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

[Stephan Gerhold]

On Tue, Sep 26, 2023 at 09:56:12PM +0200, Konrad Dybcio wrote:
> [...]
> 
> >>> +
> >>>  &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
> 

No problem, I will try to add something simple.

> 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.
> 

Translating from the QDSP6 audio setup to the LPASS one is mostly simple
but not entirely trivial (especially the patching needed for the DAI
links). Main blocker for DTBOs is that you can only add or change, but
AFAIK there is no mechanism to _delete_ or fully recreate nodes.

I guess I could maybe derive this from the QDSP6 definitions using
custom magic code, but the code complexity for that is much higher than
adding these nodes here for completeness.

Let me try to add some comment first.

Thanks,
Stephan