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>

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