[alsa-devel] Duplicate wake-ups in pcm_lib.c

[pl bossart]

> The improved and more clean fix in now in my tree:
>
> http://git.alsa-project.org/?p=alsa-kernel.git;a=commitdiff;h=1250932e48d3b698415b1f04775433cf1da688d6
>

The race condition is definitely gone. However I still see a NULL
event in snd_pcm_playback_poll() in pcm_native.c.

[13312.575829] snd_intel8x0_interrupt
[13312.575839]  snd_intel8x0_update
[13312.575847] snd_pcm_period_elapsed
[13312.575853]  snd_pcm_update_hw_ptr0
[13312.575862]    snd_pcm_update_state
[13312.575870]     wakeup in snd_pcm_update_state: avail 8194, avail_min 8192
[13312.575894] ALSA: poll event  POLLOUT|POLLWRNORM, avail 8194 avail_min 8192
[13312.581259] snd_pcm_lib_write1: in
[13312.581267] snd_pcm_lib_write1 loop
[13312.581273]  snd_pcm_update_hw_ptr0
[13312.581282]    snd_pcm_update_state
[13312.581312] snd_pcm_lib_write1: end
[13312.581317]    snd_pcm_update_state
[13312.581328] ALSA: null poll event, avail 262 avail_min 8192
[13312.746498] snd_intel8x0_interrupt
[13312.746511]  snd_intel8x0_update
[13312.746520] snd_pcm_period_elapsed
[13312.746526]  snd_pcm_update_hw_ptr0
[13312.746536]    snd_pcm_update_state
[13312.746544]     wakeup in snd_pcm_update_state: avail 8194, avail_min 8192
[13312.746571] ALSA: poll event  POLLOUT|POLLWRNORM, avail 8194 avail_min 8192
[13312.751990] snd_pcm_lib_write1: in
[13312.751998] snd_pcm_lib_write1 loop
[13312.752004]  snd_pcm_update_hw_ptr0
[13312.752013]    snd_pcm_update_state
[13312.755285] snd_pcm_lib_write1: end
[13312.755293]    snd_pcm_update_state
[13312.755305] ALSA: null poll event, avail 265 avail_min 8192

I couldn't figure out who generates this null event, and even better
it does not show in the report printed by my test app. I am somewhat
lost here as to how snd_pcm_playback_poll works.
- Pierre