[libcamera-devel] Userspace API for controlling the focus of the Surface Go [2] main/back-camera

[Hans Verkuil]

On 25/10/2021 12:30, Laurent Pinchart wrote:
> Hi Hans,
> 
> On Mon, Oct 25, 2021 at 12:06:30PM +0200, Hans de Goede wrote:
>> Hi All,
>>
>> With my (and Dan's) kernel patch-series to enable the back camera on
>> the Surface Go shaping up (and hopefully going upstream soon),
>> the next step is to enable control of the focus lens for the back
>> camera.
>>
>> The focus is controlled through a separate i2c-client which is
>> described by a 2nd I2cSerialBusV2 resource entry in the ACPI
>> device for the ov8865 sensor. By default the kernel only instantiates
>> an i2c-client for the first I2cSerialBusV2 resource entry for an
>> ACPI device, getting an i2c-client for the 2nd one is easy and
>> out of scope for this discussion.
>>
>> The question which I have is, assuming we have the 2nd i2c-client
>> instantiated and we have a i2c-driver binding to it, how do we
>> represent the focus control to userspace.
>>
>> I see 2 possible directions we can go here:
>>
>> 1. Somehow inject an extra v4l2ctrl for this into the v4l2ctrl
>> list of the sensor. AFAIK we don't have infra for this atm, but
>> we could add some generic mechanism to do this to the v4l2-ctrls
>> core. IMHO from a userspace pov this is the cleanest, but at the
>> cost of some extra work / possible ugliness on the kernel side.
>>
>> 2. Register a separate v4l2_subdev for the focus-ctrl and in
>> some way provide information to userspace to which sensor this
>> belongs.
> 
> The second approach is what V4L2 does already. We have a set of drivers
> for VCMs already (search for V4L2_CID_FOCUS_ABSOLUTE in
> drivers/media/i2c/).
> 
>> I believe that both are valid approaches. So before diving into
>> this I wonder what others are thinking about this.
>>
>> Specific questions:
>>
>> 1. Hans Verkuil, what do you think about adding
>> support for another driver to inject ctrls into the ctrl
>> list of another v4l2(sub)dev ? Maybe something like this
>> already exists ? If not do you think this is feasible
>> and desirable to add ?
>>
>> 2. If we go with a separate v4l2_subdev, how do we communicate
>> to which sensor the focus-control belongs to userspace ?
> 
> The information was initially envisioned to be conveyed to userspace
> through the media controller API, using the entity group ID to group the
> camera sensor, lens controller and flash controller, but the
> media_entity_desc.group_id field is now obsolete. No other mechanism
> exist to replace that as far as I know, so we'll have to create
> something. There have been some talks about using a special kind of link
> to expose the relationship between the camera sensor and other
> components.
> 

I thought this was implemented: there should be an interface link from the
sensor entity to the subdev for the flash or focus control.

To my knowledge, this is all available.

Regards,

	Hans