Using alsa midi with SuperCollider

Right, so I have a workaround which leads to a pretty usable solution. Takes about 10 minutes to setup.
This entails:

• installing a kernel with support for virmidi (see below)
• running modprobe snd-virmidi. This creates 4 virtual midi ports
• in a terminal or a script, run stdbuf -i 0 -o 0 -e 0 cat /dev/snd/midiC1D0 > /dev/snd/midiC0D0 and leave it running. This will pipe the inputs from hw:1,0 to hw:0,0. In my case, these correspond to a USB MIDI keyboard and the first virmidi port, you'll have to check for your setup.
• now Supercollider should be able to access MIDI by connecting to the first virmidi port. What I run in SC is:

  MIDIClient.init;
  MIDIIn.connectAll;
  (MIDIdef.noteOn(\noteOnTest, {arg vel, nn, chan, src;	[vel, nn, chan, src].postln;}))

The kernel that came with the image does not support virmidi. This one does. Installing it should be as easy as:

• download the zip archive
• copy it to the board
• unzip xenomai-r81-interrupts-virmidi.zip && rm xenomai-r81-interrupts-virmidi.zip
• needless to say, backup your important files before doing the following

  for a in linux-*3.8.13xenomai-xenomai-r81_1cross_armhf.deb;
  do
    dpkg -i $a;
    rm $a; done;
  done
  mv /boot/uboot/zImage /boot/uboot/zImage.bak
  cp /boot/vmlinuz-3.8.13xenomai-xenomai-r81 /boot/uboot/zImage

  (may take a few minutes, especially when installing the headers)
• reboot
• if you are lucky enough that it reboots (lol), you can check you are running the new kernel:

  root@bela ~$ uname -a
  Linux bela 3.8.13xenomai-xenomai-r81 #4 Wed Oct 26 20:25:54 BST 2016 armv7l GNU/Linux

The script that pipes the data could be something like this

#!/bin/bash
modprobe snd-virmidi
INPUT=/dev/snd/midiC1D0
OUTPUT=/dev/snd/midiC0D0
while sleep 1
do
  ls $INPUT &>/dev/null && ls $OUTPUT &> /dev/null &&\
  stdbuf -i 0 -o 0 -e 0 cat $INPUT > $OUTPUT
done

and this could go, e.g.: into ~/Bela_startup.sh (unless you have a project set to start on boot in there)
This will test whether the input and output ports exist before piping the one to the other. If it fails (at startup or while running), it will sleep for 1 second before retrying.

Let me know how it goes.

Im a bit confused here... on other distros (e.g. ubuntu) a usb midi device is registered as both a raw device and a sequencer device.

I had a quick look at the alsa modules source (3.8.13) and snd_midi.c does seem to have a method to register raw devices in the sequencer , see snd_seq_midisynth_register_port(), so I'm assuming snd-seq-midi should do this 'automatically'

also comparing to another linux distro, I can see snd_rawmidi module is 'missing' - i guess because its been compiled into the kernel rather than a module, but perhaps that causes an issue with snd-seq-midi, which seems to be the 'problematic one' .... (you can see the dependancy is there, in alsa-base.conf in /etc/modprobe.c)

cross compiling on Linux, this kernel with this config file. Note: make sure you install ccache on your Linux machine, it will make newer builds much faster!