[alsa-devel] snd_pcm_delay, hw buffers and driver api (v2)

[James Courtier-Dutton]

2009/8/13 Kai Vehmanen <kvehmanen at eca.cx>:
> Hi,
>
>
> Any update/ideas on this topic? Personally I think adding a new driver
> callback would make most sense, as that would allow to take full benefit
> from hardware that allows to query sample-accurate position of playout
> (i.e. not just support exposing a fixed latency). Of course that's
> potentially a big change. In alsa-lib, snd_pcm_hwsync() could call this
> driver callback, and a new variant of snd_pcm_delay() could present the
> information to the applications.

snd_pcm_delay() does not just support exposing a fixed latency. The
value returned is dynamic.

In the current ALSA implementation, the value returns a real time
count of how many samples are already in the hardware buffer.
So, if one makes a call to snd_pcm_delay(), waits for a period of a
few samples, the new value returned in snd_pcm_delay() is going to be
different, but only for sound cards that support this level of
accuracy.

So, I think the snd_pcm_delay() function is already doing what you want.
It was me who originally requested the snd_pcm_delay() function to be
introduced into the API, for the purposes of audio/video sync in the
media player xine.

Kind Regards

James