linux-kernel - Re: [PATCH RFC v2 2/6] media: v4l2-ctrls: clarify documentation of V4L2_CID_FOCUS

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Message-ID: <20230607064207.GG14101@pendragon.ideasonboard.com>
Date:   Wed, 7 Jun 2023 09:42:07 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Michael Riesch <michael.riesch@...fvision.net>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        libcamera-devel@...ts.libcamera.org,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Matthias Fend <Matthias.Fend@...fvision.net>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v2 2/6] media: v4l2-ctrls: clarify documentation of
 V4L2_CID_FOCUS_RELATIVE

Hi Michael,

On Tue, Jun 06, 2023 at 03:15:33PM +0200, Michael Riesch wrote:
> On 6/6/23 12:36, Laurent Pinchart wrote:
> > On Tue, Apr 25, 2023 at 11:45:12AM +0200, Michael Riesch wrote:
> >> The control V4L2_CID_FOCUS_RELATIVE only makes sense if the device cannot
> >> handle absolute focal point positioning with V4L2_CID_FOCUS_ABSOLUTE.
> >> Clarify this in the documentation.
> >>
> >> Signed-off-by: Michael Riesch <michael.riesch@...fvision.net>
> >> ---
> >>  Documentation/userspace-api/media/v4l/ext-ctrls-camera.rst | 4 +++-
> >>  1 file changed, 3 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-camera.rst b/Documentation/userspace-api/media/v4l/ext-ctrls-camera.rst
> >> index df29150dce7b..42cf4c3cda0c 100644
> >> --- a/Documentation/userspace-api/media/v4l/ext-ctrls-camera.rst
> >> +++ b/Documentation/userspace-api/media/v4l/ext-ctrls-camera.rst
> >> @@ -147,7 +147,9 @@ enum v4l2_exposure_metering -
> >>      This control moves the focal point of the camera by the specified
> >>      amount. The unit is undefined. Positive values move the focus closer
> >>      to the camera, negative values towards infinity. This is a
> >> -    write-only control.
> >> +    write-only control. It should be implemented only if the device cannot
> >> +    handle absolute values.
> >> +
> > 
> > Extra blank line.
> 
> Will fix that.
> 
> > I don't think this is right. The control was added for the UVC driver,
> > and there are devices that implement both absolute and relative focus.
> 
> I am by no means an expert here and just following Sakari's suggestion
> here (see [0]). I can drop the patch, leave it as-is, or modify it.
> Whatever makes sense to you guys. But maybe I should leave this to
> someone more knowledgeable in this area and drop the patch from my
> series. The changes above are completely orthogonal to my work.

V4L2_CID_FOCUS_RELATIVE is an annoying control. It was introduced for
UVC, and to my surprise, it turns out it has never been implemented in
the uvcvideo driver. The 3 devices I know of that implement the UVC
relative focus control also implement the UVC absolute focus control.

I'm tempted to deprecate this control completely. Sakari, any opinion ?

> >>  
> >>  ``V4L2_CID_FOCUS_AUTO (boolean)``
> >>      Enables continuous automatic focus adjustments. The effect of manual
> >>
> > 
> 
> [0] https://lore.kernel.org/all/ZDbChgZJHVaaX3%2Fx@kekkonen.localdomain/

-- 
Regards,

Laurent Pinchart