[alsa-devel] Disabling buffer fill level preprocessing by ALSA

Lennart Poettering mznyfn at 0pointer.de
Mon Jan 7 23:38:50 CET 2008

On Mon, 07.01.08 19:33, Jaroslav Kysela (perex at perex.cz) wrote:

> > > > I assume that I can enable a mode like that with one of the SW
> > > > params. But quite frankly the docs for it are not enlighening at all.
> > > 
> > > Set the stop_threshould sw_params to the boundary size.
> > > 
> > > 	snd_pcm_sw_params_get_boundary(sw_params, &boundary);
> > > 	snd_pcm_sw_params_set_stop_threshold(pcm, sw_params, boundary);
> > > 
> > > then the driver behaves in the "freewheel" mode.  The dmix plugin uses
> > > this technique.
> > 
> > That's not what I was looking for. This will only disable automatic
> > stopping on buffer underrun. I am using that already in PA (however I
> > pass -1 as stop threshold, which should work, too, shouldn't it?)
> > 
> > What I am really looking for is a way to disable that ALSA reports via
> > poll() the buffer fill level, but instead only reports whether an
> > interrupt happened.
> Note that you can control fill level using snd_pcm_forward/rewind 
> without any R/W calls (of course if supported in whole chain).
> ALSA supports only "controlled" I/O not dumb I/O as OSS driver for mmap.
> If you look for timing source, use timer API - you may try 
> alsa-lib/test/timer.c:
> ./timer class=3 card=0 device=0 subdevice=0

How does this timer depend on the PCM clock? Is its wakeup granularity
dependant on the period parameters of the matching PCM device? Or am I
supposed to first initialize PCM, and chose some period parameters the
hw likes and than pass that on to the timer subsystem?

I assume I don't have any guarantee that all alsa devices have such a
timer attached? So I'd need some major non-trivial fallback code if I
make use of these timers? 

> > The background why I want this is this: As mentioned I am now
> > scheduling audio in PA mostly based on system timers. To be able to do
> > that I need to be able to translate timespans from the sound card
> > clock to the system clock. Which requires me to get the sample time
> > from the sound card from time to time and filter it through some code
> > that estimates how the sound card clock and the system clock
> > deviates. I'd prefer to do that only once or maybe twice everytime the
> > playback buffer is fully played, and only shortly after an IRQ
> > happened, under the assumption that this is the best time to get the
> > most accurate timing information from the sound card.
> It's not really necessary. You can use only one timing source (system 
> timer) and use position timestamps to do corrections.

Position timestamps? You mean status->tstamp, right? I'd like to use
that.  But this still has two problems:

1) As mentioned, CLOCK_MONOTONIC support is still missing in ALSA(-lib)

2) I'd like to correct my estimations as quickly as possible, i.e. as
   soon as a new update is available, and not only when I ask for
   it. So basically, I want to be able to sleep in a poll() for timing

> But your example does not explain, why you don't move r/w pointer in the 
> ring buffer (use mmap_commit), thus why you don't fullfill the avail_min 
> requirement for poll wakeup. It seems to me that you're trying to do some 
> crazy things with the ring buffer which are not allowed.

As mentioned, when PA starts up it configures the audio hw buffer to
2s or so with the minimal number of periods (2 on my sound
cards). Then, clients come and go. Depending on the what the minimal latency
constraints of the clients are, I however will only fill up part of
the buffer.

Scenario #1: 

Only one simple MP3 playing music application is connected. It doesn't
have any real latency constraints. We always fill up the whole 2s
buffer, then sleep for 1990 ms, and then fill it up again, and so
on. If the MP3 player pauses or seeks, we rewrite the audio buffer
with _rewind(). Thus alsthough we buffer two full seconds the user
interfaces still reacts snappy.

Now, because the user starts and stops applications all the time, we
dynamically change into scenario #2:

The MP3 playing application is still running. However, now a VoIP
application is running too. It wants a worst case latency of let's say
20ms. When this applications starts up we don't want to interrupt
playback of the MP3 application. So from now on we only use 20ms of
the previously configured 2s hw buffer. And as soon as we wrote 20ms,
we sleep for 10ms, and then fill it up again, and so on.

Now, after a while the VoIP call is over, we enter scenario #3:

This is identical to #1, we again use the full 2s hw buffer, and sleep
for 1990ms.

So, depending on what clients are connected, we dynamically change the
wakeups. Now, on ALSA (and with a lot of sound hw, as I understood
Takashi) you cannot reconfigure the period sizes dynamically without
interruptions of audio output. That's why I want to disable the whole
period/buffer fill level management of ALSA, and do all myself with
system timers, which I thankfully now can due to the advent of
hrtimers (at least on Linux/x86). System timers nowadays are a lot
more flexibe than the PCM timer, because they can be reconfigured all
the time without any drawbacks. They are not dependant on period sizes
or other stuff which may only be reconfigured by resetting the audio
devices. The only drawback is that we need to determinine how the
sound card clock and the system clock deviate.

Does that make sense to you?


Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net         ICQ# 11060553
http://0pointer.net/lennart/           GnuPG 0x1A015CC4

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