[alsa-devel] PulseAudio and SNDRV_PCM_INFO_BATCH

[Raymond Yau]

> > > Well, USB-audio has another problem.  USB-audio uses the intermediate
> > > audio ring buffer, and the samples are copied to each URB buffer.  At
> > > each packet complete, the driver copies the rest of sample chunk
> > > again, and advances the hwptr when the packets.  So, the hwptr of
> > > USB-audio is in advance of the actual sample position.  But we provide
> > > the runtime delay information for user-space to correct to the more
> > > accurate sample position.  So far, so good.
> > >
> > > A missing piece in this picture is, however, the position of the
> > > not-yet-queued samples in ring buffer.  Basically you can rewrite /
> > > rewind the sample at most this point, but not farther -- such
> > > in-flight samples can't be modified any longer.  This can be seen a
> > > kind of hardware fifo with a pretty big and non-continuously variable
> > > size during operation.
> > >
> > > In that sense, get_fifo() looks like a candidate for giving such
> > > information, indeed.  But reviving the old (and rather bad working)
> > > API appears dangerous to me.  I'd prefer creating a new API function
> > > instead, if any.
> > >
> > > BTW, because of its design like above, a large (or no) period size
> > > doesn't help for saving power at all with USB-audio.  This should be
> > > considered before the further discussion...
> >
> > Hmm...I was trying to understand this power save argument. I tried to
> > figure out a "typical" URB size by just plugging my headset in, and I
> > saw wMaxPacketSize being 96 and/or 192 bytes.
> > Then, MAX_PACKS is set to either 6 (or 48 for USB 2.0 devices, but this
> > is just a headset).
> >
> > Can this be correct? Does it mean that we are getting interrupts every
> > 192 * 6 bytes (i e, every 6 ms for a 48kHz/stereo/16bit stream)?
>
> The driver can build up a URB containing multiple packets, so the
> wakeups can be reduced in some level.  But, then the hwptr update also
> suffers, and more badly, the in-flight size also increases -- both are
> bad for sample mixing, obviously.
>

http://lxr.free-electrons.com/ident?i=SNDRV_PCM_INFO_BATCH

If info batch represent exact one period,

do you mean snd-intel8x0 is worst or better ?

How about firewire which also use packets ?