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

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Message-ID: <20250617100730.GA10006@pendragon.ideasonboard.com>
Date: Tue, 17 Jun 2025 13:07:30 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Ricardo Ribalda <ribalda@...omium.org>
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 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.

> 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.

> > 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