linux-kernel - Re: [PATCH v4 5/5] media: uvcvideo: Use prio state from v4l2

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Message-ID: <CANiDSCvfbB+wwFqNGJKBbSGNhXWvxxK=dvGuej7VmHc+hAUNEA@mail.gmail.com>
Date: Tue, 17 Jun 2025 13:09:38 +0200
From: Ricardo Ribalda <ribalda@...omium.org>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Hans de Goede <hdegoede@...hat.com>, Hans Verkuil <hans@...erkuil.nl>, 
	Mauro Carvalho Chehab <mchehab@...nel.org>, linux-media@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 5/5] media: uvcvideo: Use prio state from v4l2_device

On Tue, 17 Jun 2025 at 12:07, Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
>
> On Mon, Jun 16, 2025 at 08:30:08PM +0200, Ricardo Ribalda wrote:
> > On Mon, 16 Jun 2025 at 17:24, Ricardo Ribalda <ribalda@...omium.org> wrote:
> > >
> > > Currently, a UVC device can have multiple chains, and each chain maintains
> > > its own priority state. While this behavior is technically correct for UVC,
> > > uvcvideo is the *only* V4L2 driver that does not utilize the priority state
> > > defined within `v4l2_device`.
> > >
> > > This patch modifies uvcvideo to use the `v4l2_device` priority state. While
> > > this might not be strictly "correct" for uvcvideo's multi-chain design, it
> > > aligns uvcvideo with the rest of the V4L2 drivers, providing "correct enough"
> > > behavior and enabling code cleanup in v4l2-core. Also, multi-chain
> > > devices are extremely rare, they are typically implemented as two
> > > independent usb devices.
> >
> > As the cover letter says, this last patch 5/5 is a RFC. We can decide
> > if it is worth to keep it or not.
> >
> > The pros is that we can do some cleanup in the core,
>
> What cleanups would that be ?
>
> > the cons is that it might break kAPI.
>
> Multi-chain devices are essentially multiple video devices inside a
> single USB function. They are exposed as completely separate devices to
> userspace, having the priority ioctls on one chain impact the other
> chain wouldn't make much sense to me. I think we should drop this patch.

Ack, let's drop it.

PS: Do you know about a multi chain device that is commercially
available? I would love to buy one for testing.
Also do you know any "output" device that I can buy?

>
> > I checked in the debian sourcecode and I could only find a user of
> > PRIORITY for dvb and was optional.
>
> We could discuss deprecating the priority ioctls overall if we think
> they're not useful (and used) by userspace. I was however considering
> using them in libcamera though, to prevent other applications from
> modifying the camera configuration behind the library's back.

For the record:
From: https://codesearch.debian.net/search?q=VIDIOC_S_PRIORITY
If I am not wrong, this is the only relevant usecase:
https://sources.debian.org/src/zvbi/0.2.44-1/daemon/proxyd.c/?hl=1523#L1523

O_EXCL does not work for you?


>
> > > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > > ---
> > >  drivers/media/usb/uvc/uvc_driver.c | 2 --
> > >  drivers/media/usb/uvc/uvcvideo.h   | 1 -
> > >  2 files changed, 3 deletions(-)
> > >
> > > diff --git a/drivers/media/usb/uvc/uvc_driver.c b/drivers/media/usb/uvc/uvc_driver.c
> > > index accfb4ca3c72cb899185ddc8ecf4e29143d58fc6..e3795e40f14dc325e5bd120f5f45b60937841641 100644
> > > --- a/drivers/media/usb/uvc/uvc_driver.c
> > > +++ b/drivers/media/usb/uvc/uvc_driver.c
> > > @@ -1728,7 +1728,6 @@ static struct uvc_video_chain *uvc_alloc_chain(struct uvc_device *dev)
> > >         INIT_LIST_HEAD(&chain->entities);
> > >         mutex_init(&chain->ctrl_mutex);
> > >         chain->dev = dev;
> > > -       v4l2_prio_init(&chain->prio);
> > >
> > >         return chain;
> > >  }
> > > @@ -2008,7 +2007,6 @@ int uvc_register_video_device(struct uvc_device *dev,
> > >         vdev->fops = fops;
> > >         vdev->ioctl_ops = ioctl_ops;
> > >         vdev->release = uvc_release;
> > > -       vdev->prio = &stream->chain->prio;
> > >         vdev->queue = &queue->queue;
> > >         vdev->lock = &queue->mutex;
> > >         if (type == V4L2_BUF_TYPE_VIDEO_OUTPUT)
> > > diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
> > > index 3e6d2d912f3a1cfcf63b2bc8edd3f86f3da305db..5ed9785d59c698cc7e0ac69955b892f932961617 100644
> > > --- a/drivers/media/usb/uvc/uvcvideo.h
> > > +++ b/drivers/media/usb/uvc/uvcvideo.h
> > > @@ -354,7 +354,6 @@ struct uvc_video_chain {
> > >                                                  * uvc_fh.pending_async_ctrls
> > >                                                  */
> > >
> > > -       struct v4l2_prio_state prio;            /* V4L2 priority state */
> > >         u32 caps;                               /* V4L2 chain-wide caps */
> > >         u8 ctrl_class_bitmap;                   /* Bitmap of valid classes */
> > >  };
>
> --
> Regards,
>
> Laurent Pinchart



-- 
Ricardo Ribalda