[alsa-devel] Buffer size for ALSA USB PCM audio

[Takashi Iwai]

At Wed, 24 Jul 2013 11:22:00 -0400 (EDT),
Alan Stern wrote:
> 
> On Wed, 24 Jul 2013, Takashi Iwai wrote:
> 
> > At Wed, 24 Jul 2013 10:41:43 -0400 (EDT),
> > Alan Stern wrote:
> > > 
> > > I have been studying the data_ep_set_params() function in
> > > sound/usb/endpoint.c.  This is the routine that calculates the number
> > > of samples and I/O requests to keep on the USB hardware queue for PCM
> > > audio, based on the ALSA parameters.
> > > 
> > > It uses the PERIOD_BYTES parameter but not BUFFER_BYTES.  In simplified
> > > terms (ignoring rounding, boundary cases, and other things), the number
> > > of periods per buffer is fixed at 24 for recording and 1 for playback,
> > > completely ignoring the user's setting.  If you look at the parameters
> > > copied in snd_usb_hw_params() in sound/usb/pcm.c, you'll see what I
> > > mean.
> > > 
> > > Is this really the intended behavior?  It doesn't seem right at all.
> > 
> > The buffer size doesn't matter for urb setup because the usb-audio
> > driver transfers the data by the driver itself at urb completes.
> > The buffer size is considered in these callbacks,
> > i.e. prepare_playback_urb() and retire_capture_urb().
> 
> I don't understand.  Consider a simple playback example.  Suppose the
> user wants to keep the latency low, so he requests 2 periods per
> buffer.  snd-usb-audio ignores this value and decides to use 10 URBs,
> which is equivalent to setting the buffer size to 10 periods.

You don't have to fill all 10 URBs to start the stream...


Takashi