linux-kernel - Re: [PATCH v15 14/19] media: uvcvideo: Use the camera to clamp compound controls

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Message-ID: <CANiDSCsN8UjxnCWmvuKzXnvSFY6YO02N24ha6W4PNTkUVZTzHg@mail.gmail.com>
Date: Mon, 9 Dec 2024 15:46:38 +0100
From: Ricardo Ribalda <ribalda@...omium.org>
To: Hans de Goede <hdegoede@...hat.com>
Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>, 
	Mauro Carvalho Chehab <mchehab@...nel.org>, Ricardo Ribalda <ribalda@...nel.org>, 
	Sakari Ailus <sakari.ailus@...ux.intel.com>, Hans Verkuil <hverkuil@...all.nl>, 
	Yunke Cao <yunkec@...omium.org>, linux-media@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v15 14/19] media: uvcvideo: Use the camera to clamp
 compound controls

On Mon, 9 Dec 2024 at 15:05, Hans de Goede <hdegoede@...hat.com> wrote:
>
> Hi,
>
> On 14-Nov-24 8:10 PM, Ricardo Ribalda wrote:
> > Compound controls cannot e reliable clamped. There is plenty of space
> > for interpretation for the device manufacturer.
> >
> > When we write a compound control, let the camera do the clamping and
> > return back to the user the value used by the device.
> >
> > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > ---
> >  drivers/media/usb/uvc/uvc_ctrl.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> >
> > diff --git a/drivers/media/usb/uvc/uvc_ctrl.c b/drivers/media/usb/uvc/uvc_ctrl.c
> > index 0dae5e8c3ca0..72ed7dc9cfc1 100644
> > --- a/drivers/media/usb/uvc/uvc_ctrl.c
> > +++ b/drivers/media/usb/uvc/uvc_ctrl.c
> > @@ -2339,6 +2339,18 @@ int uvc_ctrl_set(struct uvc_fh *handle, struct v4l2_ext_control *xctrl)
> >
> >       ctrl->dirty = 1;
> >       ctrl->modified = 1;
> > +
> > +     /*
> > +      * Compound controls cannot reliable clamp the value when they are
> > +      * written to the device. Let the device do the clamping and read back
> > +      * the value that the device is using. We do not need to return an
> > +      * error if this fails.
> > +      */
> > +     if (uvc_ctrl_mapping_is_compound(mapping) &&
> > +         uvc_ctrl_is_readable(V4L2_CTRL_WHICH_CUR_VAL, ctrl, mapping))
> > +             uvc_mapping_get_xctrl_compound(chain, ctrl, mapping,
> > +                                            V4L2_CTRL_WHICH_CUR_VAL, xctrl);
> > +
>
> I do not believe that this actually works / does what you want it to do.
>
> At this point we have only updated in memory structures for the control
> and not send anything to camera.
>
> Querying the control to return the actual achieved values to userspace
> only makes sense after uvc_ctrl_commit() has succeeded, unless I am
> missing something ?

You are absolutely correct. Sorry about that.

Let's drop it for now.

Thanks!

>
> Regards,
>
> Hans
>
>


-- 
Ricardo Ribalda