profiles/midi: Added MIDI over BLE profile implementation

This plugin implements the Central role of MIDI over Bluetooth
Low-Energy (BLE-MIDI) 1.0 specification as published by MMA in
November/2015.

It was implmemented as a bluetoothd plugin because of latency requirements
of MIDI. There are still room for improvements on this regard.

Like previsouly mentioned, it only implements the Central role, but
since all parsing and state-machine code is in libmidi.[hc] it should be
simple to implement the Peripheral role as a GATT service as well.

Files added:
 * profiles/midi/midi.c: Actual GATT plugin
 * profiles/midi/libmidi.[ch]: MIDI parsers

Techinal notes
==============

This plugin doesn't require any new threads. It relies on notifications
from a device to parse and render proper events that are queued in the
kernel, causing no blocks at all. Even if an error occur, it will be
handled and returned control to bluetoothd.

It also adds a new file descriptor to be read using struct io. That is
necessary to read events from applications and render raw BLE packets to
be sent to the device with a write without response command. It doesn't
block as well.

This patch introduces ALSA as dependency. But this feature is disabled
by default. To enable it, pass --enable-midi to the configure script.

Even though this introduces ALSA dependency, it is not an audio plugin.
It is rather a MIDI plugin, which is a byte stream protocol with low
throughput but requires low-latency.

Observations
============

I have tested on a normal laptop Arch-linux (x86_64) and a Raspberry Pi 2
(ARM Cortex-A8) and it works very well. As I mentioned, the latency can
always be improved.

I will still maintain a personal branch on my github[1] so others can
contribute there and I can test before sending to BlueZ.

IMPORTAT: the timestamp support is incomplete since ALSA doesn't support the
way MIDI over BLE expects (asign timestamp to an event without scheduling).
We are working on ALSA to support this.

Credits
=======

I would like to send kudos to ROLI Ltd. which allowed my to work
on this as part of my full-time job.

[1] https://github.com/ftonello/bluez/

1 files changed, 8 insertions, 0 deletions

diff --git a/Makefile.plugins b/Makefile.plugins
index 59342c0cb..3a9e27c65 100644
--- a/Makefile.plugins
+++ b/Makefile.plugins
@@ -95,6 +95,14 @@ builtin_sources += profiles/scanparam/scan.c
 builtin_modules += deviceinfo
 builtin_sources += profiles/deviceinfo/deviceinfo.c
 
+if MIDI
+builtin_modules += midi
+builtin_sources += profiles/midi/midi.c \
+			profiles/midi/libmidi.h \
+			profiles/midi/libmidi.c
+builtin_ldadd += @ALSA_LIBS@
+endif
+
 if SIXAXIS
 plugin_LTLIBRARIES += plugins/sixaxis.la
 plugins_sixaxis_la_SOURCES = plugins/sixaxis.c