[alsa-devel] Latency of mixer reconfiguration

Lennart Poettering mznyfn at 0pointer.de
Sun Feb 21 20:26:21 CET 2010

On Sat, 20.02.10 11:59, Raymond Yau (superquad.vortex2 at gmail.com) wrote:

> 2010/2/19 Lennart Poettering <mznyfn at 0pointer.de>
> >
> > On Thu, 18.02.10 10:01, Mark Brown (broonie at opensource.wolfsonmicro.com)
> > wrote:
> >
> >
> > The current logic is to not do any software adjustment if the hardware
> > adjustment is "close enough" to the total adjustment we want to do,
> > tested against a threshold. Which I think is quite a reasonable
> > approach because it enables/disables this feature not globally, but
> > looks at each case and enables this logic only if it really has a
> > benefit.

> PA 's watermark is already 20 ms if you want to do any software adjustment
> by PA server

It's not necessarily 20ms. That's just the default (which we
unfortunately had to pick very large, since ALSA right now does not
inform us about transfer block granularity).

But anyway, whatever the watermark is: this is a latency we know
about, which means that we can delay the hw mixer updating
accordingly. But that works correctly only if the hw mixer updating is
immediate. WHich it most likely is not. 

So we now the software volume adjustment latency. We don't know the
hardware volume adjustment latency, and assume it is 0. If we knew
both we could sync up both changes perfectly, but this way our model
has a weakness.


Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net
http://0pointer.net/lennart/           GnuPG 0x1A015CC4

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