[alsa-devel] ALSA always writes _whole_ periods, right?

Joachim Förster mls.JOFT at gmx.de
Sat Jun 16 23:35:15 CEST 2007

Hi Takashi,

thanks for your answer,

On Thu, 2007-06-14 at 12:39 +0200, Takashi Iwai wrote:
> > I have a quick question regarding the filling of the usual ring buffer
> > in RAM (used by sound controllers via DMA), just to be really sure:
> > 
> > Is it true, that ALSA always writes (via mmap'ing) _whole_ periods?
> The whole buffer is, of course, used.  Otherwise you'll get obvious
> noises :)
> But if you're asking about the timinig, then no.  How much data is
> written at which timing isn't strictly defined in mmap mode.  The
> PCM core just checks the available size at each
> snd_pcm_period_elapsed(), and reports XRUN if needed.  That's all.

Ok, ahm, I think my question was not about what I really wanted to know
- sorry. I have to reformulate it: In fact I would like to know, how the
ALSA-library mmap code behaves regarding the last chunk of samples, for
example: Consider aplay, playing a wav file and the wav file's
size/number of samples does _not_ fit to a multiple of a period's size.
Then, does the mmap code copy these last samples as is _or_ does it copy
one whole last period (which is filled with zeros)?


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