linux-kernel - Re: [PATCH v6 5/5] media: uvcvideo: Flush the control cache when we get an event

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Message-ID: <20241219003130.GJ5518@pendragon.ideasonboard.com>
Date: Thu, 19 Dec 2024 02:31:30 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Ricardo Ribalda <ribalda@...omium.org>
Cc: Hans de Goede <hdegoede@...hat.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Guennadi Liakhovetski <guennadi.liakhovetski@...el.com>,
	Hans Verkuil <hverkuil@...all.nl>,
	Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 5/5] media: uvcvideo: Flush the control cache when we
 get an event

Hi Ricardo,

Thank you for the patch.

On Tue, Dec 03, 2024 at 09:20:12PM +0000, Ricardo Ribalda wrote:
> Asynchronous controls trigger an event when they have completed their
> operation.
> 
> This can make that the control cached value does not match the value in
> the device.
> 
> Let's flush the cache to be on the safe side.
> 
> Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> ---
>  drivers/media/usb/uvc/uvc_ctrl.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/media/usb/uvc/uvc_ctrl.c b/drivers/media/usb/uvc/uvc_ctrl.c
> index 3dc9b7a49f64..db29e0e8bfd4 100644
> --- a/drivers/media/usb/uvc/uvc_ctrl.c
> +++ b/drivers/media/usb/uvc/uvc_ctrl.c
> @@ -1622,6 +1622,9 @@ void uvc_ctrl_status_event(struct uvc_video_chain *chain,
>  
>  	mutex_lock(&chain->ctrl_mutex);
>  
> +	/* Flush the control cache, the data might have changed. */
> +	ctrl->loaded = 0;
> +

That's better, covered by ctrl_mutex. There are however quite a few code
paths through which ctrl->loaded is accessed without holding the mutex
:-( Not a candidate for this series, but this should eventually be
fixed.

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

>  	handle = ctrl->handle;
>  	if (handle)
>  		uvc_ctrl_set_handle(handle, ctrl, NULL);
> 

-- 
Regards,

Laurent Pinchart