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

From: Lars-Peter Clausen
Date: Thu Mar 20 2014 - 15:40:28 EST

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formatting, references to unpublished documents and so on) but that's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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