On Fri, 29 Nov 2024 at 11:36, Hans Verkuil <hverkuil-cisco@xxxxxxxxx> wrote: > > Hi Laurent, Ricardo, > > 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. 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 - 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? > > Regards, > > Hans > > > > >>>> 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: > > > -- Ricardo Ribalda
