USB: EHCI: start new isochronous streams ASAP

This patch changes the initial delay before the startup of a newly
scheduled isochronous stream.  Currently the stream doesn't start
for at least 5 ms (40 microframes).  This value is just an estimate;
it has no real justification.

Instead, we can start the stream as soon as possible after the
scheduling computations are complete.  Essentially this requires
nothing more than reading the frame counter after the stream is
scheduled, instead of before.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/ehci-sched.c

@@ -1491,8 +1491,6 @@ sitd_slot_ok (
  * given EHCI_TUNE_FLS and the slop).  Or, write a smarter scheduler!
  */
 
-#define SCHEDULING_DELAY	40	/* microframes */
-
 static int
 iso_stream_schedule (
 	struct ehci_hcd		*ehci,
@@ -1506,27 +1504,13 @@ iso_stream_schedule (
 	unsigned		mod = ehci->periodic_size << 3;
 	struct ehci_iso_sched	*sched = urb->hcpriv;
 	bool			empty = list_empty(&stream->td_list);
+	bool			new_stream = false;
 
 	period = stream->uperiod;
 	span = sched->span;
 	if (!stream->highspeed)
 		span <<= 3;
 
-	now = ehci_read_frame_index(ehci) & (mod - 1);
-
-	/* Take the isochronous scheduling threshold into account */
-	if (ehci->i_thresh)
-		next = now + ehci->i_thresh;	/* uframe cache */
-	else
-		next = (now + 2 + 7) & ~0x07;	/* full frame cache */
-
-	/*
-	 * Use ehci->last_iso_frame as the base.  There can't be any
-	 * TDs scheduled for earlier than that.
-	 */
-	base = ehci->last_iso_frame << 3;
-	next = (next - base) & (mod - 1);
-
 	/* Start a new isochronous stream? */
 	if (unlikely(empty && !hcd_periodic_completion_in_progress(
 			ehci_to_hcd(ehci), urb->ep))) {
@@ -1542,7 +1526,7 @@ iso_stream_schedule (
 			}
 			compute_tt_budget(ehci->tt_budget, tt);
 
-			start = (now & ~0x07) + SCHEDULING_DELAY;
+			start = ((-(++ehci->random_frame)) << 3) & (period - 1);
 
 			/* find a uframe slot with enough bandwidth.
 			 * Early uframes are more precious because full-speed
@@ -1585,17 +1569,35 @@ iso_stream_schedule (
 			start = (stream->ps.phase << 3) + stream->ps.phase_uf;
 		}
 
-		start = (start - base) & (mod - 1);
-		goto use_start;
+		stream->next_uframe = start;
+		new_stream = true;
 	}
 
+	now = ehci_read_frame_index(ehci) & (mod - 1);
+
+	/* Take the isochronous scheduling threshold into account */
+	if (ehci->i_thresh)
+		next = now + ehci->i_thresh;	/* uframe cache */
+	else
+		next = (now + 2 + 7) & ~0x07;	/* full frame cache */
+
+	/*
+	 * Use ehci->last_iso_frame as the base.  There can't be any
+	 * TDs scheduled for earlier than that.
+	 */
+	base = ehci->last_iso_frame << 3;
+	next = (next - base) & (mod - 1);
+	start = (stream->next_uframe - base) & (mod - 1);
+
+	if (unlikely(new_stream))
+		goto do_ASAP;
+
 	/*
 	 * Typical case: reuse current schedule, stream may still be active.
 	 * Hopefully there are no gaps from the host falling behind
 	 * (irq delays etc).  If there are, the behavior depends on
 	 * whether URB_ISO_ASAP is set.
 	 */
-	start = (stream->next_uframe - base) & (mod - 1);
 	now2 = (now - base) & (mod - 1);
 
 	/* Is the schedule already full? */