ALSA: pcsp - Fix locking messes in snd-pcsp

snd-pcsp driver takes chip->substream_lock together with PCM substream
lock.  These are even mixed up with hrtimer's lock, resulting in messy
lock depencies.  Right now, snd-pcsp driver resolves the deadlock by
using HRTIMER_CB_SOFTIRQ.  However, this isn't nice for a really fast
path like bit-flipping.

This patch introduces a tasklet for PCM period handling so that the
hrtimer callback can be handled fast.  This also reduce the use of
chip->substream_lock to avoid deadlocks.  It's still used in pointer
callback, but even this could be removed with a proper barrier.

Another good solution is to introduce async trigger callback.  But,
this will involve with a major rewrite of the PCM core code, so I
take first this easy fix.

Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/drivers/pcsp/pcsp.h

@@ -77,6 +77,7 @@ struct snd_pcsp {
 extern struct snd_pcsp pcsp_chip;
 
 extern enum hrtimer_restart pcsp_do_timer(struct hrtimer *handle);
+extern void pcsp_sync_stop(struct snd_pcsp *chip);
 
 extern int snd_pcsp_new_pcm(struct snd_pcsp *chip);
 extern int snd_pcsp_new_mixer(struct snd_pcsp *chip);