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USB: update documentation for URB_ISO_ASAP

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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>
This commit is contained in:
Alan Stern
2012-10-01 10:31:53 -04:00
committed by Greg Kroah-Hartman
parent 7267547992
commit a03bede5c7
3 changed files with 37 additions and 17 deletions
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27
include/linux/usb.h
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@@ -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