On Thu, Dec 19, 2024 at 09:18:30AM +0100, Ricardo Ribalda wrote: > On Thu, 19 Dec 2024 at 00:28, Laurent Pinchart wrote: > > On Wed, Dec 18, 2024 at 03:38:34PM +0100, Ricardo Ribalda wrote: > > > On Fri, 13 Dec 2024 at 12:21, Ricardo Ribalda 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. > > I'd rather not introduce more differences between uvc and the rest of > the drivers. > > the ctrl framework only seems to return -EINVAL or 0. True, but that's likely because there's no driver today other than uvcvideo that needs to interogate the hardware in a way that can fail to implement QUERYCTRL. I wish we didn't have to, but UVC is special from that point of view. Anyway, as mentioned in a separate answer in this mail thread, we may not need this if we hide the problematic controls from applications. > > > 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
