Current implementation of ALSA PCM core has a kernel API,
snd_pcm_period_elapsed(), for drivers to queue event to awaken processes
from waiting for available frames. The function voluntarily acquires lock
of PCM substream, therefore it is not called in process context for any
PCM operation since the lock is already acquired.

It is convenient for packet-oriented driver, at least for drivers to audio
and music unit in IEEE 1394 bus. The drivers are allowed by Linux
FireWire subsystem to process isochronous packets queued till recent
isochronous cycle in process context in any time.

This commit adds snd_pcm_period_elapsed() variant,
snd_pcm_period_elapsed_without_lock(), for drivers to queue the event in
the process context.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210610031733.56297-2-o-takashi@sakamocchi.jpSigned-off-by: Takashi Iwai <tiwai@suse.de>