SysEx overflow when using the MIDI sequencer event interface

[Haakon Riiser <haakon.riiser@fys.uio.no>]

I'm trying to fix a problem with the DOSBox emulator that occurs
when it's sending SysEx events to an external Roland MIDI device
(CM-32L in my case).

After a little investigation, I discovered that the problem is that
SysEx messages sent via the sequencer event interface are pushed out
faster than my Roland CM-32L can process them.

This bug only affects the event interface:  Sending the SysEx
commands using the rawmidi interface (for example by using 'amidi
-s <sysex-file>') works perfectly.  I can reproduce DOSBox's
problems using a toy version of amidi that uses the event interface.
Here's what my implementation does:

  snd_seq_open(&handle, "hw", SND_SEQ_OPEN_DUPLEX, 0);
  snd_seq_create_simple_port(handle, NULL, SND_SEQ_PORT_CAP_WRITE |
                                           SND_SEQ_PORT_CAP_SUBS_WRITE |
                                           SND_SEQ_PORT_CAP_READ,
                             SND_SEQ_PORT_TYPE_MIDI_GENERIC);
  snd_seq_connect_to(handle, 0, 16, 0);

  while (get_next_sysex_command(sysex_data, &sysex_len)) {
          snd_seq_event_t ev;
          snd_seq_ev_clear(&ev);
          snd_seq_ev_set_source(&ev, 0);
          snd_seq_ev_set_subs(&ev);
          snd_seq_ev_set_direct(&ev);
          snd_seq_ev_set_sysex(&ev, sysex_len, sysex_data);
          snd_seq_event_output(handle, &ev);
          snd_seq_drain_output(handle);
          snd_seq_sync_output_queue(handle);
  } 

As an example of how much data is lost, consider that it took less
than 1.5 seconds to send 18 kB of SysEx messages using my program,
while it takes amidi over 6 seconds.

If I insert a delay of, say, 100 ms between each sent SysEx
command, my program appears to work, but there _has_ to be a
better way to ensure that the SysEx message is processed before
starting to send a new one.

Is this a bug in the ALSA code, or is there a synchronization
call or something like that I can use?

-- 
 Haakon

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