Re: [PATCH] media: uvcvideo: Filter hw errors while enumerating controls

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Wed, Dec 18, 2024 at 03:38:34PM +0100, Ricardo Ribalda wrote:
> On Fri, 13 Dec 2024 at 12:21, Ricardo Ribalda <ribalda@chromium.org> wrote:
> >
> > To implement VIDIOC_QUERYCTRL, we need to read from the hardware all the
> > values that were not cached previously. If that read fails, we used to
> > report back the error to the user.
> >
> > Unfortunately this does not play nice with userspace. When they are
> > enumerating the contols, the only expect an error when there are no
> > "next" control.
> >
> > This is probably a corner case, and could be handled in userspace, but
> > both v4l2-ctl and yavta fail to enumerate all the controls if we return
> > then -EIO during VIDIOC_QUERYCTRL. I suspect that there are tons of
> > userspace apps handling this wrongly as well.
> 
> Actually it CANNOT be handled in userspace.
> 
> If we return anything different than 0, the structure is not copied to
> userspace:

That could be fixed, we do copy data back to userspace in case of
failure for some ioctls. I don't think that would be needed though, I
believe we can either mark controls as broken in the uvcvideo driver
through quirks, or in a dynamic fashion.

> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/media/v4l2-core/v4l2-ioctl.c#n2929
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/media/v4l2-core/v4l2-ioctl.c#n3490

-- 
Regards,

Laurent Pinchart