Focusrite Scarlett 18i6 mixer

[Robin Gareus <robin@gareus.org>]

Hi,

I've started to reverse engineer the 18i6 mixer protocol and came up
with a proof-of-concept test script which includes documentation:

  https://github.com/x42/scarlettmixer

Except for a few fixed controls and switches, the 18i6's mixer and
router is [likely] too complex to be exposed via ALSA's mixer interface.
AFAICT it'll require a dedicated control application - like hdspmixer -
in order to make full use of it.

The current proof-of-concept uses pyUSB to send URB control messages and
therefore requires root access. It will also not work while the device
is in use (by snd-usb-audio). A dedicated ioctl interface - similar to
SNDRV_HDSP_ - should be added to ALSA. However, it is yet unclear to me
how much of the protocol should be moved to kernel-space.

Anyway, the script can be used to configure the device, save settings to
it and subsequently use those settings with snd-usb-audio and Martin's
patch.

Thoughts, comments?
robin

PS. the same protocol may apply to other Focusrite Scalett USB2 devices
such as the 8i6 - some of the factory-reset traces hint at that, but I
would not know.