V4L2/libCEC support

[kkjdroid]

I don't know if this is remotely within scope, but if this device could show up as a /dev/cec* via V4L2's driver or libCEC, it would integrate with a pretty decent amount of software that's already compatible with devices that have built-in CEC (e.g. Raspberry Pi, Steam Machine). Even Konsole can use my remote's directional buttons on the Pi.

[gkoh]

@kkjdroid Hmm, very, very interesting suggestion.
I did a quick look around and I actually think this is within scope and without insane effort.
In particular, if we can emulate a Rainshadow CEC device, we could leverage the existing Linux kernel driver.

I'll do some more research on this when I get a chance to confirm feasibility.

[gkoh]

So, I did said research and recalled this idea was floated waaay back here

I found at least 2 CEC protocols that should work with a generic Linux kernel:

• Rainshadow
• Pulse-Eight

both appear as USB-CDC, rainshadow is ASCII, pulse-eight is binary.
Pulse-Eight is the likely target as the most common, both protocols are in the kernel.

If I reframe this project as 'HDMI CEC to USB adapter' (and drop the HID), then it starts to make more sense.
A user would need to disable the HID output for the CDC interface, otherwise there would be interesting interactions.

With the recent exposure of exec_send it is now possible to do this in a usable way.
I would propose that:

• the project defaults to USB HID as per the original "plug'n'play" goal
• add configuration ability (and/or build alternative?) to swap to USB P8 mode
• implement USB P8

This will take some time to investigate, design and implement, but does indeed seem reasonable.
The difficult part (the CEC handling) seems reasonably stable, so this effort is mostly protocol/interface matching.

[gkoh]

I have created an issue to track this as I believe there is a clean way to support.
Please add comments to #98 as necessary.