linux-kernel - Re: [RFC 2/5] media: v4l2-ctrl: Document V4L2_CID

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Date:   Thu, 15 Aug 2019 16:19:59 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     Jacopo Mondi <jacopo@...ndi.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        "open list:MEDIA INPUT INFRASTRUCTURE (V4L/DVB)" 
        <linux-media@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 2/5] media: v4l2-ctrl: Document V4L2_CID_LOCATION

On Thu, Aug 15, 2019 at 04:15:10PM +0300, Sakari Ailus wrote:
> On Thu, Aug 15, 2019 at 04:10:53PM +0300, Laurent Pinchart wrote:
> > On Thu, Aug 15, 2019 at 04:08:49PM +0300, Sakari Ailus wrote:
> >> On Thu, Aug 15, 2019 at 03:59:38PM +0300, Laurent Pinchart wrote:
> >>> On Thu, Aug 15, 2019 at 10:00:25AM +0300, Sakari Ailus wrote:
> >>>> On Wed, Aug 14, 2019 at 10:28:12PM +0200, Jacopo Mondi wrote:
> >>>>> Add documentation for the V4L2_CID_LOCATION camera control. The newly
> >>>>> added read-only control reports the camera device mounting position.
> >>>>> 
> >>>>> Signed-off-by: Jacopo Mondi <jacopo@...ndi.org>
> >>>>> ---
> >>>>>  .../media/uapi/v4l/ext-ctrls-camera.rst       | 23 +++++++++++++++++++
> >>>>>  1 file changed, 23 insertions(+)
> >>>>> 
> >>>>> diff --git a/Documentation/media/uapi/v4l/ext-ctrls-camera.rst b/Documentation/media/uapi/v4l/ext-ctrls-camera.rst
> >>>>> index 51c1d5c9eb00..fc0a02eee6d4 100644
> >>>>> --- a/Documentation/media/uapi/v4l/ext-ctrls-camera.rst
> >>>>> +++ b/Documentation/media/uapi/v4l/ext-ctrls-camera.rst
> >>>>> @@ -510,6 +510,29 @@ enum v4l2_scene_mode -
> >>>>>      value down. A value of zero stops the motion if one is in progress
> >>>>>      and has no effect otherwise.
> >>>>> 
> >>>>> +``V4L2_CID_LOCATION (integer)``
> >>>>> +    This read-only control describes the camera location by reporting its
> >>>>> +    mounting position on the device where the camera is installed. This
> >>>>> +    control is particularly meaningful for devices which have a well defined
> >>>>> +    orientation, such as phones, laptops and portable devices as the camera
> >>>>> +    location is expressed as a position relative to the device intended
> >>>>> +    usage position. In example, a camera installed on the user-facing side
> >>>> 
> >>>> s/In/For/
> >>>> 
> >>>>> +    of a phone device is said to be installed in the ``V4L2_LOCATION_FRONT``
> >>>>> +    position.
> >>>>> +
> >>>>> +
> >>>>> +
> >>>>> +.. flat-table::
> >>>>> +    :header-rows:  0
> >>>>> +    :stub-columns: 0
> >>>>> +
> >>>>> +    * - ``V4L2_LOCATION_FRONT``
> >>>>> +      - The camera device is located on the front side of the device.
> >>>>> +    * - ``V4L2_LOCATION_BACK``
> >>>>> +      - The camera device is located on the back side of the device.
> >>>>> +
> >>>>> +
> >>>>> +
> >>>>>  .. [#f1]
> >>>>>     This control may be changed to a menu control in the future, if more
> >>>>>     options are required.
> >>>> 
> >>>> There's an effective limit of 64 for menus. ACPI has less than ten
> >>>> different locations for a device, I think 64 will be enough here.
> >>>> 
> >>>> So I'd be actually in favour of switching to a menu.
> >>> 
> >>> Why ? As you explained yourself, it's a static read-only control, all it
> >>> needs to report is a single value.
> >> 
> >> Yes. That's true. It wasn't meant for this but it's nevertheless a
> >> convenient API to get that information, both as integer and string.
> > 
> > But why is that needed ? The integer seems enough to me.
> 
> Because it's a qualitative control, not a quantitative one.

And ? :-) The integer values are defined in the V4L2 spec, they map to a
usage, and a name can easily be derived from that in userspace if
desired.

-- 
Regards,

Laurent Pinchart