On Thu, 28 Nov 2024 at 23:22, Laurent Pinchart
<laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
>
> Hi Ricardo,
>
> (CC'ing Hans Verkuil)
>
> Thank you for the patch.
>
> On Wed, Nov 27, 2024 at 12:14:50PM +0000, Ricardo Ribalda wrote:
> > If a file handle is waiting for a response from an async control, avoid
> > that other file handle operate with it.
> >
> > Without this patch, the first file handle will never get the event
> > associated with that operation, which can lead to endless loops in
> > applications. Eg:
> > If an application A wants to change the zoom and to know when the
> > operation has completed:
> > it will open the video node, subscribe to the zoom event, change the
> > control and wait for zoom to finish.
> > If before the zoom operation finishes, another application B changes
> > the zoom, the first app A will loop forever.
>
> Hans, the V4L2 specification isn't very clear on this. I see pros and
> cons for both behaviours, with a preference for the current behaviour,
> as with this patch the control will remain busy until the file handle is
> closed if the device doesn't send the control event for any reason. What
> do you think ?
Just one small clarification. The same file handler can change the
value of the async control as many times as it wants, even if the
operation has not finished.
It will be other file handles that will get -EBUSY if they try to use
an async control with an unfinished operation started by another fh.
>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Fixes: e5225c820c05 ("media: uvcvideo: Send a control event when a Control Change interrupt arrives")
> > Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
> > ---
> > drivers/media/usb/uvc/uvc_ctrl.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/drivers/media/usb/uvc/uvc_ctrl.c b/drivers/media/usb/uvc/uvc_ctrl.c
> > index b6af4ff92cbd..3f8ae35cb3bc 100644
> > --- a/drivers/media/usb/uvc/uvc_ctrl.c
> > +++ b/drivers/media/usb/uvc/uvc_ctrl.c
> > @@ -1955,6 +1955,10 @@ int uvc_ctrl_set(struct uvc_fh *handle,
> > if (!(ctrl->info.flags & UVC_CTRL_FLAG_SET_CUR))
> > return -EACCES;
> >
> > + /* Other file handle is waiting a response from this async control. */
> > + if (ctrl->handle && ctrl->handle != handle)
> > + return -EBUSY;
> > +
> > /* Clamp out of range values. */
> > switch (mapping->v4l2_type) {
> > case V4L2_CTRL_TYPE_INTEGER:
>
> --
> Regards,
>
> Laurent Pinchart
--
Ricardo Ribalda
