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

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Message-ID: <Z1ATxAsXFhQraQwH@linaro.org>
Date: Wed, 4 Dec 2024 09:33:08 +0100
From: Stephan Gerhold <stephan.gerhold@...aro.org>
To: 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 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.

Thanks,
Stephan