Re: [PATCH v3 4/6] media: uvcvideo: Do not add clock samples with small sof delta

[Ricardo Ribalda]

Hi Hans

On Wed, 13 May 2026 at 13:49, Ricardo Ribalda <ribalda@xxxxxxxxxxxx> wrote:
>
> Some UVC 1.1 cameras running in fast isochronous mode tend to spam the
> USB host with a lot of empty packets. These packets contain clock
> information and are added to the clock buffer but do not add any
> accuracy to the calculation. In fact, it is quite the opposite, in our
> calculations, only the first and the last timestamp is used, and we only
> have 32 slots.
>
> Ignore the samples that will produce less than MIN_HW_TIMESTAMP_DIFF
> data.
>
> Fixes: 141270bd95d4 ("media: uvcvideo: Refactor clock circular buffer")
> Cc: stable@xxxxxxxxxxxxxxx
> Tested-by: Yunke Cao <yunkec@xxxxxxxxxx>
> Reviewed-by: Hans de Goede <johannes.goede@xxxxxxxxxxxxxxxx>
> Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
> ---
>  drivers/media/usb/uvc/uvc_video.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index 355b9bfb799e..63850b779e24 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -544,6 +544,15 @@ static void uvc_video_clock_add_sample(struct uvc_clock *clock,
>         spin_unlock_irqrestore(&clock->lock, flags);
>  }
>
> +static inline u16 sof_diff(u16 a, u16 b)
> +{
> +       /*
> +        * Because the result is modulo 2048 (via & 2047), we do not need a
> +        * special case for a < b.
> +        */
> +       return (a - b) & 2047;
> +}
I have modified this function but kept your R-b. Hope that it is fine.

Thanks!


> +
>  static void
>  uvc_video_clock_decode(struct uvc_streaming *stream, struct uvc_buffer *buf,
>                        const u8 *data, int len)
> @@ -664,12 +673,13 @@ uvc_video_clock_decode(struct uvc_streaming *stream, struct uvc_buffer *buf,
>         sample.dev_sof = (sample.dev_sof + stream->clock.sof_offset) & 2047;
>
>         /*
> -        * To limit the amount of data, drop SCRs with an SOF identical to the
> +        * To limit the amount of data, drop SCRs with an SOF similar to the
>          * previous one. This filtering is also needed to support UVC 1.5, where
>          * all the data packets of the same frame contains the same SOF. In that
>          * case only the first one will match the host_sof.
>          */
> -       if (sample.dev_sof == stream->clock.last_sof)
> +       if (sof_diff(sample.dev_sof, stream->clock.last_sof) <=
> +           (UVC_MIN_HW_TIMESTAMP_DIFF / stream->clock.size))
>                 return;
>
>         uvc_video_clock_add_sample(&stream->clock, &sample);
>
> --
> 2.54.0.563.g4f69b47b94-goog
>


-- 
Ricardo Ribalda