ALSA: firewire-tascam: support drain callback for MIDI playback substream

ALSA driver for TASCAM FireWire series transfers MIDI messages in system
workqueue. In current design of the driver, applications should wait for
sequence of transmission when they close ALSA rawmidi character devices.
However, when considering design of rawmidi interface, it's preferable
to wait in drain ioctl.

This commit adds support for the drain ioctl to wait for the end of
the transmission.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/firewire/tascam/tascam-midi.c

@@ -30,12 +30,15 @@ static int midi_capture_close(struct snd_rawmidi_substream *substream)
 }
 
 static int midi_playback_close(struct snd_rawmidi_substream *substream)
+{
+	return 0;
+}
+
+static void midi_playback_drain(struct snd_rawmidi_substream *substream)
 {
 	struct snd_tscm *tscm = substream->rmidi->private_data;
 
 	snd_fw_async_midi_port_finish(&tscm->out_ports[substream->number]);
-
-	return 0;
 }
 
 static void midi_capture_trigger(struct snd_rawmidi_substream *substrm, int up)
@@ -77,6 +80,7 @@ int snd_tscm_create_midi_devices(struct snd_tscm *tscm)
 	static const struct snd_rawmidi_ops playback_ops = {
 		.open		= midi_playback_open,
 		.close		= midi_playback_close,
+		.drain		= midi_playback_drain,
 		.trigger	= midi_playback_trigger,
 	};
 	struct snd_rawmidi *rmidi;