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

[Konrad Dybcio]

On 26.09.2023 22:09, Stephan Gerhold wrote:
> 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.
Correct.

> 
> 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.
I hate to be the bearer of bad news, but this is probably more
of a "do we reasonably want this upstream" type question, as
you'll probably get some grumpy emails about upstream not caring
about what's outside the mainline tree..

> 
> Let me try to add some comment first.
Please try to explicitly explain the reasoning of why one would
want this change and what are the drawbacks etc.

Konrad