[alsa-devel] What does snd_pcm_delay() actually return?

[Takashi Iwai]

At Fri, 13 Jun 2008 17:44:24 +0200 (CEST),
Jaroslav Kysela wrote:
> 
> On Fri, 13 Jun 2008, Takashi Iwai wrote:
> 
> > > >     Ditto, or we may reuse snd_pcm_hw_params_fifo_size()?
> > > 
> > > Yes, fifo_size was designed to announce possible extra latency to 
> > > applications.
> > 
> > On the second thought, it's better *not* to query this value as
> > hw_params.  The latency may be variable.  And the word "FIFO" isn't
> > appropriate in every case.
> > 
> > And, above all, reviving an old API is bad...
> 
> This value should define maximum latency - not actual latency. 
> snd_pcm_delay() should give actual overall latency to apps.
> 
> > > I think that the current PCM API concept is tightly period based. You 
> > > cannot change it easily. It would be probably better to move to 
> > > "byte-stream" in next revision of PCM API.
> > 
> > Not that difficult, I guess, from API POV.  The major work is in the
> > PCM core part and some alsa-lib plugins.  But, it's not API.
> >
> > What I once worked on is an extra timinig queue.  Suppose that we
> > provide an API to access a timing queue that holds the wake-up
> > schedule, either in time or sample unit.  The poll / read / write
> > syscalls are woken up at the time of this schedule.  In the case of
> > period model, it means that the queue is automatically filled up with
> > a constant period.  If app wants to schedule by itself, it can use
> > this queue manually.  (Of course, this means that the timing queue
> > must be filled before starting the stream.)
> 
> It look like more complicated sleep_min implementation we already had:
> 
> http://git.alsa-project.org/?p=alsa-kernel.git;a=commit;h=31e8960b35975ed235d283d6fb95d0e28dffded0

Yes and no.  Yes, it updates with a system timer.  No, it's not bound
with the period like the current framework.

The point is that the wake-up timing isn't defined as constant but via
a timing queue (or a request queue).  This is more suitable for
pull-style apps like JACK.  Which irq source is used doesn't matter.


Takashi