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pcm: Skip avail_min check during draining
snd_pcm_wait() & co checks the current avail value and returns
immediately if it satisfies <= avail_min condition. It's good in
general except for one situation: draining. When the draining is
being performed in the non-blocking mode, apps are supposed to wait
via poll(), typically via snd_pcm_wait(). So this ends up with the
busy loop because of the immediate return from snd_pcm_wait().
A simple workaround is to put the PCM state check and ignore the
avail_min condition if it's DRAINING state. The equivalent check is
found in the kernel xfer code, too.
Signed-off-by: Takashi Iwai <tiwai@suse.de>