Re: [PATCH 1/3] media: uvcvideo: report AUTO_UPDATE controls as volatile

[Ricardo Ribalda]

Hi Michael


On Fri, 28 Aug 2026 at 17:26, Michael Jordan <jordan.mymail@xxxxxxxxx> wrote:
>
> A control with UVC_CTRL_FLAG_AUTO_UPDATE is one whose value the device
> changes on its own: the driver never trusts its cached value for it,
> re-reading the device on every VIDIOC_G_EXT_CTRLS (the rollback at the
> end of the ioctl runs uvc_ctrl_commit_entity(), which clears ctrl->loaded
> for these controls) and re-reading it after each write. That is exactly
> what V4L2_CTRL_FLAG_VOLATILE describes to userspace, but the driver never
> reported it, so applications had no way to know that the value they read
> can change under them and that a fresh read is worth issuing.
>
> Report V4L2_CTRL_FLAG_VOLATILE for AUTO_UPDATE controls. The uAPI
> documents writes to a volatile control as ignored unless
> V4L2_CTRL_FLAG_EXECUTE_ON_WRITE is also set, and this driver sends every
> write of a writable control to the device, so report EXECUTE_ON_WRITE
> alongside it whenever the control is settable.
>
> Suggested-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
Reviewed-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
> Signed-off-by: Michael Jordan <jordan.mymail@xxxxxxxxx>
> ---
>  drivers/media/usb/uvc/uvc_ctrl.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/drivers/media/usb/uvc/uvc_ctrl.c b/drivers/media/usb/uvc/uvc_ctrl.c
> index 3ca108b83..aceb26310 100644
> --- a/drivers/media/usb/uvc/uvc_ctrl.c
> +++ b/drivers/media/usb/uvc/uvc_ctrl.c
> @@ -1840,6 +1840,17 @@ static int __uvc_query_v4l2_ctrl(struct uvc_video_chain *chain,
>         if ((ctrl->info.flags & UVC_CTRL_FLAG_GET_MAX) &&
>             (ctrl->info.flags & UVC_CTRL_FLAG_GET_MIN))
>                 v4l2_ctrl->flags |= V4L2_CTRL_FLAG_HAS_WHICH_MIN_MAX;
> +       if (ctrl->info.flags & UVC_CTRL_FLAG_AUTO_UPDATE) {
> +               v4l2_ctrl->flags |= V4L2_CTRL_FLAG_VOLATILE;
> +               /*
> +                * Writes to a volatile control are documented to be ignored
> +                * unless EXECUTE_ON_WRITE is also reported. The driver sends
> +                * every write of a writable control to the device, so report
> +                * the flag accordingly.
> +                */
> +               if (ctrl->info.flags & UVC_CTRL_FLAG_SET_CUR)
> +                       v4l2_ctrl->flags |= V4L2_CTRL_FLAG_EXECUTE_ON_WRITE;
> +       }
>
>         if (mapping->master_id)
>                 __uvc_find_control(ctrl->entity, mapping->master_id,
> --
> 2.43.0
>


-- 
Ricardo Ribalda