Re: periods*period_size vs. buffer_size; should they be equal?
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2010/12/18 Jassi Brar <jassisinghbrar@xxxxxxxxx> > On Sat, Dec 18, 2010 at 5:04 AM, Stephen Warren <swarren@xxxxxxxxxx> > 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 ? _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel