PCM: Avoid busy loop in snd_pcm_write_areas() with rate plugin

snd_pcm_write_areas() tries to wait until avail >= avail_min condition
is satisfied.  This doesn't work always well when a rate plugin is in
the play.

When a partial data with a smaller size than a period is written, the
rate plugin doesn't transfer the data immediately to the slave PCM,
but kept in an internal buffer and it changes only the hwptr of the
plugin.  Thus, the condition "avail < avail_min" is triggered for a
wait check although the underlying slave PCM has enough room.  This
results in a call of snd_pcm_wait() which returns immediately after
poll() call, and the snd_pcm_write_areas() loop continues.  As a
consequence, it falls into a CPU hog.

This patch fixes that busy loop by introducing a new fast_ops to check
the availability for wait of avail_min.  Then a plugin can ask the
slave PCM whether the wait is required (or possible).

A few plugins like multi plugin need a special handling.  Otherwise a
generic plugin function can be used.

Reported-by: Trent Piepho <tpiepho@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

src/pcm/pcm_meter.c

@@ -545,6 +545,7 @@ static const snd_pcm_fast_ops_t snd_pcm_meter_fast_ops = {
 	.poll_descriptors_count = snd_pcm_generic_poll_descriptors_count,
 	.poll_descriptors = snd_pcm_generic_poll_descriptors,
 	.poll_revents = snd_pcm_generic_poll_revents,
+	.may_wait_for_avail_min = snd_pcm_generic_may_wait_for_avail_min,
 };
 
 /**