[linux-uvc-devel] How do I properly expose control unit to toggle camera's infrared emitter

[Peter-Simon D. <pet...@po...>]

Hi,

I recently bought a Lenovo T490s with an integrated IR-camera that is
compatible with Windows Hello face authentication. The camera shows up  as:

04f2:b67c Chicony Electronics Co., Ltd Integrated Camera

The camera has an integrated IR emitter which is off by default. By
looking at Wireshark USB dumps while an app in Windows turned on the
LED, I found a call to the Realtek Extended Control Unit
26b8105a-0713-4870-979d-da79444bb68e (to which I couldn't find any
documentation), writing 2 bytes to the selector 0x0e. The data sent was
0x0219 and would indeed turn on the IR LED. The first byte (0x19) seems
to control the intensity (0x19 is seemingly already the maximum
intensity). I don't know what the second byte does: 0x01 seems to do the
same, 0x00 turns off the LED.

I wrote a simple tool that can send the corresponding UVCIOC_CTRL_QUERY
to the device (https://github.com/PetePriority/chicony-ir-toggle/),
however, this feels more of a workaround to me. One could use udev rules
to do this at boot. However, another call is necessary after waking from
suspend. I don't know if this is possible with udev.

I tried mapping this control with UVCIOC_CTRL_MAP,
(UVC_CTRL_DATA_TYPE_UNSIGNED -> V4L2_CTRL_TYPE_INTEGER) but the control
reports a range from 0 to 2 and only sets the the second byte. I don't
know how I would go about exposing the raw bytes to v4l2.

I'm writing to this list because
* I thought someone might find these findings interesting/useful,
* I'm unsure of what would be the right way to expose the functionality
of enabling the IR emitters (I haven't done anything regarding uvcvideo
or v4l2 before)? Would turning on the LED be the responsibility of the
user applications by calling the ioctl directly? Should this go into the
uvcvideo driver or the v4l2 layer?

Thanks!
Peter