[alsa-devel] using all channels of RME Babyface Pro in class compliant mode

[Be]

Hello, I just got an RME Babyface Pro USB sound card. It can operate 
either as a USB audio class compliant device (with mixing controlled by 
the device's physical controls) or make use of a proprietary driver on 
Windows and Mac OS X. The proprietary driver allows for control of the 
mixing capabilities of the device and using onboard DSP EQ. It would be 
great if someone wrote a Linux driver to make use of these. I'd be happy 
to send debugging info or whatever about the device and test kernel 
patches. I could set up a Windows VM to intercept USB communication.

In the meantime, I am trying to use its class compliant mode to use with 
Mixxx for DJing. However, I cannot figure out how to use channels 3-4 
for my headphones. The device has 4 analog channels out and in plus 8 
digital out and in (via ADAT). Its manual says, regarding class 
compliant mode, "in Mac OS X, either the first two or all channels will 
work, depending on the choice in the Audio MIDI Setup." I believe the 
two channel operation exists so when the Babyface Pro is plugged into an 
iPad, the iPad sends stereo output from iTunes and other programs to the 
Babyface Pro's main output. However, I do not know how to select between 
this 2 channel mode and the 12 channel mode with Linux. PulseAudio only 
shows it as a stereo device. Mixxx, which uses the PortAudio library, 
displays all 12 channels as available but doesn't work when any channels 
other than 1-2 are selected. Here is debugging output of Mixxx:

Debug [Main]: SoundManager::setupDevices()
Debug [Main]: SoundDevicePortAudio::open() "2, Babyface Pro (70785713): 
USB Audio (hw:1,0)"
Debug [Main]: framesPerBuffer: 256
Debug [Main]: Requested sample rate:  44100 Hz, latency: 5.80499 ms
Debug [Main]: Output channels: 4 | Input channels: 0
Debug [Main]: Opening stream with id 2
Expression 'alsa_snd_pcm_hw_params_set_channels( pcm, hwParams, 
self->numHostChannels )' failed in 'src/hostapi/alsa/pa_linux_alsa.c', 
line: 2051
Expression 'PaAlsaStreamComponent_InitialConfigure( &self->playback, 
outParams, self->primeBuffers, hwParamsPlayback, &realSr )' failed in 
'src/hostapi/alsa/pa_linux_alsa.c', line: 2717
Expression 'PaAlsaStream_Configure( stream, inputParameters, 
outputParameters, sampleRate, framesPerBuffer, &inputLatency, 
&outputLatency, &hostBufferSizeMode )' failed in 
'src/hostapi/alsa/pa_linux_alsa.c', line: 2838
Warning [Main]: Error opening stream: Invalid number of channels

aplay -l output:
**** List of PLAYBACK Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: ALC272 Analog [ALC272 Analog]
   Subdevices: 1/1
   Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 3: HDMI 0 [HDMI 0]
   Subdevices: 1/1
   Subdevice #0: subdevice #0
card 1: Pro70785713 [Babyface Pro (70785713)], device 0: USB Audio [USB 
Audio]
   Subdevices: 1/1
   Subdevice #0: subdevice #0

aplay -L output:
null
     Discard all samples (playback) or generate zero samples (capture)
pulse
     PulseAudio Sound Server
default
     Default ALSA Output (currently PulseAudio Sound Server)
sysdefault:CARD=PCH
     HDA Intel PCH, ALC272 Analog
     Default Audio Device
front:CARD=PCH,DEV=0
     HDA Intel PCH, ALC272 Analog
     Front speakers
surround21:CARD=PCH,DEV=0
     HDA Intel PCH, ALC272 Analog
     2.1 Surround output to Front and Subwoofer speakers
surround40:CARD=PCH,DEV=0
     HDA Intel PCH, ALC272 Analog
     4.0 Surround output to Front and Rear speakers
surround41:CARD=PCH,DEV=0
     HDA Intel PCH, ALC272 Analog
     4.1 Surround output to Front, Rear and Subwoofer speakers
surround50:CARD=PCH,DEV=0
     HDA Intel PCH, ALC272 Analog
     5.0 Surround output to Front, Center and Rear speakers
surround51:CARD=PCH,DEV=0
     HDA Intel PCH, ALC272 Analog
     5.1 Surround output to Front, Center, Rear and Subwoofer speakers
surround71:CARD=PCH,DEV=0
     HDA Intel PCH, ALC272 Analog
     7.1 Surround output to Front, Center, Side, Rear and Woofer speakers
hdmi:CARD=PCH,DEV=0
     HDA Intel PCH, HDMI 0
     HDMI Audio Output
sysdefault:CARD=Pro70785713
     Babyface Pro (70785713), USB Audio
     Default Audio Device
front:CARD=Pro70785713,DEV=0
     Babyface Pro (70785713), USB Audio
     Front speakers
surround21:CARD=Pro70785713,DEV=0
     Babyface Pro (70785713), USB Audio
     2.1 Surround output to Front and Subwoofer speakers
surround40:CARD=Pro70785713,DEV=0
     Babyface Pro (70785713), USB Audio
     4.0 Surround output to Front and Rear speakers
surround41:CARD=Pro70785713,DEV=0
     Babyface Pro (70785713), USB Audio
     4.1 Surround output to Front, Rear and Subwoofer speakers
surround50:CARD=Pro70785713,DEV=0
     Babyface Pro (70785713), USB Audio
     5.0 Surround output to Front, Center and Rear speakers
surround51:CARD=Pro70785713,DEV=0
     Babyface Pro (70785713), USB Audio
     5.1 Surround output to Front, Center, Rear and Subwoofer speakers
surround71:CARD=Pro70785713,DEV=0
     Babyface Pro (70785713), USB Audio
     7.1 Surround output to Front, Center, Side, Rear and Woofer speakers
iec958:CARD=Pro70785713,DEV=0
     Babyface Pro (70785713), USB Audio
     IEC958 (S/PDIF) Digital Audio Output

/proc/asound/card1/stream0 when playing to channels 1-2 with PulseAudio:
RME Babyface Pro (70785713) at usb-0000:00:1d.0-1.2, high speed : USB Audio

Playback:
   Status: Running
     Interface = 1
     Altset = 1
     Packet Size = 47
     Momentary freq = 44118 Hz (0x5.83c9)
     Feedback Format = 16.16
   Interface 1
     Altset 1
     Format: S24_3LE
     Channels: 2
     Endpoint: 3 OUT (ASYNC)
     Rates: 44100, 48000, 88200, 96000, 176400, 192000
     Data packet interval: 125 us
   Interface 1
     Altset 2
     Format: S24_3LE
     Channels: 12
     Endpoint: 3 OUT (ASYNC)
     Rates: 44100, 48000, 88200, 96000, 176400, 192000
     Data packet interval: 125 us

Capture:
   Status: Stop
   Interface 2
     Altset 1
     Format: S24_3LE
     Channels: 12
     Endpoint: 4 IN (ASYNC)
     Rates: 44100, 48000, 88200, 96000, 176400, 192000
     Data packet interval: 125 us
   Interface 2
     Altset 2
     Format: S24_3LE
     Channels: 2
     Endpoint: 4 IN (ASYNC)
     Rates: 44100, 48000, 88200, 96000, 176400, 192000
     Data packet interval: 125 us

So I guess the question is how to get PortAudio to use the "Altset 2" 
option. Could this be accomplished with some asoundrc sorcery? Does 
something need to change in snd-usb-audio? Is the issue with PortAudio 
and PulseAudio? How can I test channels 3-4 with aplay or another program?