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

[Ricardo Ribalda]

On Fri, 13 Dec 2024 at 12:21, Ricardo Ribalda <ribalda@xxxxxxxxxxxx> 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:
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