linux-kernel - [PATCH v4 3/5] media: uvcvideo: Allow hw clock updates with buffers not full

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Message-Id: <20220920-resend-hwtimestamp-v4-3-a8ddc1358a29@chromium.org>
Date:   Mon, 09 Jan 2023 11:00:50 +0100
From:   Ricardo Ribalda <ribalda@...omium.org>
To:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Ricardo Ribalda <ribalda@...omium.org>,
        "hn.chen" <hn.chen@...plusit.com>, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org
Subject: [PATCH v4 3/5] media: uvcvideo: Allow hw clock updates with buffers not full

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@...plusit.com>
Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
---
 drivers/media/usb/uvc/uvc_video.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
index ecd3c1292182..ee0f40b9eae3 100644
--- a/drivers/media/usb/uvc/uvc_video.c
+++ b/drivers/media/usb/uvc/uvc_video.c
@@ -747,10 +747,10 @@ 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. */
@@ -765,6 +765,15 @@ 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
+	 * precision. This value was determined by running Android CTS
+	 * on different devices.
+	 */
+	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);

-- 
2.39.0.314.g84b9a713c41-goog-b4-0.11.0-dev-696ae