[PATCH 6/6] usb: musb: Decrease URB starting latency in musb_advance_schedule()

["Matwey V. Kornilov" <matwey@sai.msu.ru>]

Previously, the algorithm was the following:

 1. giveback current URB
 2. if current qh is not empty
    then start next URB
 3. if current qh is empty
    then dispose the qh, find next qh if any, and start URB.

It may take a while to run urb->callback inside URB giveback which is
run synchronously in musb. In order to improve the latency we rearrange
the function behaviour for the case when qh is not empty: next URB is
started before URB giveback. When qh is empty then the behaviour is
intentionally kept in order not to break existing inter qh scheduling:
URB giveback could potentionally enqueue other URB to the empty qh
preventing it from being disposed.

Before this patch, time spent in urb->callback led to the following
glitches between the host and a hub during isoc transfer (line 4):

    11.624492 d=  0.000124 [130.6 +  1.050] [  4] SPLIT
    11.624492 d=  0.000000 [130.6 +  1.467] [  3] IN   : 3.5
    11.624493 d=  0.000000 [130.6 +  1.967] [ 37] DATA0: aa 08 [skipped...]
    11.625617 d=  0.001124 [131.7 +  1.050] [  4] SPLIT
    11.625617 d=  0.000000 [131.7 +  1.467] [  3] IN   : 3.5
    11.625867 d=  0.000250 [132.1 +  1.050] [  4] SPLIT
    11.625867 d=  0.000000 [132.1 +  1.467] [  3] IN   : 3.5
    11.625868 d=  0.000001 [132.1 +  1.983] [  3] DATA0: 00 00
    11.626617 d=  0.000749 [132.7 +  1.050] [  4] SPLIT
    11.626617 d=  0.000000 [132.7 +  1.467] [  3] IN   : 3.5
    11.626867 d=  0.000250 [133.1 +  1.050] [  4] SPLIT
    11.626867 d=  0.000000 [133.1 +  1.467] [  3] IN   : 3.5
    11.626868 d=  0.000000 [133.1 +  1.967] [  3] DATA0: 00 00

After the hub, they look as the following and may lead to broken
perepherial transfer (as in case of PWC based webcam):

    11.332004 d=  0.000997 [ 30.0 +  3.417] [  3] IN   : 5.5
    11.332007 d=  0.000003 [ 30.0 +  6.833] [800] DATA0: 8a 1c [skipped...]
    11.334004 d=  0.001997 [ 32.0 +  3.417] [  3] IN   : 5.5
    11.334007 d=  0.000003 [ 32.0 +  6.750] [  3] DATA0: 00 00
    11.335004 d=  0.000997 [ 33   +  3.417] [  3] IN   : 5.5
    11.335007 d=  0.000003 [ 33   +  6.750] [  3] DATA0: 00 00

Removing this glitches makes us able to successfully run 10fps
video stream from the webcam attached via USB hub. That was
previously impossible.

Signed-off-by: Matwey V. Kornilov <matwey@sai.msu.ru>
---
 drivers/usb/musb/musb_host.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/usb/musb/musb_host.c b/drivers/usb/musb/musb_host.c
index ed99ecd4e63a..75be92873b5b 100644
--- a/drivers/usb/musb/musb_host.c
+++ b/drivers/usb/musb/musb_host.c
@@ -85,6 +85,11 @@ static bool musb_qh_empty(struct musb_qh *qh)
 	return list_empty(&qh->hep->urb_list);
 }
 
+static bool musb_qh_singular(struct musb_qh *qh)
+{
+	return list_is_singular(&qh->hep->urb_list);
+}
+
 static void musb_qh_unlink_hep(struct musb_qh *qh)
 {
 	if (!qh->hep)
@@ -362,6 +367,19 @@ static void musb_advance_schedule(struct musb *musb, struct urb *urb,
 		break;
 	}
 
+	if (ready && !musb_qh_singular(qh)) {
+		struct urb *next_urb = list_next_entry(urb, urb_list);
+
+		musb_dbg(musb, "... next ep%d %cX urb %p", hw_ep->epnum, is_in ? 'R' : 'T', next_urb);
+		musb_start_urb(musb, is_in, qh, next_urb);
+
+		qh->is_ready = 0;
+		musb_giveback(musb, urb, status);
+		qh->is_ready = ready;
+
+		return;
+	}
+
 	qh->is_ready = 0;
 	musb_giveback(musb, urb, status);
 	qh->is_ready = ready;
-- 
2.16.4