Re: [PATCH v1 2/2] dt-bindings: ASoC: simple-card: Add system-clock-id property

[Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>]

On 23/10/2022 09:47, Aidan MacDonald wrote:
> 
> Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org> writes:
> 
>> On 22/10/2022 12:27, Aidan MacDonald wrote:
>>> This is a new per-DAI property used to specify the clock ID argument
>>> to snd_soc_dai_set_sysclk().
>>
>> You did no show the use of this property and here you refer to some
>> specific Linux driver implementation, so in total this does no look like
>>  a hardware property.
>>
>> You also did not explain why do you need it (the most important piece of
>> commit msg).
>>
>>>
>>> Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
>>> ---
>>>  Documentation/devicetree/bindings/sound/simple-card.yaml | 8 ++++++++
>>>  1 file changed, 8 insertions(+)
>>>
>>> diff --git a/Documentation/devicetree/bindings/sound/simple-card.yaml b/Documentation/devicetree/bindings/sound/simple-card.yaml
>>> index ed19899bc94b..cb7774e235d0 100644
>>> --- a/Documentation/devicetree/bindings/sound/simple-card.yaml
>>> +++ b/Documentation/devicetree/bindings/sound/simple-card.yaml
>>> @@ -57,6 +57,12 @@ definitions:
>>>        single fixed sampling rate.
>>>      $ref: /schemas/types.yaml#/definitions/flag
>>>
>>> +  system-clock-id:
>>> +    description: |
>>> +      Specify the clock ID used for setting the DAI system clock.
>>
>>
>> With lack of explanation above, I would say - use common clock framework
>> to choose a clock...
>>
>>
>> Best regards,
>> Krzysztof
> 
> Sorry, I didn't explain things very well. The system clock ID is indeed
> a property of the DAI hardware. The ID is not specific to Linux in any
> way, and really it's an enumeration that requires a dt-binding.
> 
> A DAI may support multiple system clock inputs or outputs identified by
> the clock ID. In the case of outputs, these could be distinct clocks
> that have their own I/O pins, or the clock ID could select the internal
> source clock used for a clock generator. For inputs, the system clock ID
> may inform the DAI how or where the system clock is being provided so
> hardware registers can be configured appropriately.
> 
> Really the details do not matter, except that in a particular DAI link
> configuration a specific clock ID must be used. This is determined by
> the actual hardware connection between the DAIs; if the wrong clock is
> used, the DAI may not function correctly.
> 
> Currently the device tree is ambiguous as to which system clock should
> be used when the DAI supports more than one, because there is no way to
> specify which clock was intended. Linux just treats the ID as zero, but
> that's currently a Linux-specific numbering so there's guarantee that
> another OS would choose the same clock as Linux.
> 
> The system-clock-id property is therefore necessary to fully describe
> the hardware connection between DAIs in a DAI link when a DAI offers
> more than one choice of system clock.
> 
> I will resend the patch with the above in the commit message.

For example if you want to define which input pin to use (so you have
internal mux), it's quite unspecific to give them some indexes. What is
0? What is 1? Number of pin? Number of pin counting from where?

Since this is unanswered, the IDs are also driver and implementation
dependent, thus you still have the same problem - another OS can choose
different clock. That's not then a hardware description, but software
configuration.

Best regards,
Krzysztof