On Wed May 13 11:49:21 2026 +0000, Ricardo Ribalda wrote:
> In some situations, even with a full clock buffer, it does not contain
> 250msec of data. This results in the driver jumping back from software
> to hardware timestapsing creating a nasty artifact in the video.
>
> If the clock buffer is full, use it to calculate the timestamp instead
> of defaulting to software stamps, the reduced accuracy is less visible
> than jumping from one timestamping mechanism to the other.
>
> Fixes: 6243c83be6ee8 ("media: uvcvideo: Allow hw clock updates with buffers
> not full")
> Cc: [email protected]
> Reviewed-by: Hans de Goede <[email protected]>
> Tested-by: Yunke Cao <[email protected]>
> Signed-off-by: Ricardo Ribalda <[email protected]>
> Link:
> https://patch.msgid.link/[email protected]
> Signed-off-by: Hans de Goede <[email protected]>
> Signed-off-by: Hans Verkuil <[email protected]>
Patch committed.
Thanks,
Hans Verkuil
drivers/media/usb/uvc/uvc_video.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)
---
diff --git a/drivers/media/usb/uvc/uvc_video.c
b/drivers/media/usb/uvc/uvc_video.c
index efd746dd5db0..bb3ee942e570 100644
--- a/drivers/media/usb/uvc/uvc_video.c
+++ b/drivers/media/usb/uvc/uvc_video.c
@@ -834,15 +834,22 @@ void uvc_video_clock_update(struct uvc_streaming *stream,
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 the buffer is not full, we want to gather at least 1/4th of
+ * timestamps before using HW timestamping. We do this to avoid jitter
+ * on the initial frames.
+ *
+ * If the buffer is full we would use it regardless of how much data
+ * it represents. This could be solved with an infinite big circular
+ * buffer, but RAM is expensive these days, specially the infinitely
+ * big.
+ *
+ * The value of 1/4th of a second was determined by running Android's
+ * 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))
+ if (clock->size != clock->count && (y2 - y1) < ((1000 / 4) << 16))
goto done;
y = (u64)(y2 - y1) * (1ULL << 31) + (u64)y1 * (u64)x2
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