[PATCH v3 8/8] media: uvcvideo: Fix hw timestamp handling for slow FPS

[Ricardo Ribalda]

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@xxxxxxxxxxxxx>
Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
---
 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 2c6da49ff84b..b0b93e218288 100644
--- a/drivers/media/usb/uvc/uvc_video.c
+++ b/drivers/media/usb/uvc/uvc_video.c
@@ -471,6 +471,20 @@ 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->last_sof = sample->dev_sof;
 	clock->head = (clock->head + 1) % clock->size;
@@ -595,6 +609,7 @@ 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 24c3e9411415..6dc015754e99 100644
--- a/drivers/media/usb/uvc/uvcvideo.h
+++ b/drivers/media/usb/uvc/uvcvideo.h
@@ -503,6 +503,7 @@ 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