[PATCH] Add basic capture support for Pioneer DJ DJM-250MK2

[František Kučera]

Hello,

in February I added playback support for Pioneer DJ DJM-250MK2 (commit 73d8c94084341e2895169a0462dbc18167f01683).
Then I gave up being disgusted by Pioneer's quality and support...

Now I returned back to the unfinished work and added basic support for recording.
It is now possible to record from MIC, AUX and PHONO/LINE (but just post fader).
Thanks Dmitry Panchenko and Fabian Lesniak for inspiration.

There are two remaining open questions/issues:

1) The direct (before fader) PHONO/LINE recording channels are silent until some (yet unknown) special command is send to the mixer. There is quite a lot communication over USB in both directions and I have to dig through the dumps. This would be a separate commit and I do not know, how long it could take.

2) The channels 1-2, 2-3 and 3-5, both recording and playback, can be configured/mapped (by an USB URB command) to various sources. I wrote some userspace C code for testing purposes that uses libusb-1.0/libusb.h, so I can switch the channels. How this feature should be provided to the user? Should the USB commands be issued from the kernel driver or from userspace? Is there any existing similar example?

Franta