linux-kernel - [PATCH v5 5/5] media: uvcvideo: Fix hw timestamp handling for slow FPS

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Message-Id: <20220920-resend-hwtimestamp-v5-5-660679c6e148@chromium.org>
Date:   Fri, 13 Jan 2023 14:48:51 +0100
From:   Ricardo Ribalda <ribalda@...omium.org>
To:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     "hn.chen" <hn.chen@...plusit.com>, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, Ricardo Ribalda <ribalda@...omium.org>
Subject: [PATCH v5 5/5] media: uvcvideo: Fix hw timestamp handling for slow FPS

In UVC 1.5 we get a single clock value per frame. With the current
buffer size of 32, FPS slowers than 32 might roll-over twice.

The current code cannot handle two roll-over and provide invalid
timestamps.

Revome all the samples from the circular buffer that are more than two
rollovers old, so the algorithm always provides good timestamps.

Note that we are removing values that are more than one second old,
which means that there is enough distance between the two points that
we use for the interpolation to provide good values.

Tested-by: HungNien Chen <hn.chen@...plusit.com>
Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
---
 drivers/media/usb/uvc/uvc_video.c | 15 +++++++++++++++
 drivers/media/usb/uvc/uvcvideo.h  |  1 +
 2 files changed, 16 insertions(+)

diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
index 22255bfa7db9..ae692122d5dc 100644
--- a/drivers/media/usb/uvc/uvc_video.c
+++ b/drivers/media/usb/uvc/uvc_video.c
@@ -473,6 +473,20 @@ static void uvc_video_clock_add_sample(struct uvc_clock *clock,
 
 	spin_lock_irqsave(&clock->lock, flags);
 
+	/* Delete last overflows */
+	if (clock->head == clock->last_sof_overflow)
+		clock->last_sof_overflow = -1;
+
+	/* Handle overflows */
+	if (clock->count > 0 && clock->last_sof > sample->dev_sof) {
+		/* Remove data from the last^2 overflows */
+		if (clock->last_sof_overflow != -1)
+			clock->count = (clock->head - clock->last_sof_overflow)
+								% clock->count;
+		clock->last_sof_overflow = clock->head;
+	}
+
+	/* Add sample */
 	memcpy(&clock->samples[clock->head], sample, sizeof(*sample));
 	clock->head = (clock->head + 1) % clock->size;
 	clock->count = min(clock->count + 1, clock->size);
@@ -600,6 +614,7 @@ static void uvc_video_clock_reset(struct uvc_clock *clock)
 	clock->head = 0;
 	clock->count = 0;
 	clock->last_sof = -1;
+	clock->last_sof_overflow = -1;
 	clock->sof_offset = -1;
 }
 
diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
index 699c33bc24ed..f2a7e1507492 100644
--- a/drivers/media/usb/uvc/uvcvideo.h
+++ b/drivers/media/usb/uvc/uvcvideo.h
@@ -501,6 +501,7 @@ struct uvc_streaming {
 		unsigned int head;
 		unsigned int count;
 		unsigned int size;
+		unsigned int last_sof_overflow;
 
 		u16 last_sof;
 		u16 sof_offset;

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