[PATCH RESEND v2 6/8] media: uvcvideo: Allow hw clock updates with buffers not full

[Ricardo Ribalda <ribalda@chromium.org>]

With UVC 1.5 we get as little as one clock sample per frame. Which means
that it takes 32 frames to move from the software timestamp to the
hardware timestamp method.

This results in abrupt changes in the timestamping after 32 frames (~1
second), resulting in noticeable artifacts when used for encoding.

With this patch we modify the update algorithm to work with whatever
amount of values are available.

Tested-by: HungNien Chen <hn.chen@sunplusit.com>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
---
 drivers/media/usb/uvc/uvc_video.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
index 75c32e232f5d..7c6448c6d706 100644
--- a/drivers/media/usb/uvc/uvc_video.c
+++ b/drivers/media/usb/uvc/uvc_video.c
@@ -742,10 +742,10 @@ void uvc_video_clock_update(struct uvc_streaming *stream,
 
 	spin_lock_irqsave(&clock->lock, flags);
 
-	if (clock->count < clock->size)
+	if (clock->count < 2)
 		goto done;
 
-	first = &clock->samples[clock->head];
+	first = &clock->samples[(clock->head - clock->count) % clock->size];
 	last = &clock->samples[(clock->head - 1) % clock->size];
 
 	/* First step, PTS to SOF conversion. */
@@ -760,6 +760,14 @@ void uvc_video_clock_update(struct uvc_streaming *stream,
 	if (y2 < y1)
 		y2 += 2048 << 16;
 
+	/*
+	 * Have at least 1/4 of a second of timestamps before we
+	 * try to do any calculation. Otherwise we do not have enough
+	 * precission.
+	 */
+	if ((y2 - y1) < (256 << 16))
+		goto done;
+
 	y = (u64)(y2 - y1) * (1ULL << 31) + (u64)y1 * (u64)x2
 	  - (u64)y2 * (u64)x1;
 	y = div_u64(y, x2 - x1);

-- 
2.39.0.rc0.267.gcb52ba06e7-goog-b4-0.11.0-dev-696ae