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>
diff --git a/include/linux/usb.h b/include/linux/usb.h
index 10278d1..f92cdf0 100644
--- a/include/linux/usb.h
+++ b/include/linux/usb.h
@@ -1129,8 +1129,8 @@
* 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 @@
* 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