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| author |   Alan Stern <stern@rowland.harvard.edu> | 2012-10-01 10:31:53 -0400 |
|---|---|---|
| committer |   Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2012-10-22 11:10:24 -0700 |
| commit | a03bede5c73a6876fa891cfe82a65460dc9f4698 (patch) | |
| tree | 608948e319564fce6a5cf68bc3ad09c1cdf5e010 /include/linux/usb.h | |
| parent | EHCI: replace mult/div with bit-mask operation (diff) | |
| download | linux-dev-a03bede5c73a6876fa891cfe82a65460dc9f4698.tar.xz linux-dev-a03bede5c73a6876fa891cfe82a65460dc9f4698.zip | |
USB: update documentation for URB_ISO_ASAP
This patch (as1611) updates the USB documentation and kerneldoc to
give a more precise meaning for the URB_ISO_ASAP flag and to explain
more of the details of scheduling for isochronous URBs.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'include/linux/usb.h')
| -rw-r--r-- | include/linux/usb.h | 27 |
1 files changed, 16 insertions, 11 deletions
diff --git a/include/linux/usb.h b/include/linux/usb.h index 10278d18709c..f92cdf0c1457 100644 --- a/include/linux/usb.h +++ b/include/linux/usb.h @@ -1129,8 +1129,8 @@ extern int usb_disabled(void); * Note: URB_DIR_IN/OUT is automatically set in usb_submit_urb(). */ #define URB_SHORT_NOT_OK 0x0001 /* report short reads as errors */ -#define URB_ISO_ASAP 0x0002 /* iso-only, urb->start_frame - * ignored */ +#define URB_ISO_ASAP 0x0002 /* iso-only; use the first unexpired + * slot in the schedule */ #define URB_NO_TRANSFER_DMA_MAP 0x0004 /* urb->transfer_dma valid on submit */ #define URB_NO_FSBR 0x0020 /* UHCI-specific */ #define URB_ZERO_PACKET 0x0040 /* Finish bulk OUT with short packet */ @@ -1309,15 +1309,20 @@ typedef void (*usb_complete_t)(struct urb *); * the transfer interval in the endpoint descriptor is logarithmic. * Device drivers must convert that value to linear units themselves.) * - * Isochronous URBs normally use the URB_ISO_ASAP transfer flag, telling - * the host controller to schedule the transfer as soon as bandwidth - * utilization allows, and then set start_frame to reflect the actual frame - * selected during submission. Otherwise drivers must specify the start_frame - * and handle the case where the transfer can't begin then. However, drivers - * won't know how bandwidth is currently allocated, and while they can - * find the current frame using usb_get_current_frame_number () they can't - * know the range for that frame number. (Ranges for frame counter values - * are HC-specific, and can go from 256 to 65536 frames from "now".) + * If an isochronous endpoint queue isn't already running, the host + * controller will schedule a new URB to start as soon as bandwidth + * utilization allows. If the queue is running then a new URB will be + * scheduled to start in the first transfer slot following the end of the + * preceding URB, if that slot has not already expired. If the slot has + * expired (which can happen when IRQ delivery is delayed for a long time), + * the scheduling behavior depends on the URB_ISO_ASAP flag. If the flag + * is clear then the URB will be scheduled to start in the expired slot, + * implying that some of its packets will not be transferred; if the flag + * is set then the URB will be scheduled in the first unexpired slot, + * breaking the queue's synchronization. Upon URB completion, the + * start_frame field will be set to the (micro)frame number in which the + * transfer was scheduled. Ranges for frame counter values are HC-specific + * and can go from as low as 256 to as high as 65536 frames. * * Isochronous URBs have a different data transfer model, in part because * the quality of service is only "best effort". Callers provide specially |