SIZE);
                return -EINVAL;
        }
        return 0;
}

  </programlisting>
  <para>
        This is little changed from the needs of the radio card. We specify
        VFL_TYPE_GRABBER this time as we want to be allocated a /dev/video name.
  </para>
  </sect1>
  <sect1 id="opvid">
  <title>Opening And Closing The Capture Device</title>
  <programlisting>


static int users = 0;

static int camera_open(stuct video_device *dev, int flags)
{
        if(users)
                return -EBUSY;
        if(request_irq(irq, camera_irq, 0, "camera", dev)&lt;0)
                return -EBUSY;
        users++;
        return 0;
}


static int camera_close(struct video_device *dev)
{
        users--;
        free_irq(irq, dev);
}
  </programlisting>
  <para>
        The open and close routines are also quite similar. The only real change is
        that we now request an interrupt for the camera device interrupt line. If we
        cannot get the interrupt we report EBUSY to the application and give up.
  </para>
  </sect1>
  <sect1 id="irqvid">
  <title>Interrupt Handling</title>
  <para>
        Our example handler is for an ISA bus device. If it was PCI you would be
        able to share the interrupt and would have set SA_SHIRQ to indicate a 
        shared IRQ. We pass the device pointer as the interrupt routine argument. We
        don't need to since we only support one card but doing this will make it
        easier to upgrade the driver for multiple devices in the future.
  </para>
  <para>
        Our interrupt routine needs to do little if we assume the card can simply
        queue one frame to be read after it captures it. 
  </para>
  <programlisting>


static struct wait_queue *capture_wait;
static int capture_ready = 0;

static void camera_irq(int irq, void *dev_id, 
                          struct pt_regs *regs)
{
        capture_ready=1;
        wake_up_interruptible(&amp;capture_wait);
}
  </programlisting>
  <para>
        The interrupt handler is nice and simple for this card as we are assuming
        the card is buffering the frame for us. This means we have little to do but
        wake up        anybody interested. We also set a capture_ready flag, as we may
        capture a frame before an application needs it. In this case we need to know
        that a frame is ready. If we had to collect the frame on the interrupt life
        would be more complex.
  </para>
  <para>
        The two new routines we need to supply are camera_read which returns a
        frame, and camera_poll which waits for a frame to become ready.
  </para>
  <programlisting>


static int camera_poll(struct video_device *dev, 
	struct file *file, struct poll_table *wait)
{
        poll_wait(file, &amp;capture_wait, wait);
        if(capture_read)
                return POLLIN|POLLRDNORM;
        return 0;
}

  </programlisting>
  <para>
        Our wait queue for polling is the capture_wait queue. This will cause the
        task to be woken up by our camera_irq routine. We check capture_read to see
        if there is an image present and if so report that it is readable.
  </para>
  </sect1>
  <sect1 id="rdvid">
  <title>Reading The Video Image</title>
  <programlisting>


static long camera_read(struct video_device *dev, char *buf,
                                unsigned long count)
{
        struct wait_queue wait = { current, NULL };
        u8 *ptr;
        int len;
        int i;

        add_wait_queue(&amp;capture_wait, &amp;wait);

        while(!capture_ready)
        {
                if(file->flags&amp;O_NDELAY)
                {
                        remove_wait_queue(&amp;capture_wait, &amp;wait);
                        current->state = TASK_RUNNING;
                        return -EWOULDBLOCK;
                }
                if(signal_pending(current))
                {
                        remove_wait_queue(&amp;capture_wait, &amp;wait);
                        current->state = TASK_RUNNING;
                        return -ERESTARTSYS;
                }
                schedule();
                current->state = TASK_INTERRUPTIBLE;
        }
        remove_wait_queue(&amp;capture_wait, &amp;wait);
        current->state = TASK_RUNNING;

  </programlisting>
  <para>
        The first thing we have to do is to ensure that the application waits until
        the next frame is ready. The code here is almost identical to the mouse code
        we used earlier in this chapter. It is one of the common building blocks of
        Linux device driver code and probably one which you will find occurs in any
        drivers you write.
  </para>
  <para>
        We wait for a frame to be ready, or for a signal to interrupt our waiting. If a
        signal occurs we need to return from the system call so that the signal can
        be sent to the application itself. We also check to see if the user actually
        wanted to avoid waiting - ie  if they are using non-blocking I/O and have other things 
        to get on with.
  </para>
  <para>
        Next we copy the data from the card to the user application. This is rarely
        as easy as our example makes out. We will add capture_w, and capture_h here
        to hold the width and height of the captured image. We assume the card only
        supports 24bit RGB for now.
  </para>
  <programlisting>



        capture_ready = 0;

        ptr=(u8 *)buf;
        len = capture_w * 3 * capture_h; /* 24bit RGB */

        if(len>count)
                len=count;  /* Doesn't all fit */

        for(i=0; i&lt;len; i++)
        {
                put_user(inb(io+IMAGE_DATA), ptr);
                ptr++;
        }

        hardware_restart_capture();
                
        return i;
}

  </programlisting>
  <para>
        For a real hardware device you would try to avoid the loop with put_user().
        Each call to put_user() has a time overhead checking whether the accesses to user
        space are allowed. It would be better to read a line into a temporary buffer
        then copy this to user space in one go.
  </para>
  <para>
        Having captured the image and put it into user space we can kick the card to
        get the next frame acquired.
  </para>
  </sect1>
  <sect1 id="iocvid">
  <title>Video Ioctl Handling</title>
  <para>
        As with the radio driver the major control interface is via the ioctl()
        function. Video capture devices support the same tuner calls as a radio
        device and also support additional calls to control how the video functions
        are handled. In this simple example the card has no tuners to avoid making
        the code complex. 
  </para>
  <programlisting>



static int camera_ioctl(struct video_device *dev, unsigned int cmd, void *arg)
{
        switch(cmd)
        {
                case VIDIOCGCAP:
                {
                        struct video_capability v;
                        v.type = VID_TYPE_CAPTURE|\
                                 VID_TYPE_CHROMAKEY|\
                                 VID_TYPE_SCALES|\
                                 VID_TYPE_OVERLAY;
                        v.channels = 1;
                        v.audios = 0;
                        v.maxwidth = 640;
                        v.minwidth = 16;
                        v.maxheight = 480;
                        v.minheight = 16;
                        strcpy(v.name, "My Camera");
                        if(copy_to_user(arg, &amp;v, sizeof(v)))
                                return -EFAULT;
                        return 0;
                }


  </programlisting>
  <para>
        The first ioctl we must support and which all video capture and radio
        devices are required to support is VIDIOCGCAP. This behaves exactly the same
        as with a radio device. This time, however, we report the extra capabilities
        we outlined earlier on when defining our video_dev structure.
  </para>
  <para>
        We now set the video flags saying that we support overlay, capture,
        scaling and chromakey. We also report size limits - our smallest image is
        16x16 pixels, our largest is 640x480. 
  </para>
  <para>
        To keep things simple we report no audio and no tuning capabilities at all.
  </para>
  <programlisting>        

                case VIDIOCGCHAN:
                {
                        struct video_channel v;
                        if(copy_from_user(&amp;v, arg, sizeof(v)))
                                return -EFAULT;
                        if(v.channel != 0)
                                return -EINVAL;
                        v.flags = 0;
                        v.tuners = 0;
                        v.type = VIDEO_TYPE_CAMERA;
                        v.norm = VIDEO_MODE_AUTO;
                        strcpy(v.name, "Camera Input");break;
                        if(copy_to_user(&amp;v, arg, sizeof(v)))
                                return -EFAULT;
                        return 0;
                }


  </programlisting>
  <para>
        This follows what is very much the standard way an ioctl handler looks
        in Linux. We copy the data into a kernel space variable and we check that the
        request is valid (in this case that the input is 0). Finally we copy the
        camera info back to the user.
  </para>
  <para>
        The VIDIOCGCHAN ioctl allows a user to ask about video channels (that is
        inputs to the video card). Our example card has a single camera input. The
        fields in the structure are
  </para>
   <table frame="all"><title>struct video_channel fields</title>
   <tgroup cols="2" align="left">
   <tbody>
   <row>

   <entry>channel</entry><entry>The channel number we are selecting</entry>
   </row><row>
   <entry>name</entry><entry>The name for this channel. This is intended
                   to describe the port to the user.
                   Appropriate names are therefore things like
                   "Camera" "SCART input"</entry>
   </row><row>
   <entry>flags</entry><entry>Channel properties</entry>
   </row><row>
   <entry>type</entry><entry>Input type</entry>
   </row><row>
   <entry>norm</entry><entry>The current television encoding being used
                   if relevant for this channel.
    </entry>
    </row>
    </tbody>
    </tgroup>
    </table>
    <table frame="all"><title>struct video_channel flags</title>
    <tgroup cols="2" align="left">
    <tbody>
    <row>
        <entry>VIDEO_VC_TUNER</entry><entry>Channel has a tuner.</entry>
   </row><row>
        <entry>VIDEO_VC_AUDIO</entry><entry>Channel has audio.</entry>
    </row>
    </tbody>
    </tgroup>
    </table>
    <table frame="all"><title>struct video_channel types</title>
    <tgroup cols="2" align="left">
    <tbody>
    <row>
        <entry>VIDEO_TYPE_TV</entry><entry>Television input.</entry>
   </row><row>
        <entry>VIDEO_TYPE_CAMERA</entry><entry>Fixed camera input.</entry>
   </row><row>
	<entry>0</entry><entry>Type is unknown.</entry>
    </row>
    </tbody>
    </tgroup>
    </table>
    <table frame="all"><title>struct video_channel norms</title>
    <tgroup cols="2" align="left">
    <tbody>
    <row>
        <entry>VIDEO_MODE_PAL</entry><entry>PAL encoded Television</entry>
   </row><row>
        <entry>VIDEO_MODE_NTSC</entry><entry>NTSC (US) encoded Television</entry>
   </row><row>
        <entry>VIDEO_MODE_SECAM</entry><entry>SECAM (French) Television </entry>
   </row><row>
        <entry>VIDEO_MODE_AUTO</entry><entry>Automatic switching, or format does not
                                matter</entry>
    </row>
    </tbody>
    </tgroup>
    </table>
    <para>
        The corresponding VIDIOCSCHAN ioctl allows a user to change channel and to
        request the norm is changed - for example to switch between a PAL or an NTSC
        format camera.
  </para>
  <programlisting>


                case VIDIOCSCHAN:
                {
                        struct video_channel v;
                        if(copy_from_user(&amp;v, arg, sizeof(v)))
                                return -EFAULT;
                        if(v.channel != 0)
                                return -EINVAL;
                        if(v.norm != VIDEO_MODE_AUTO)
                                return -EINVAL;
                        return 0;
                }


  </programlisting>
  <para>
        The implementation of this call in our driver is remarkably easy. Because we
        are assuming fixed format hardware we need only check that the user has not
        tried to change anything. 
  </para>
  <para>
        The user also needs to be able to configure and adjust the picture they are
        seeing. This is much like adjusting a television set. A user application
        also needs to know the palette being used so that it knows how to display
        the image that has been captured. The VIDIOCGPICT and VIDIOCSPICT ioctl
        calls provide this information.
  </para>
  <programlisting>


                case VIDIOCGPICT
                {
                        struct video_picture v;
                        v.brightness = hardware_brightness();
                        v.hue = hardware_hue();
                        v.colour = hardware_saturation();
                        v.contrast = hardware_brightness();
                        /* Not settable */
                        v.whiteness = 32768;
                        v.depth = 24;           /* 24bit */
                        v.palette = VIDEO_PALETTE_RGB24;
                        if(copy_to_user(&amp;v, arg, 
                             sizeof(v)))
                                return -EFAULT;
                        return 0;
                }


  </programlisting>
  <para>
        The brightness, hue, color, and contrast provide the picture controls that
        are akin to a conventional television. Whiteness provides additional
        control for greyscale images. All of these values are scaled between 0-65535
        and have 32768 as the mid point setting. The scaling means that applications
        do not have to worry about the capability range of the hardware but can let
        it make a best effort attempt.
  </para>
  <para>
        Our depth is 24, as this is in bits. We will be returning RGB24 format. This
        has one byte of red, then one of green, then one of blue. This then repeats
        for every other pixel in the image. The other common formats the interface 
        defines are
  </para>
   <table frame="all"><title>Framebuffer Encodings</title>
   <tgroup cols="2" align="left">
   <tbody>
   <row>
   <entry>GREY</entry><entry>Linear greyscale. This is for simple cameras and the
                        like</entry>
   </row><row>
   <entry>RGB565</entry><entry>The top 5 bits hold 32 red levels, the next six bits
                        hold green and the low 5 bits hold blue. </entry>
   </row><row>
   <entry>RGB555</entry><entry>The top bit is clear. The red green and blue levels
                        each occupy five bits.</entry>
    </row>
    </tbody>
    </tgroup>
    </table>
  <para>
        Additional modes are support for YUV capture formats. These are common for
        TV and video conferencing applications.
  </para>
  <para>
        The VIDIOCSPICT ioctl allows a user to set some of the picture parameters.
        Exactly which ones are supported depends heavily on the card itself. It is
        possible to support many modes and effects in software. In general doing
        this in the kernel is a bad idea. Video capture is a performance-sensitive
        application and the programs can often do better if they aren't being
        'helped' by an overkeen driver writer. Thus for our device we will report
        RGB24 only and refuse to allow a change.
  </para>
  <programlisting>


                case VIDIOCSPICT:
                {
                        struct video_picture v;
                        if(copy_from_user(&amp;v, arg, sizeof(v)))
                                return -EFAULT;
                        if(v.depth!=24 || 
                           v.palette != VIDEO_PALETTE_RGB24)
                                return -EINVAL;
                        set_hardware_brightness(v.brightness);
                        set_hardware_hue(v.hue);
                        set_hardware_saturation(v.colour);
                        set_hardware_brightness(v.contrast);
                        return 0;
                }


  </programlisting>
  <para>
        We check the user has not tried to change the palette or the depth. We do
        not want to carry out some of the changes and then return an error. This may
        confuse the application which will be assuming no change occurred.
  </para>
  <para>
        In much the same way as you need to be able to set the picture controls to
        get the right capture images, many cards need to know what they are
        displaying onto when generating overlay output. In some cases getting this
        wrong even makes a nasty mess or may crash the computer. For that reason
        the VIDIOCSBUF ioctl used to set up the frame buffer information may well
        only be usable by root.
  </para>
  <para>
        We will assume our card is one of the old ISA devices with feature connector
        and only supports a couple of standard video modes. Very common for older
        cards although the PCI devices are way smarter than this.
  </para>
  <programlisting>


static struct video_buffer capture_fb;

                case VIDIOCGFBUF:
                {
                        if(copy_to_user(arg, &amp;capture_fb, 
                             sizeof(capture_fb)))
                                return -EFAULT;
                        return 0;
                        
                }


  </programlisting>
  <para>
        We keep the frame buffer information in the format the ioctl uses. This
        makes it nice and easy to work with in the ioctl calls.
  </para>
  <programlisting>

                case VIDIOCSFBUF:
                {
                        struct video_buffer v;

                        if(!capable(CAP_SYS_ADMIN))
                                return -EPERM;

                        if(copy_from_user(&amp;v, arg, sizeof(v)))
                                return -EFAULT;
                        if(v.width!=320 &amp;&amp; v.width!=640)
                                return -EINVAL;
                        if(v.height!=200 &amp;&amp; v.height!=240 
                                &amp;&amp; v.height!=400
                                &amp;&amp; v.height !=480)
                                return -EINVAL;
                        memcpy(&amp;capture_fb, &amp;v, sizeof(v));
                        hardware_set_fb(&amp;v);
                        return 0;
                }



  </programlisting>
  <para>
        The capable() function checks a user has the required capability. The Linux
        operating system has a set of about 30 capabilities indicating privileged
        access to services. The default set up gives the superuser (uid 0) all of
        them and nobody else has any.
  </para>
  <para>
        We check that the user has the SYS_ADMIN capability, that is they are
        allowed to operate as the machine administrator. We don't want anyone but
        the administrator making a mess of the display.
  </para>
  <para>
        Next we check for standard PC video modes (320 or 640 wide with either
        EGA or VGA depths). If the mode is not a standard video mode we reject it as
        not supported by our card. If the mode is acceptable we save it so that
        VIDIOCFBUF will give the right answer next time it is called.  The
        hardware_set_fb() function is some undescribed card specific function to
        program the card for the desired mode.
  </para>
  <para>
        Before the driver can display an overlay window it needs to know where the
        window should be placed, and also how large it should be. If the card
        supports clipping it needs to know which rectangles to omit from the
        display. The video_window structure is used to describe the way the image 
        should be displayed. 
   </para>
   <table frame="all"><title>struct video_window fields</title>
   <tgroup cols="2" align="left">
   <tbody>
   <row>
        <entry>width</entry><entry>The width in pixels of the desired image. The card
                        may use a smaller size if this size is not available</entry>
	</row><row>
        <entry>height</entry><entry>The height of the image. The card may use a smaller
                        size if this size is not available.</entry>
	</row><row>
        <entry>x</entry><entry>   The X position of the top left of the window. This
                        is in pixels relative to the left hand edge of the
                        picture. Not all cards can display images aligned on
                        any pixel boundary. If the position is unsuitable
                        the card adjusts the image right and reduces the
                        width.</entry>
	</row><row>
        <entry>y</entry><entry>   The Y position of the top left of the window. This
                        is counted in pixels relative to the top edge of the
                        picture. As with the width if the card cannot
                        display  starting on this line it will adjust the
                        values.</entry>
	</row><row>
        <entry>chromakey</entry><entry>The colour (expressed in RGB32 format) for the
                        chromakey colour if chroma keying is being used. </entry>
	</row><row>
        <entry>clips</entry><entry>An array of rectangles that must not be drawn
			over.</entry>
	</row><row>
        <entry>clipcount</entry><entry>The number of clips in this array.</entry>
    </row>
    </tbody>
    </tgroup>
    </table>
    <para>
        Each clip is a struct video_clip which has the following fields
   </para>
   <table frame="all"><title>video_clip fields</title>
   <tgroup cols="2" align="left">
   <tbody>
   <row>
        <entry>x, y</entry><entry>Co-ordinates relative to the display</entry>
	</row><row>
        <entry>width, height</entry><entry>Width and height in pixels</entry>
	</row><row>
        <entry>next</entry><entry>A spare field for the application to use</entry>
    </row>
    </tbody>
    </tgroup>
    </table>
    <para>
        The driver is required to ensure it always draws in the area requested or a        smaller area, and that it never draws in any of the areas that are clipped.
        This may well mean it has to leave alone. small areas the application wished to be
        drawn.
  </para>
  <para>
        Our example card uses chromakey so does not have to address most of the
        clipping.  We will add a video_window structure to our global variables to
        remember our parameters, as we did with the frame buffer.
  </para>
  <programlisting>


                case VIDIOCGWIN:
                {
                        if(copy_to_user(arg, &amp;capture_win, 
                            sizeof(capture_win)))
                                return -EFAULT;
                        return 0;
                }


                case VIDIOCSWIN:
                {
                        struct video_window v;
                        if(copy_from_user(&amp;v, arg, sizeof(v)))
                                return -EFAULT;
                        if(v.width &gt; 640 || v.height &gt; 480)
                                return -EINVAL;
                        if(v.width &lt; 16 || v.height &lt; 16)
                                return -EINVAL;
                        hardware_set_key(v.chromakey);
                        hardware_set_window(v);
                        memcpy(&amp;capture_win, &amp;v, sizeof(v));
                        capture_w = v.width;
                        capture_h = v.height;
                        return 0;
                }


  </programlisting>
  <para>
        Because we are using Chromakey our setup is fairly simple. Mostly we have to
        check the values are sane and load them into the capture card.
  </para>
  <para>
        With all the setup done we can now turn on the actual capture/overlay. This
        is done with the VIDIOCCAPTURE ioctl. This takes a single integer argument
        where 0 is on and 1 is off.
  </para>
  <programlisting>


                case VIDIOCCAPTURE:
                {
                        int v;
                        if(get_user(v, (int *)arg))
                                return -EFAULT;
                        if(v==0)
                                hardware_capture_off();
                        else
                        {
                                if(capture_fb.width == 0 
                                    || capture_w == 0)
                                        return -EINVAL;
                                hardware_capture_on();
                        }
                        return 0;
                }


  </programlisting>
  <para>
        We grab the flag from user space and either enable or disable according to
        its value. There is one small corner case we have to consider here. Suppose
        that the capture was requested before the video window or the frame buffer
        had been set up. In those cases there will be unconfigured fields in our
        card data, as well as unconfigured hardware settings. We check for this case and
        return an error if the frame buffer or the capture window width is zero.
  </para>
  <programlisting>


                default:
                        return -ENOIOCTLCMD;
        }
}
  </programlisting>
  <para>

        We don't need to support any other ioctls, so if we get this far, it is time
        to tell the video layer that we don't now what the user is talking about.
  </para>
  </sect1>
  <sect1 id="endvid">
  <title>Other Functionality</title>
  <para>
        The Video4Linux layer supports additional features, including a high
        performance mmap() based capture mode and capturing part of the image. 
        These features are out of the scope of the book.  You should however have enough 
        example code to implement most simple video4linux devices for radio and TV
        cards.
  </para>
  </sect1>
  </chapter>
  <chapter id="bugs">
     <title>Known Bugs And Assumptions</title>
  <para>
  <variablelist>
    <varlistentry><term>Multiple Opens</term>
    <listitem>
    <para>
        The driver assumes multiple opens should not be allowed. A driver
        can work around this but not cleanly.
    </para>
    </listitem></varlistentry>

    <varlistentry><term>API Deficiencies</term>
    <listitem>
    <para>
        The existing API poorly reflects compression capable devices. There
        are plans afoot to merge V4L, V4L2 and some other ideas into a
        better interface.
    </para>
    </listitem></varlistentry>
  </variablelist>

  </para>
  </chapter>

  <chapter id="pubfunctions">
     <title>Public Functions Provided</title>
!Edrivers/media/video/videodev.c
  </chapter>

</book>