Re: [PATCH v2 2/4] media: uvcvideo: Do not set an async control owned by other fh

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Fri, Nov 29, 2024 at 11:59:27AM +0100, Ricardo Ribalda wrote:
> On Fri, 29 Nov 2024 at 11:36, Hans Verkuil wrote:
> > On 28/11/2024 23:33, Laurent Pinchart wrote:
> > > On Thu, Nov 28, 2024 at 11:28:29PM +0100, Ricardo Ribalda wrote:
> > >> On Thu, 28 Nov 2024 at 23:22, Laurent Pinchart 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.
> > >
> > > Yes, I should have been more precised. If the device doesn't send the
> > > control event, then all other file handles will be prevented from
> > > setting the control until the file handle that set it first gets closed.
> >
> > I think I need a bit more background here:
> >
> > First of all, what is an asynchronous control in UVC? I think that means
> > you can set it, but it takes time for that operation to finish, so you
> > get an event later when the operation is done. So zoom and similar operations
> > are examples of that.
> >
> > And only when the operation finishes will the control event be sent, correct?
> 
> You are correct.  This diagrams from the spec is more or less clear:
> https://ibb.co/MDGn7F3
> 
> > While the operation is ongoing, if you query the control value, is that reporting
> > the current position or the final position?
> 
> I'd expect hardware will return either the current position, the start
> position or the final position. I could not find anything in the spec
> that points in one direction or the others.

Figure 2-21 in UVC 1.5 indicates that the device should STALL the
GET_CUR and SET_CUR requests if a state change is in progress.

> And in the driver I believe that we might have a bug handling this
> case (will send a patch if I can confirm it)
> the zoom is at 0 and you set it 10
> if you read the value 2 times before the camera reaches value 10:
> - First value will come from the hardware and the response will be cached

Only if the control doesn't have the auto-update flag set, so it will be
device-dependent. As GET_CUR should stall that's not really relevant,
except for the fact that devices may not stall the request.

> - Second value will be the cached one
> 
> now the camera  is at zoom 10
> If you read the value, you will read the cached value
>
> > E.g.: the zoom control is at value 0 and I set it to 10, then I poll the zoom control
> > value: will that report the intermediate values until it reaches 10? And when it is
> > at 10, the control event is sent?
> >
> > >>>> Cc: stable@vger.kernel.org
> > >>>> Fixes: e5225c820c05 ("media: uvcvideo: Send a control event when a Control Change interrupt arrives")
> > >>>> Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
> > >>>> ---
> > >>>>  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