linux-kernel - Re: [PATCH v5 5/5] media: uvcvideo: Flush the control cache when we get an event

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Message-ID: <20241203203748.GD5196@pendragon.ideasonboard.com>
Date: Tue, 3 Dec 2024 22:37:48 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Hans de Goede <hdegoede@...hat.com>
Cc: Ricardo Ribalda <ribalda@...omium.org>,
	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 v5 5/5] media: uvcvideo: Flush the control cache when we
 get an event

On Mon, Dec 02, 2024 at 03:45:57PM +0100, Hans de Goede wrote:
> Hi,
> 
> On 2-Dec-24 3:24 PM, 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 e90bf2aeb5e4..75d534072f50 100644
> > --- a/drivers/media/usb/uvc/uvc_ctrl.c
> > +++ b/drivers/media/usb/uvc/uvc_ctrl.c
> > @@ -1672,6 +1672,9 @@ bool uvc_ctrl_status_event_async(struct urb *urb, struct uvc_video_chain *chain,
> >  	struct uvc_device *dev = chain->dev;
> >  	struct uvc_ctrl_work *w = &dev->async_ctrl;
> >  
> > +	/* Flush the control cache, the data might have changed. */
> > +	ctrl->loaded = 0;
> > +
> >  	if (list_empty(&ctrl->info.mappings))
> >  		return false;
> >  
> 
> Thank you for the patch.
> 
> I'm not familiar enough with UVC yet to really have an opinion on this one,
> so I'll defer reviewing this one to Laurent.

Conceptually this change looks fine, but the commit message needs to
explain why this is safe to do without protecting ctrl->loaded with a
lock.

-- 
Regards,

Laurent Pinchart