[alsa-devel] periods*period_size vs. buffer_size; should they be equal?

[Takashi Iwai]

At Sat, 18 Dec 2010 11:12:09 +0800,
Raymond Yau wrote:
> 
> 2010/12/18 Jassi Brar <jassisinghbrar at gmail.com>
> 
> > On Sat, Dec 18, 2010 at 5:04 AM, Stephen Warren <swarren at nvidia.com>
> > wrote:
> > > I'm attempting to understand the layout of the DMA buffer used by ASoC
> > > devices. It seems like the buffer should consist of runtime->periods
> > entries
> > > each of runtime->period_size frames of audio data. Hence,
> > runtime->buffer_size
> > > should be equal to runtime->periods * runtime->period_size in frames.
> > >
> > > This assumption holds true in many cases. However, it doesn't in others:
> > >
> > > (sizes in hex)
> > >
> > > Playing WAVE '8000.wav' : Signed 16 bit Little Endian, Rate 8000 Hz,
> > Stereo
> > > [  217.910867] Trigger: runtime->periods=4
> > > [  217.914696] Trigger: runtime->period_size=3e8
> > > [  217.919043] Trigger: runtime->buffer_size=fa0
> > > [  217.923389] Trigger: f2b(4)=10
> > >
> > > Playing WAVE '11025.wav' : Signed 16 bit Little Endian, Rate 11025 Hz,
> > Stereo
> > > [  268.543821] Trigger: runtime->periods=5
> > > [  268.547651] Trigger: runtime->period_size=400
> > > [  268.551998] Trigger: runtime->buffer_size=1589
> > >
> > > Playing WAVE '16000.wav' : Signed 16 bit Little Endian, Rate 16000 Hz,
> > Stereo
> > > [  315.889280] Trigger: runtime->periods=7
> > > [  315.893109] Trigger: runtime->period_size=400
> > > [  315.897456] Trigger: runtime->buffer_size=1f40
> > >
> > > Yet for any higher sample rate the layout is identical:
> > >
> > > Playing WAVE '96000.wav' : Signed 16 bit Little Endian, Rate 96000 Hz,
> > Stereo
> > > [  385.494764] Trigger: runtime->periods=8
> > > [  385.498594] Trigger: runtime->period_size=400
> > > [  385.502941] Trigger: runtime->buffer_size=2000
> > >
> > > Is my assumption wrong? Is there some bug somewhere?
> >
> > You assumption doesn't seem to be accurate.
> > Theoretically, your buffer_size may not be a multiple of period_size. But
> > that
> > would mean points shifting on the ring buffer at which interrupts are
> > generated.
> >
> > Usually embedded systems don't have dedicated ring buffer DMA for use
> > with ASoC... and it is cumbersome to reprogram the next interrupt point if
> > buffer_size is not a multiple of period_size. So, we usually call
> > snd_pcm_hw_constraint_integer(runtime, SNDRV_PCM_HW_PARAM_PERIODS);
> > in the stream open callback, which ensures constrainst buffer_size to
> > be a multiple of period_size.
> >
> >
> How about the DMA page size in PCI/PCIe ?
> 
> It seem that some PCI card (e.g. au88x0) does not allow a period split
> across two DMA page and require period size must be power of two.
> 
> Is this a requirement of using scatther gather buffer or hardware limitation
> of au88x0 ?

It's au88x0 specific issue.


Takashi