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

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Message-ID: <CANiDSCs2VXdM5pekzEHTqddR=1G5U2wa+zdkRgDKvVhfxw73Mw@mail.gmail.com>
Date: Tue, 3 Dec 2024 22:10:50 +0100
From: Ricardo Ribalda <ribalda@...omium.org>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
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 v5 5/5] media: uvcvideo: Flush the control cache when we
 get an event

On Tue, 3 Dec 2024 at 21:38, Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
>
> 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.

Just realised that loaded is a bit field... and we shall not be
creative with lockless writes with those.

I am going to move it to uvc_ctrl_status_event(). It will not work
with controls without mappings... but I guess we do not care about the
cache for those.

Feel free to land the whole series without this patch if we need
further discussion or changes.

>
> --
> Regards,
>
> Laurent Pinchart



-- 
Ricardo Ribalda