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

Jaroslav Kysela perex at perex.cz
Thu Jan 7 15:59:41 CET 2010

On Wed, 6 Jan 2010, pl bossart wrote:

>>> A wake-up is generated during the period interrupt, and a second
>>> wake-up is generated during the write loop, after the application was
>>> awaken but just before the pointers are updated. This second wake-up
>>> shouldn't exist, since the write loop actually fills the ring buffer.
>>> By the time the second wake-up is actually handled, there's really no
>>> space left in the buffer and a null event is generated; it'll wake-up
>>> the application a second time for nothing. Maybe we should move the
>>> call to snd_pcm_update_hw_ptr() after the transfer took place?
>> The right fix should be to preserve wakeups when write operation is in
>> progress (also for interrupts). Something like this (untested):
> Thanks Jaroslav for your feedback. It seems your fix is similar to
> what I suggested, that is check the pointers and generate a wake-up
> after the write loop completes rather than right before the write
> starts. I modified your patch to correct the behavior when errors
> occur; the jumps to _end_wake compared to _end_unlock weren't self
> explanatory and not always consistent. I only perform a pointer update
> if nothing wrong happened. In case of errors, I unlock or just return
> the status as before. I am testing the changes and should have a patch
> shortly.
> Cheers

The improved and more clean fix in now in my tree:


Note that you should apply these 4 patches (in reverse order):

4: ALSA: pcm_lib - optimize wake_up() calls for PCM I/O
3: ALSA: pcm_lib - cleanup & merge hw_ptr update functions
2: ALSA: pcm_lib - add possibility to log last 10 DMA ring buffer positions
1: ALSA: pcm_lib.c - convert second xrun_debug() parameter to use defines

Patch #3 should definitely fix problems with large avail or delay values 
reported in PA. I finally figured the culprit (and decide to cleanup all
relevant code rather do just another workaround). In other words, new 
hw_ptr pointer should never be less than previous one now.


Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.

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