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

Alan Stern stern at rowland.harvard.edu
Tue Aug 27 22:11:00 CEST 2013

On Tue, 27 Aug 2013, Clemens Ladisch wrote:

> Alan Stern wrote:
> > On Tue, 27 Aug 2013, Clemens Ladisch wrote:
> >> The current algorithm uses very short capture URBs to ensure that _some_
> >> URB is completed as soon as possible after a period ends.
> >> [...]
> >> I'd suggest to keep the old calculation for capture URBs.  It would
> >> make sense to use longer capture URBs only if the period size is
> >> relatively large.
> >
> > Okay.  What about playback endpoints with implicit sync?  The current
> > driver treats them the same as capture endpoints.  Is that really the
> > right thing to do?
> The only reason to not have an interrupt after each packet is to avoid
> the interrupt overhead (for both CPU and power); shorter URBs would
> result in a more precise DMA position reported to user space.  If there
> is already an interrupt for some capture URB (or for the feedback packet
> in case of explicit feedback), we might as well handle the playback
> packets so far.

I don't quite understand.  Are you saying that playback endpoints with 
implicit sync may as well use the same values for urb_packs and 
ep->nurbs as other playback endpoints, rather than using the values 
calculated for capture endpoints (which is what the current driver 

Alan Stern

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