[alsa-devel] PulseAudio and SNDRV_PCM_INFO_BATCH

[Takashi Iwai]

At Wed, 17 Jun 2015 17:09:41 +0200,
David Henningsson wrote:
> 
> 
> 
> On 2015-06-17 11:19, Takashi Iwai wrote:
> > 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.


Takashi