[alsa-devel] HDAudio period/buffer sizes

[Takashi Iwai]

At Sun, 14 Aug 2011 09:44:26 +0800,
Raymond Yau wrote:
> 
> 2011/7/27 Takashi Iwai <tiwai at suse.de>:
> > At Tue, 26 Jul 2011 14:23:40 -0500,
> > Pierre-Louis Bossart wrote:
> >>
> >> that the buffer itself was already aligned to 128 bytes from the
> >> > very beginning as it's allocated via dma_alloc_coherent().
> >> > The fact that the commit fixes the real problem means that the buffer
> >> > size must be aligned, too, at least for some hardware.
> >>
> >> I checked internally and the reply was "There is no requirement the buffer
> >> must be 128 byte multiples.  As long as it is integer number of samples, we
> >> are good."
> >>
> >> What's the best way to deal with it? Add a driver option? A compilation
> >> option?
> >
> > I'd say a driver option since this must be dynamically adjustable.
> > Care to make a patch?
> >
> > For 3.2 (after 3.1 merge window), we can disable 128-bytes limit as
> > default again for testing.  If someone hits the problem, he can switch
> > easily and we can investigate more at this time.
> >
> >
> The patch seem fix the problem of
> snd_pcm_hw_params_set_period_time_near() to 120ms at 11025Hz for those
> ad198x codec which are the only HDA codecs which support 11025Hz and
> 22050Hz
> 
> Does it mean that the period_bytes_min =128 can also be removed to
> achieve lowest latency ?

It depends on the sample rate (of course, 128byte itself defines the
limit, though).  The problem is for sample rates that can't be divided
by 8khz.


Takashi