Re: Bug#1121718: linux-image-6.17.8+deb14-amd64: Logitech C920 HD Pro Webcam shows flickering artifacts (sometimes)

[Salvatore Bonaccorso <carnil@debian.org>]

Hi Ralf,

On Fri, Jan 02, 2026 at 02:24:20PM +0100, Ralf Jung wrote:
> Hi all,
> 
> I did a (lengthy) bisect session, and came out with this commit:

Thanks for doing so and for the time invested!

> 52fbe173baa4df9d14bd733f42ee6b9ceab8299b is the first bad commit
> commit 52fbe173baa4df9d14bd733f42ee6b9ceab8299b (HEAD)
> Author: Ricardo Ribalda <ribalda@chromium.org>
> Date:   Wed Dec 18 21:39:09 2024 +0000
> 
>     media: uvcvideo: Invert default value for nodrop module param
> 
>     The module param `nodrop` defines what to do with frames that contain an
>     error: drop them or sending them to userspace.
> 
>     The default in the rest of the media subsystem is to return buffers with
>     an error to userspace with V4L2_BUF_FLAG_ERROR set in v4l2_buffer.flags.
>     In UVC we drop buffers with errors by default.
> 
>     Change the default behaviour of uvcvideo to match the rest of the
>     drivers and maybe get rid of the module parameter in the future.
> 
>     Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
>     Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
>     Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
>     Reviewed-by: Hans de Goede <hdegoede@redhat.com>
>     Link: https://lore.kernel.org/r/20241218-uvc-deprecate-v2-2-ab814139e983@chromium.org
>     Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
>     Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
> 
>  drivers/media/usb/uvc/uvc_driver.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> I added Ricardo in Cc. Ricardo, see
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1121718 for context.
> 
> The bug does not always immediately manifest, so there is a small chance
> that for some of the commits that I marked "good", artifacts would have
> started appearing if I had waited a bit longer. But the commit seems
> reasonably plausible to be able to cause the kind of error I am seeing.
> 
> Given that this is about a module parameter, I assume I could test this by
> booting the latest kernel and setting the parameter back to its previous
> value... but I don't know enough about how the kernel works to actually do
> that.^^ Happy to try that if someone gives me some pointers.

You can create a modprobe.d file /etc/modprobe.d/uvcvideo.conf with

options uvcvideo nodrop=0

to pass 'nodrop=0' parameter when loading the uvcvideo module (then
unload and load the module).

But it already warns in dmesg when doing so with:

uvcvideo: [Deprecated]: nodrop parameter will be eventually removed.

Regards,
Salvatore