[alsa-devel] [PATCH 4/4] ASoC: TWL4030: Add pre-DAC digital mux controls

[Mark Brown]

On Mon, Dec 01, 2008 at 02:09:40PM +0200, Peter Ujfalusi wrote:

> Well, I'm not sure how often this can be used. I was reading the wm* codec
> drivers as you suggested a while ago. Those also implement the digital routing
> outside of DAPM. So I thought why not.

Right, this is one reason I know it can cause issues.  For the Wolfson
codecs it's mostly sidetone paths that are implemented which tends to
work OK since it's relatively rare to want to run a digital sidetone
without also doing a record and playback.  It's when you start
implementing DAC source and ADC output selection that things can get
tricky - DAPM needs to have a method for figuring out which DACs and
ADCs to power up and currently that's done by hard coding.

> Most of the times these routings are static, so one can set these from the actual
> board file, it is kind of hackish, but could do it:
> codec->write(codec, TWL4030_REG_RX_PATH_SEL, _something_);
> I don't see to much harm on having it as runtime configurable. As I said I think
> it is good to have 'full control' over things.

There are definitely use cases for having the control, it's just the
problem with working out what's actually active that means it's safer to
let people make local modifications to the driver for now.  Having parts
of the chip you're trying to use fail to power on doesn't tend to work
so well.

> > Implementing digial routing support is on my radar but it's firmly below
> > the V2 merge.

> You obviously know about ASoCv2... Do you have some pointers on what is coming and when?

The main outstanding bit is the ability to probe machine, codec and
platform drivers independently.  The asoc-v2-dev branch of
git://opensource.wolfsonmicro.com/linux-2.6-asoc has the model for where
we're heading, but there are likely to be some differences due to merge
and review.  The for-tiwai branch will have whatever backports have not
yet been submitted to Takashi.

BTW, there seems to be some problem with your mail client - many of your
lines are over 80 characters, making it hard to read your mails.  It
looks awfully like it's wrapping at ~90 characters or something;