Message-ID: <20241129214721.GC2652@pendragon.ideasonboard.com>
Date: Fri, 29 Nov 2024 23:47:21 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Ricardo Ribalda <ribalda@...omium.org>
Cc: Hans Verkuil <hverkuil-cisco@...all.nl>,
	Hans de Goede <hdegoede@...hat.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Guennadi Liakhovetski <guennadi.liakhovetski@...el.com>,
	Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH v2 2/4] media: uvcvideo: Do not set an async control
 owned by other fh

On Fri, Nov 29, 2024 at 02:37:21PM +0100, Ricardo Ribalda wrote:
> On Fri, 29 Nov 2024 at 14:10, Hans Verkuil wrote:
> > On 29/11/2024 12:06, Laurent Pinchart wrote:
> > > 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.
> >
> > OK, that helps a lot.
> >
> > If an operation is in progress, then setting a new control value should
> > result in -EBUSY. Based on the description above, I gather that even the
> > same fh that made the request cannot update it while the operation is
> > ongoing?
> 
> That is correct according to the spec. But both Laurent (correct me if
> I am wrong) and me suspect that there are devices that do not
> implement this properly.

With UVC it's more than suspicion. Whatever the feature, there's bound
to be a device that gets it wrong. The more niche the feature is, the
worse it can get.

> > Getting the control should just return the value that was set. I assume
> > that is cached in uvc?
> 
> If I get the code right... we only return the cached value when the
> field "loaded" is true. That happens when we read the device. So if
> the driver is just loaded
> 
> write(full controlA)
> read(full controlA)
> read(full controlA)
> 
> The first read will get the value from the hardware, the second will be cached.

It depends if the control is marked as auto-update or not. This is
orthogonal to asynchronous controls, so we shouldn't rely on it.

> > >> - 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@...r.kernel.org
> > >>>>>>> Fixes: e5225c820c05 ("media: uvcvideo: Send a control event when a Control Change interrupt arrives")
> > >>>>>>> Signed-off-by: Ricardo Ribalda <ribalda@...omium.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

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