[alsa-devel] Question on wait_for_avail function

Takashi Iwai tiwai at suse.de
Thu Jun 6 08:53:53 CEST 2013

At Wed, 05 Jun 2013 23:25:15 -0700,
Patrick Lai wrote:
> + << Jaroslav >>
> On 6/5/2013 2:17 AM, Clemens Ladisch wrote:
> > gsantosh at codeaurora.org wrote:
> >> 2) HAL issues pause in parallel to the write
> >
> > This is illegal.
> > <http://www.alsa-project.org/main/index.php/SMP_Design> says:
> > | The use of returned handles [such as snd_pcm_t*] must be serialized in
> > | the application using own locking scheme.
> >
> > If you want to issue a pause while waiting for data to be written, use
> > non-blocking writes.
> >
> Based on my understanding of SMP_Design and response above, pause and 
> write to same
> PCM device are expected to be serialized. However, my interpretation of
> following patch is that pause and stop are expected to wakes up process
> waiting in pcm_lib.c wait_for_avail(). It seems to give me the
> indication that multiple threads acting on same PCM device is expected.
> ALSA: pcm_core: Fix wake_up() optimization
> This change fixes the "ALSA: pcm_lib - optimize wake_up() calls for PCM
> I/O" commit. New sleeping queue is introduced to separate user space
> and kernel space wake_ups. runtime->nowake is renamed to twake
> (transfer wake).
> Jaroslav: Can you share your thought as you are author of this patch?

I don't understand what's the problem in this thread...

The behavior of PAUSED stream in wait_for_avail() is clear.  If it's
woken up by some events, it simply goes to sleep again without exiting
the loop.


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