linux-kernel - Re: [PATCH 2/2] arm64: dts: qcom: x1e80100-crd: Drop duplicate DMIC supplies

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Message-ID: <afd010c9-8c24-482e-a479-2396f08c972b@oss.qualcomm.com>
Date: Thu, 5 Dec 2024 18:11:47 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Stephan Gerhold <stephan.gerhold@...aro.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc: Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>, Rob Herring <robh@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Abel Vesa <abel.vesa@...aro.org>,
        Johan Hovold <johan@...nel.org>
Subject: Re: [PATCH 2/2] arm64: dts: qcom: x1e80100-crd: Drop duplicate DMIC
 supplies

On 4.12.2024 9:33 AM, Stephan Gerhold wrote:
> On Wed, Dec 04, 2024 at 08:20:15AM +0100, Krzysztof Kozlowski wrote:
>> On 03/12/2024 18:44, Stephan Gerhold wrote:
>>> The WCD938x codec provides two controls for each of the MIC_BIASn outputs:
>>>
>>>  - "MIC BIASn" enables an internal regulator to generate the output
>>>    with a configurable voltage (qcom,micbiasN-microvolt).
>>>
>>>  - "VA MIC BIASn" enables "pull-up mode" that bypasses the internal
>>>    regulator and directly outputs fixed 1.8V from the VDD_PX pin.
>>>    This is intended for low-power VA (voice activation) use cases.
>>>
>>> The audio-routing setup for the X1E80100 CRD currently specifies both
>>> as power supplies for the DMICs, but only one of them can be active
>>> at the same time. In practice, only the internal regulator is used
>>> with the current setup because the driver prefers it over pull-up mode.
>>>
>>> Make this more clear by dropping the redundant routes to the pull-up
>>> "VA MIC BIASn" supply. There is no functional difference except that we
>>> skip briefly switching to pull-up mode when shutting down the microphone.
>>>
>>> Fixes: 4442a67eedc1 ("arm64: dts: qcom: x1e80100-crd: add sound card")
>>
>> If there is no functional difference and this is just redundant, then
>> there is nothing to fix, so drop the tag. But the point is that users
>> might want the low-power VA. You claim they don't want... sure, I am
>> fine with that but there is nothing to fix in such case.
>>
> 
> The fix here is that two mutually exclusive power supplies for the DMIC
> are specified in the device tree. You can only have one of them active
> at the same time. The Linux driver handles that gracefully, but the
> device tree is still wrong and IMO deserves a fixes tag.
> 
> The functional difference is that we skip briefly switching to pull-up
> mode when shutting down the microphone. Users won't notice that, but
> it's not the intended behavior.
> 
> I don't claim that users don't want to switch to the low-power pull-up
> mode (VA MIC BIASn). However, we would need a different mechanism to
> give them the option to switch at runtime. "audio-routing" just
> specifies static routes, so the current description does not allow
> switching between the two modes either.

Is there no existing mechanism to alter this at runtime?

Konrad