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

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Message-ID: <5c3d9cc8-65cf-46e9-bbf5-da9705ff3f66@jjverkuil.nl>
Date: Tue, 17 Jun 2025 11:35:10 +0200
From: Hans Verkuil <hans@...erkuil.nl>
To: Ricardo Ribalda <ribalda@...omium.org>,
 Laurent Pinchart <laurent.pinchart@...asonboard.com>,
 Hans de Goede <hdegoede@...hat.com>,
 Mauro Carvalho Chehab <mchehab@...nel.org>
Cc: 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 16/06/2025 17:24, Ricardo Ribalda 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.

I don't think this change is needed. I'm fine with the current code.

If someone has a multi-chain USB device, then it would be nice to see if
there are any v4l2-compliance problems. I don't think so, but it would be
good to have confirmation of that.

Regards,

	Hans

> 
> 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 */
>  };
>