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

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Message-ID: <cb36e8a0-b941-ff37-e58c-0f9b7f62116a@xs4all.nl>
Date:   Thu, 15 Aug 2019 16:40:03 +0200
From:   Hans Verkuil <hverkuil-cisco@...all.nl>
To:     Jacopo Mondi <jacopo@...ndi.org>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        "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 8/15/19 4:34 PM, Jacopo Mondi wrote:
> Hi Hans,
> 
> On Thu, Aug 15, 2019 at 04:14:38PM +0200, Hans Verkuil wrote:
>> On 8/15/19 4:10 PM, Hans Verkuil wrote:
>>> On 8/15/19 12:43 AM, Laurent Pinchart wrote:
>>>> Hi Jacopo,
>>>>
>>>> Thank you for the patch.
>>>>
>>>> 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)``
>>>>
>>>> Maybe V4L2_CID_CAMERA_SENSOR_LOCATION ? Same for the values below.
>>>
>>> Probably a better name, if a bit long. But we might need other location
>>> controls in the future (e.g. flash location), so CID_LOCATION is just too
>>> generic.
>>
> 
> Thanks for the feedback.
> 
>> Note that the location defines themselves can most likely be used with any
>> LOCATION control, so V4L2_LOCATION_FRONT would be fine with any control.
>>
> 
> What do you think instead of the control type? Would a single integer
> control do or an integer menu one would be better? I see merit in both
> proposals actually...

Single integer. It's read-only, so it just reports the location.

It would be different if this was a writable control: then you need to
know which locations are possible to set, and that requires a menu type.

But it doesn't make sense to set the location from software. However, the
location might change as a result of other changes: e.g. if the camera
has motor control of the tilt and the tilt changes from forward facing to
downward facing, then the driver might change the location from FRONT
to DOWN. A convoluted example perhaps, but this is just brainstorming.

Regards,

	Hans

> 
> Once this is clarified, I can send a proper v1.
> 
> Thanks
>   j
> 
>> Regards,
>>
>> 	Hans
>>
>>>
>>> Regards,
>>>
>>> 	Hans
>>>
>>>>
>>>>> +    This read-only control describes the camera location by reporting its
>>>>
>>>> Here too I would mention camera sensor instead of just camera (or
>>>> possibly imaging sensor).
>>>>
>>>>> +    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
>>>>> +    of a phone device is said to be installed in the ``V4L2_LOCATION_FRONT``
>>>>> +    position.
>>>>
>>>> The DT bindings could use such an example :-) I would extend this to
>>>> tablets and laptops.
>>>>
>>>>> +
>>>>> +
>>>>> +
>>>>
>>>> Do we need three blank lines ?
>>>>
>>>>> +.. 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.
>>>>
>>>
>>