linux-kernel - Re: [PATCH v3 5/8] media: uvcvideo: Turn on the camera if V4L2_EVENT_SUB_FL_SEND

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Message-ID: <20250630220923.GA15184@pendragon.ideasonboard.com>
Date: Tue, 1 Jul 2025 01:09:23 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Ricardo Ribalda <ribalda@...omium.org>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>,
	Hans Verkuil <hverkuil@...all.nl>, Hans de Goede <hansg@...nel.org>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 5/8] media: uvcvideo: Turn on the camera if
 V4L2_EVENT_SUB_FL_SEND_INITIAL

On Mon, Jun 30, 2025 at 02:20:30PM +0000, Ricardo Ribalda wrote:
> If we subscribe to an event with V4L2_EVENT_SUB_FL_SEND_INITIAL, the
> driver needs to report back some values that require the camera to be
> powered on. But VIDIOC_SUBSCRIBE_EVENT is not part of the ioctls that
> turn on the camera.
> 
> We could unconditionally turn on the camera during
> VIDIOC_SUBSCRIBE_EVENT, but it is more efficient to turn it on only
> during V4L2_EVENT_SUB_FL_SEND_INITIAL, which we believe is not a common
> usecase.
> 
> To avoid a list_del if uvc_pm_get() fails, we move list_add_tail to the
> end of the function.
> 
> Reviewed-by: Hans de Goede <hansg@...nel.org>
> Fixes: d1b618e79548 ("media: uvcvideo: Do not turn on the camera for some ioctls")
> Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> ---
>  drivers/media/usb/uvc/uvc_ctrl.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_ctrl.c b/drivers/media/usb/uvc/uvc_ctrl.c
> index 303b7509ec47964dc1bf0e28127075b4a3867511..e84aaf130c73403a6c818deceadf404a2975c816 100644
> --- a/drivers/media/usb/uvc/uvc_ctrl.c
> +++ b/drivers/media/usb/uvc/uvc_ctrl.c
> @@ -2072,18 +2072,23 @@ static int uvc_ctrl_add_event(struct v4l2_subscribed_event *sev, unsigned elems)
>  		goto done;
>  	}
>  
> -	list_add_tail(&sev->node, &mapping->ev_subs);
>  	if (sev->flags & V4L2_EVENT_SUB_FL_SEND_INITIAL) {
>  		struct v4l2_event ev;
>  		u32 changes = V4L2_EVENT_CTRL_CH_FLAGS;
>  		s32 val = 0;
>  
> +		ret = uvc_pm_get(handle->chain->dev);
> +		if (ret)
> +			goto done;
> +
>  		if (uvc_ctrl_mapping_is_compound(mapping) ||
>  		    __uvc_ctrl_get(handle->chain, ctrl, mapping, &val) == 0)
>  			changes |= V4L2_EVENT_CTRL_CH_VALUE;
>  
>  		uvc_ctrl_fill_event(handle->chain, &ev, ctrl, mapping, val,
>  				    changes);
> +
> +		uvc_pm_put(handle->chain->dev);

Add a blank line here.

>  		/*
>  		 * Mark the queue as active, allowing this initial event to be
>  		 * accepted.
> @@ -2092,6 +2097,7 @@ static int uvc_ctrl_add_event(struct v4l2_subscribed_event *sev, unsigned elems)
>  		v4l2_event_queue_fh(sev->fh, &ev);
>  	}
>  
> +	list_add_tail(&sev->node, &mapping->ev_subs);

And here.

Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>

>  done:
>  	mutex_unlock(&handle->chain->ctrl_mutex);
>  	return ret;
> 

-- 
Regards,

Laurent Pinchart