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

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Date:	Thu, 20 Mar 2014 20:05:39 +0100
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Mark Brown <broonie@...nel.org>
CC:	Stephen Warren <swarren@...dotorg.org>,
	Songhee Baek <sbaek@...dia.com>,
	Arun Shamanna Lakshmi <aruns@...dia.com>,
	"alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
	"tiwai@...e.de" <tiwai@...e.de>,
	"lgirdwood@...il.com" <lgirdwood@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [alsa-devel] [PATCH] ASoC: Add support for multi register mux

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 formatting, references to unpublished documents and so on) but that's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

- Lars

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