ALSA: seq: virmidi: Add a drain operation

If a driver does not supply a drain operation for outputs, a default code
path will execute msleep(50). Especially for a virtual midi device
this severely limmits the throughput.

This implementation for the virtual midi driver simply flushes the output
workqueue.

Signed-off-by: Stefan Sauer <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

sound/core/seq/seq_virmidi.c

···
 }
 
 /*
+ * drain output work queue
+ */
+static void snd_virmidi_output_drain(struct snd_rawmidi_substream *substream)
+{
+	struct snd_virmidi *vmidi = substream->runtime->private_data;
+
+	flush_work(&vmidi->output_work);
+}
+
+/*
  * subscribe callback - allow output to rawmidi device
  */
 static int snd_virmidi_subscribe(void *private_data,
···
 	.open = snd_virmidi_output_open,
 	.close = snd_virmidi_output_close,
 	.trigger = snd_virmidi_output_trigger,
+	.drain = snd_virmidi_output_drain,
 };
 
 /*