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

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Hans,

On Mon, Oct 25, 2021 at 12:47:05PM +0200, Hans Verkuil wrote:
> On 25/10/2021 12:30, Laurent Pinchart wrote:
> > 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.

Interface links are only meant to link entities to interfaces, not
entities to entities. media_create_intf_link() is only called for that
purpose today.

> To my knowledge, this is all available.

To my knowledge, it isn't :-) I'd be happy to be proven wrong though.

-- 
Regards,

Laurent Pinchart