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From: Ricardo Ribalda <ribalda@chromium.org>
Date: Sat, 23 Mar 2024 10:48:05 +0000
Subject: [PATCH v10 4/6] media: uvcvideo: Allow hw clock updates with
 buffers not full
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Message-Id: <20240323-resend-hwtimestamp-v10-4-b08e590d97c7@chromium.org>
References: <20240323-resend-hwtimestamp-v10-0-b08e590d97c7@chromium.org>
In-Reply-To: <20240323-resend-hwtimestamp-v10-0-b08e590d97c7@chromium.org>
To: Laurent Pinchart <laurent.pinchart@ideasonboard.com>, 
 Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: linux-media@vger.kernel.org, linux-kernel@vger.kernel.org, 
 "hn.chen" <hn.chen@sunplusit.com>, Hans Verkuil <hverkuil@xs4all.nl>, 
 Ricardo Ribalda <ribalda@chromium.org>, 
 Sergey Senozhatsky <senozhatsky@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>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
---
 drivers/media/usb/uvc/uvc_video.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
index d6ca383f643e3..af25b9f1b53fe 100644
--- a/drivers/media/usb/uvc/uvc_video.c
+++ b/drivers/media/usb/uvc/uvc_video.c
@@ -768,10 +768,10 @@ 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) % clock->size];
 	last = &clock->samples[(clock->head - 1 + clock->size) % clock->size];
 
 	/* First step, PTS to SOF conversion. */
@@ -786,6 +786,18 @@ 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.
+	 *
+	 * dev_sof runs at 1KHz, and we have a fixed point precision of
+	 * 16 bits.
+	 */
+	if ((y2 - y1) < ((1000 / 4) << 16))
+		goto done;
+
 	y = (u64)(y2 - y1) * (1ULL << 31) + (u64)y1 * (u64)x2
 	  - (u64)y2 * (u64)x1;
 	y = div_u64(y, x2 - x1);

-- 
2.44.0.396.g6e790dbe36-goog