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diff --git a/Documentation/sound/alsa/timestamping.txt b/Documentation/sound/alsa/timestamping.txt deleted file mode 100644 index 9d579aefbffd..000000000000 --- a/Documentation/sound/alsa/timestamping.txt +++ /dev/null @@ -1,200 +0,0 @@ -The ALSA API can provide two different system timestamps: - -- Trigger_tstamp is the system time snapshot taken when the .trigger -callback is invoked. This snapshot is taken by the ALSA core in the -general case, but specific hardware may have synchronization -capabilities or conversely may only be able to provide a correct -estimate with a delay. In the latter two cases, the low-level driver -is responsible for updating the trigger_tstamp at the most appropriate -and precise moment. Applications should not rely solely on the first -trigger_tstamp but update their internal calculations if the driver -provides a refined estimate with a delay. - -- tstamp is the current system timestamp updated during the last -event or application query. -The difference (tstamp - trigger_tstamp) defines the elapsed time. - -The ALSA API provides two basic pieces of information, avail -and delay, which combined with the trigger and current system -timestamps allow for applications to keep track of the 'fullness' of -the ring buffer and the amount of queued samples. - -The use of these different pointers and time information depends on -the application needs: - -- 'avail' reports how much can be written in the ring buffer -- 'delay' reports the time it will take to hear a new sample after all -queued samples have been played out. - -When timestamps are enabled, the avail/delay information is reported -along with a snapshot of system time. Applications can select from -CLOCK_REALTIME (NTP corrections including going backwards), -CLOCK_MONOTONIC (NTP corrections but never going backwards), -CLOCK_MONOTIC_RAW (without NTP corrections) and change the mode -dynamically with sw_params - - -The ALSA API also provide an audio_tstamp which reflects the passage -of time as measured by different components of audio hardware. In -ascii-art, this could be represented as follows (for the playback -case): - - ---------------------------------------------------------------> time - ^ ^ ^ ^ ^ - | | | | | - analog link dma app FullBuffer - time time time time time - | | | | | - |< codec delay >|<--hw delay-->|<queued samples>|<---avail->| - |<----------------- delay---------------------->| | - |<----ring buffer length---->| - -The analog time is taken at the last stage of the playback, as close -as possible to the actual transducer - -The link time is taken at the output of the SoC/chipset as the samples -are pushed on a link. The link time can be directly measured if -supported in hardware by sample counters or wallclocks (e.g. with -HDAudio 24MHz or PTP clock for networked solutions) or indirectly -estimated (e.g. with the frame counter in USB). - -The DMA time is measured using counters - typically the least reliable -of all measurements due to the bursty nature of DMA transfers. - -The app time corresponds to the time tracked by an application after -writing in the ring buffer. - -The application can query the hardware capabilities, define which -audio time it wants reported by selecting the relevant settings in -audio_tstamp_config fields, thus get an estimate of the timestamp -accuracy. It can also request the delay-to-analog be included in the -measurement. Direct access to the link time is very interesting on -platforms that provide an embedded DSP; measuring directly the link -time with dedicated hardware, possibly synchronized with system time, -removes the need to keep track of internal DSP processing times and -latency. - -In case the application requests an audio tstamp that is not supported -in hardware/low-level driver, the type is overridden as DEFAULT and the -timestamp will report the DMA time based on the hw_pointer value. - -For backwards compatibility with previous implementations that did not -provide timestamp selection, with a zero-valued COMPAT timestamp type -the results will default to the HDAudio wall clock for playback -streams and to the DMA time (hw_ptr) in all other cases. - -The audio timestamp accuracy can be returned to user-space, so that -appropriate decisions are made: - -- for dma time (default), the granularity of the transfers can be - inferred from the steps between updates and in turn provide - information on how much the application pointer can be rewound - safely. - -- the link time can be used to track long-term drifts between audio - and system time using the (tstamp-trigger_tstamp)/audio_tstamp - ratio, the precision helps define how much smoothing/low-pass - filtering is required. The link time can be either reset on startup - or reported as is (the latter being useful to compare progress of - different streams - but may require the wallclock to be always - running and not wrap-around during idle periods). If supported in - hardware, the absolute link time could also be used to define a - precise start time (patches WIP) - -- including the delay in the audio timestamp may - counter-intuitively not increase the precision of timestamps, e.g. if a - codec includes variable-latency DSP processing or a chain of - hardware components the delay is typically not known with precision. - -The accuracy is reported in nanosecond units (using an unsigned 32-bit -word), which gives a max precision of 4.29s, more than enough for -audio applications... - -Due to the varied nature of timestamping needs, even for a single -application, the audio_tstamp_config can be changed dynamically. In -the STATUS ioctl, the parameters are read-only and do not allow for -any application selection. To work around this limitation without -impacting legacy applications, a new STATUS_EXT ioctl is introduced -with read/write parameters. ALSA-lib will be modified to make use of -STATUS_EXT and effectively deprecate STATUS. - -The ALSA API only allows for a single audio timestamp to be reported -at a time. This is a conscious design decision, reading the audio -timestamps from hardware registers or from IPC takes time, the more -timestamps are read the more imprecise the combined measurements -are. To avoid any interpretation issues, a single (system, audio) -timestamp is reported. Applications that need different timestamps -will be required to issue multiple queries and perform an -interpolation of the results - -In some hardware-specific configuration, the system timestamp is -latched by a low-level audio subsystem, and the information provided -back to the driver. Due to potential delays in the communication with -the hardware, there is a risk of misalignment with the avail and delay -information. To make sure applications are not confused, a -driver_timestamp field is added in the snd_pcm_status structure; this -timestamp shows when the information is put together by the driver -before returning from the STATUS and STATUS_EXT ioctl. in most cases -this driver_timestamp will be identical to the regular system tstamp. - -Examples of typestamping with HDaudio: - -1. DMA timestamp, no compensation for DMA+analog delay -$ ./audio_time -p --ts_type=1 -playback: systime: 341121338 nsec, audio time 342000000 nsec, systime delta -878662 -playback: systime: 426236663 nsec, audio time 427187500 nsec, systime delta -950837 -playback: systime: 597080580 nsec, audio time 598000000 nsec, systime delta -919420 -playback: systime: 682059782 nsec, audio time 683020833 nsec, systime delta -961051 -playback: systime: 852896415 nsec, audio time 853854166 nsec, systime delta -957751 -playback: systime: 937903344 nsec, audio time 938854166 nsec, systime delta -950822 - -2. DMA timestamp, compensation for DMA+analog delay -$ ./audio_time -p --ts_type=1 -d -playback: systime: 341053347 nsec, audio time 341062500 nsec, systime delta -9153 -playback: systime: 426072447 nsec, audio time 426062500 nsec, systime delta 9947 -playback: systime: 596899518 nsec, audio time 596895833 nsec, systime delta 3685 -playback: systime: 681915317 nsec, audio time 681916666 nsec, systime delta -1349 -playback: systime: 852741306 nsec, audio time 852750000 nsec, systime delta -8694 - -3. link timestamp, compensation for DMA+analog delay -$ ./audio_time -p --ts_type=2 -d -playback: systime: 341060004 nsec, audio time 341062791 nsec, systime delta -2787 -playback: systime: 426242074 nsec, audio time 426244875 nsec, systime delta -2801 -playback: systime: 597080992 nsec, audio time 597084583 nsec, systime delta -3591 -playback: systime: 682084512 nsec, audio time 682088291 nsec, systime delta -3779 -playback: systime: 852936229 nsec, audio time 852940916 nsec, systime delta -4687 -playback: systime: 938107562 nsec, audio time 938112708 nsec, systime delta -5146 - -Example 1 shows that the timestamp at the DMA level is close to 1ms -ahead of the actual playback time (as a side time this sort of -measurement can help define rewind safeguards). Compensating for the -DMA-link delay in example 2 helps remove the hardware buffering but -the information is still very jittery, with up to one sample of -error. In example 3 where the timestamps are measured with the link -wallclock, the timestamps show a monotonic behavior and a lower -dispersion. - -Example 3 and 4 are with USB audio class. Example 3 shows a high -offset between audio time and system time due to buffering. Example 4 -shows how compensating for the delay exposes a 1ms accuracy (due to -the use of the frame counter by the driver) - -Example 3: DMA timestamp, no compensation for delay, delta of ~5ms -$ ./audio_time -p -Dhw:1 -t1 -playback: systime: 120174019 nsec, audio time 125000000 nsec, systime delta -4825981 -playback: systime: 245041136 nsec, audio time 250000000 nsec, systime delta -4958864 -playback: systime: 370106088 nsec, audio time 375000000 nsec, systime delta -4893912 -playback: systime: 495040065 nsec, audio time 500000000 nsec, systime delta -4959935 -playback: systime: 620038179 nsec, audio time 625000000 nsec, systime delta -4961821 -playback: systime: 745087741 nsec, audio time 750000000 nsec, systime delta -4912259 -playback: systime: 870037336 nsec, audio time 875000000 nsec, systime delta -4962664 - -Example 4: DMA timestamp, compensation for delay, delay of ~1ms -$ ./audio_time -p -Dhw:1 -t1 -d -playback: systime: 120190520 nsec, audio time 120000000 nsec, systime delta 190520 -playback: systime: 245036740 nsec, audio time 244000000 nsec, systime delta 1036740 -playback: systime: 370034081 nsec, audio time 369000000 nsec, systime delta 1034081 -playback: systime: 495159907 nsec, audio time 494000000 nsec, systime delta 1159907 -playback: systime: 620098824 nsec, audio time 619000000 nsec, systime delta 1098824 -playback: systime: 745031847 nsec, audio time 744000000 nsec, systime delta 1031847 |