Re: [alsa-devel] [PATCH] ASoC: Add support for multi register mux

[Lars-Peter Clausen]

On 03/20/2014 08:05 PM, Lars-Peter Clausen wrote:
> On 03/20/2014 07:36 PM, Mark Brown wrote:
> > On Thu, Mar 20, 2014 at 12:20:17PM -0600, Stephen Warren wrote:
> > > On 03/20/2014 05:48 AM, Mark Brown wrote:
> > > > On Wed, Mar 19, 2014 at 04:44:00PM -0700, Arun Shamanna Lakshmi wrote:
> >
> > > > > If each bit of a 32 bit register maps to an input of a mux, then with
> > > > > the current 'soc_enum' structure we cannot have more than 64 inputs
> > > > > for the mux (because of reg and reg2 only).
> >
> > > > What makes you say that?  We currently have devices in mainline which
> > > > have well over 32 inputs to muxes.
> >
> > > I think their register layout is different.
> >
> > > I found a number of large muxes where the register stores a 'integer'
> > > indicating which mux input to select, e.g. Arizona, WM2200, etc. In this
> > > case, an N-bit register could support up to 2^N inputs.
> >
> > > However, the registers in the Tegra AHUB use 1 bit position per input,
> > > and require you to set one single bit at a time. Hence, an N bit
> > > register (or string of registers) can support up to N inputs. In more
> > > recent Tegra chips, we have at least >32 inputs and I think Arun was
> > > saying even >64 inputs. That requires 2 or 3 or more .reg fields in
> > > struct soc_enum.
> >
> > Right, that was my guess too (the mail wasn't terribly clear with the
> > formatting, references to unpublished documents and so on) but that's
> > not a straight mux, it's a value mux, and the limit with the current
> > code is much lower on 32 bit systems (like at least some of the K1s)
> > since muxes only use one of the current register fields.
>
> It might make sense to add special code for supported muxes with a one-hot
> encoding instead of using a value mux. Having an large array where each
> entry is just 1<<n is a bit ugly in my opinion, especially if the value
> needs to be able to be larger than 2**64. But anyway the patch that modifies
> the soc_enum struct should also add the code that makes use of the new
> struct layout.

On the other hand this can actually already be implemented without any core 
changes by using a virtual mux and connecting a supply widget to each input 
which sets the bit for that input. DAPM will take care that only one of the 
supply widgets is enabled at a time.

- Lars

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