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
