linux-kernel - Re: [libcamera-devel] [PATCH RFC 1/4] media: v4l2-ctrls: add lens group status controls for zoom and focus

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Date:   Wed, 12 Apr 2023 17:39:02 +0300
From:   Sakari Ailus <sakari.ailus@...ux.intel.com>
To:     Michael Riesch <michael.riesch@...fvision.net>
Cc:     Dave Stevenson <dave.stevenson@...pberrypi.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Michael Riesch via B4 Relay 
        <devnull+michael.riesch.wolfvision.net@...nel.org>,
        linux-kernel@...r.kernel.org,
        Matthias Fend <Matthias.Fend@...fvision.net>,
        libcamera-devel@...ts.libcamera.org, linux-media@...r.kernel.org,
        hverkuil@...all.nl
Subject: Re: [libcamera-devel] [PATCH RFC 1/4] media: v4l2-ctrls: add lens
 group status controls for zoom and focus

Hi Michael,

On Wed, Apr 12, 2023 at 03:43:26PM +0200, Michael Riesch wrote:
> Hi Sakari,
> 
> On 4/12/23 14:05, Sakari Ailus wrote:
> > Hi Michael,
> > 
> > On Wed, Apr 12, 2023 at 01:57:36PM +0200, Michael Riesch wrote:
> >> Hi Sakari,
> >>
> >> On 4/12/23 13:50, Sakari Ailus wrote:
> >>> Hi Michael,
> >>>
> >>> On Wed, Apr 12, 2023 at 10:00:26AM +0200, Michael Riesch wrote:
> >>>>  - Different controls: If moving = (V4L2_CID_FOCUS_ABSOLUTE == current),
> >>>>    then what happens if the application performs a
> >>>>    V4L2_CID_FOCUS_RELATIVE with -3? current should reach 39,
> >>>>    V4L2_CID_FOCUS_ABSOLUTE is still at 42, the lens is still moving from
> >>>>    the application's point of view.
> >>>
> >>> Would there be a reason to implement both of these controls in a single
> >>> driver? AFAIU, the relative one should be used if there absolute value
> >>> isn't known to the driver.
> >>
> >> Probably not, but on the other hand there is nothing the prevents a
> >> driver developer from doing so, right? Point is that should there be a
> >> driver which does implement both controls, we are in trouble AFAIU.
> > 
> > I think the documentation should be improved in this regard.
> 
> The documentation of which control exactly? And what items should be added?

Both V4L2_CID_FOCUS_ABSOLUTE and V4L2_CID_FOCUS_RELATIVE. For the former,
the sentence "Positive values set the focus closer to the camera, negative
values towards infinity." doesn't make much sense in the context. For the
latter, what I mentioned earlier, i.e. this should be only implemented if
the absolute value isn't known. It's not a driver's job to do simple
arithmetics for the user space.

In fact, it appears that no driver is using V4L2_CID_FOCUS_RELATIVE at the
moment. So we could as well deprecate it (or remove from documentation
altogether).

-- 
Kind regards,

Sakari Ailus