On Wed, Dec 18, 2024 at 03:38:34PM +0100, Ricardo Ribalda wrote: > 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: 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
