[alsa-devel] [PATCH] ASoC: TWL4030: PM fix for output amplifiers

[Peter Ujfalusi]

On Monday 22 March 2010 17:06:03 ext Mark Brown wrote:
> On Mon, Mar 22, 2010 at 04:46:08PM +0200, Peter Ujfalusi wrote:
> > On Monday 22 March 2010 16:15:44 ext Mark Brown wrote:
> > > I don't see any fundamental problem here - mostly this just maps on to
> > > "if the widget is powered on write this value, otherwise write the
> > > value configured by the control.".
> > 
> > Hmm, I suppose it could be possible to pass the name of the DAPM widget,
> > a bitmask to a control...
> > Than in the handler in the core we could walk through the DAPM widgets,
> > and find the one, check the status, and if it is on, we allow the write,
> > otherwise we could use the mask to write only the things, that it is
> > needed.
> 
> Something like that, yes - the walk could happen at startup time to
> avoid having to do it repeatedly.

Make sense.

> 
> > But, in TWL case I'd like to filter out also the writes which is done by
> > the DAPM widgets (which is visible from the user space, the mixers). As
> > it is now, if user changes the mixer, which is associated with DAPM
> > widget (and it is different than the PGA, they are DAPM_MIXER), than
> > that write goes directly to the register, and this write takes the
> > cached value, makes the changes and than writes it to register. This
> > also enables the gain (which enables the amp, which takes power).
> 
> Remember, the register cache is below the controls and transparent to
> them - if the controls haven't written a value to the chip then it will
> not appear in the register cache so other controls will not be affected.

Yes, but it also means, that the change will be not visible at all.
I mean, if we did not update the reg_cache (when we should not write to the HW), 
than that change will be gone.
So you will only be able to change the gain on the output, when the path is 
active?

Also, I think, if we can somehow prevent the gain change on the HW, when the 
associated DAPM widget is not active, we would still have the problem of the 
DAPM_MIXER. That operates from the cache, and if the cache has no 0 for the 
gain, than that will be written to the chip.

How I see this: we can go and introduce new controls, DAPM widgets for all type 
just for the sake of the TWL codec. But at the end it will be still TWL 
specific.

I'm not saying that other codecs could not use some generic way (in a way, that 
you have described) to make their amps muted when they are not needed.
What I'm saying, is that I don't see a generic way which can handle the TWL 
codec.

> > > Well, I was rather hoping I could convince you to make this more
> > > generic.  But if that's not possible then yes, please do make the
> > > commit message clearer.
> > 
> > I need to study other codes as well, before I could start doing something
> > more generic, which I totally agree would be beneficial for others as
> > well. But for now, I would like to update the commit message, and come
> > back later, and revisit the possibility of similar, and more generic
> > ways of doing this.
> 
> I guess.

I hope that I can convince you, that - again - the TWL codec is so special, it 
has to handled a bit differently.

-- 
Péter