linux-kernel - Re: [PATCH v2 10/19] ASoC: tlv320aic31xx: Add MICBIAS off setting

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Date:   Fri, 1 Dec 2017 08:56:31 -0600
From:   "Andrew F. Davis" <afd@...com>
To:     Mark Brown <broonie@...nel.org>
CC:     Liam Girdwood <lgirdwood@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Benoît Cousson <bcousson@...libre.com>,
        Tony Lindgren <tony@...mide.com>,
        <alsa-devel@...a-project.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 10/19] ASoC: tlv320aic31xx: Add MICBIAS off setting

On 12/01/2017 07:30 AM, Mark Brown wrote:
> On Wed, Nov 29, 2017 at 03:32:51PM -0600, Andrew F. Davis wrote:
>> Leaving microphone bias off is a valid setting and even used in the DT
>> binding document example. Add this setting here and document the same.
> 
>>  - ai31xx-micbias-vg - MicBias Voltage setting
>> +        0 or MICBIAS_OFF - MICBIAS output is powered off
>>          1 or MICBIAS_2_0V - MICBIAS output is powered to 2.0V
>>          2 or MICBIAS_2_5V - MICBIAS output is powered to 2.5V
>>          3 or MICBIAS_AVDD - MICBIAS output is connected to AVDD
> 
> This doesn't make much sense.  If we enable the microphone with a
> voltage of 0V that's not going to work terribly well - I'd expect this
> property to control the voltage set when it's enabled, not if it's
> enabled.
> 

I agree about this not making much sense, but I'm not sure the right way
forward here.

Personally I wouldn't have put MicBias voltage in DT to begin with, it
is runtime configurable and depends on user runtime inserted microphone,
not really a hardware description :/

The enable/disable is controlled by a DAPM and should be wired to enable
whenever we are recording from the related microphone pin. So then
without MICBIAS_OFF the user is forced to select from the above voltages
whenever recording, which may not be correct if they are using a mic
that doesn't need bias.

The best option I think would be to drop this from DT, but I don't feel
like fighting that battle right now.