linux-kernel - Re: [PATCH 04/13] arm64: dts: qcom: msm8916-samsung-a2015: Add sound and modem

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Date:   Tue, 26 Sep 2023 21:56:12 +0200
From:   Konrad Dybcio <konrad.dybcio@...aro.org>
To:     Stephan Gerhold <stephan@...hold.net>
Cc:     Bjorn Andersson <andersson@...nel.org>,
        Andy Gross <agross@...nel.org>, linux-arm-msm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        phone-devel@...r.kernel.org, ~postmarketos/upstreaming@...ts.sr.ht,
        Bryan O'Donoghue <bryan.odonoghue@...aro.org>,
        "Lin, Meng-Bo" <linmengbo0689@...tonmail.com>
Subject: Re: [PATCH 04/13] arm64: dts: qcom: msm8916-samsung-a2015: Add sound
 and modem

[...]

>>> +
>>>  &blsp_i2c2 {
>>>  	status = "okay";
>>>  
>>> @@ -243,6 +258,13 @@ &gpu {
>>>  	status = "okay";
>>>  };
>>>  
>>> +&lpass {
>>> +	dai-link@3 {
>>> +		reg = <MI2S_QUATERNARY>;
>>> +		qcom,playback-sd-lines = <1>;
>>> +	};
>>> +};
>> Is that not status = reserved?
>>
> 
> Correct. This is here to simplify switching to the modem-bypass audio
> routing if someone does not need (or want) the modem. The direct audio
> path with the LPASS drivers tends to be more reliable and configurable
> (especially wrt bit formats, sampling rates, latency, channels etc).
> I know that at some point this helped someone who tried to use an old
> phone as some kind of portable musical instrument / synthesizer.
> 
> It's not too obvious that these definitions would be needed when making
> those changes (because devices using the standard SD lines (i.e <0>) do
> not need it). If you forget about this you get non-functional audio with
> no error or any hint what could be wrong.
> 
> To simplify switching between the different audio routing options, the
> lk2nd bootloader actually has an option to do this transformation in the
> DTB automagically during boot. It's sort of like a DTB overlay that
> disables the QDSP6 audio path and enables this node instead. The DAI
> links are also adjusted where necessary.
> 
> Do you think a comment would help here?
I'd say a comment would be necessary here :D

While I understand this use-case, I believe this is better suited
for an actual DTBO or something entirely kept inside lk2nd.
Otherwise this looks very confusing to an outside reader.

Konrad