Re: [PATCH 13/25] ASoC: dt-bindings: meson: axg-pdm: document 'sysrate' property

[Jerome Brunet <jbrunet@xxxxxxxxxxxx>]

On Mon 18 Mar 2024 at 11:55, Jerome Brunet <jbrunet@xxxxxxxxxxxx> wrote:

> On Sun 17 Mar 2024 at 18:52, Jan Dakinevich <jan.dakinevich@xxxxxxxxxxxxxxxxx> wrote:
>
>> On 3/15/24 13:22, Jerome Brunet wrote:
>>> 
>>> On Fri 15 Mar 2024 at 11:00, Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxx> wrote:
>>> 
>>>> On 15/03/2024 00:21, Jan Dakinevich wrote:
>>>>> This option allow to redefine the rate of DSP system clock.
>>>>
>>>> And why is it suitable for bindings? Describe the hardware, not what you
>>>> want to do in the driver.
>>>>
>>>>>
>>>>> Signed-off-by: Jan Dakinevich <jan.dakinevich@xxxxxxxxxxxxxxxxx>
>>>>> ---
>>>>>  Documentation/devicetree/bindings/sound/amlogic,axg-pdm.yaml | 4 ++++
>>>>>  1 file changed, 4 insertions(+)
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/sound/amlogic,axg-pdm.yaml b/Documentation/devicetree/bindings/sound/amlogic,axg-pdm.yaml
>>>>> index df21dd72fc65..d2f23a59a6b6 100644
>>>>> --- a/Documentation/devicetree/bindings/sound/amlogic,axg-pdm.yaml
>>>>> +++ b/Documentation/devicetree/bindings/sound/amlogic,axg-pdm.yaml
>>>>> @@ -40,6 +40,10 @@ properties:
>>>>>    resets:
>>>>>      maxItems: 1
>>>>>  
>>>>> +  sysrate:
>>>>> +    $ref: /schemas/types.yaml#/definitions/uint32
>>>>> +    description: redefine rate of DSP system clock
>>>>
>>>> No vendor prefix, so is it a generic property? Also, missing unit
>>>> suffix, but more importantly I don't understand why this is a property
>>>> of hardware.
>>> 
>>> +1.
>>> 
>>> The appropriate way to set rate of the clock before the driver take over
>>> is 'assigned-rate', if you need to customize this for different
>>> platform.
>>> 
>>
>> It would be great, but it doesn't work. Below, is what I want to see:
>>
>> 	assigned-clocks =
>> 		<&clkc_audio AUD2_CLKID_PDM_SYSCLK_SEL>,
>> 		<&clkc_audio AUD2_CLKID_PDM_SYSCLK_DIV>;
>> 	assigned-clock-parents =
>> 		<&clkc_pll CLKID_FCLK_DIV3>,
>> 		<0>;
>> 	assigned-clock-rates =
>> 		<0>,
>> 		<256000000>;
>>
>> But regardles of this declaration, PDM's driver unconditionally sets
>> sysclk'rate to 250MHz and throws away everything that was configured
>> before, reparents audio2_pdm_sysclk_mux to hifi_pll and changes
>> hifi_pll's rate.
>>
>> This value 250MHz is declared here:
>>
>> static const struct axg_pdm_cfg axg_pdm_config = {
>> 	.filters = &axg_default_filters,
>> 	.sys_rate = 250000000,
>> };
>>
>> The property 'sysrate' is intended to redefine hardcoded 'sys_rate'
>> value in 'axg_pdm_config'.
>
> What is stopping you from removing that from the driver and adding
> assigned-rate to 250M is the existing platform ?

... Also, considering how PDM does work, I'm not sure I get the point of
the doing all this to go from 250MHz to 256Mhz.

PDM value is sampled at ~75% of the half period. That clock basically
feeds a counter and the threshold is adjusted based on the clock rate.

So there is no need to change the rate. Changing it is only necessary
when the captured audio rate is extremely slow (<8kHz) and the counter
may overflow. The driver already adjust this automatically.

So changing the input rate from 250MHz to 256MHz should not make any
difference.

>
>>
>>> Then you don't have to deal with it in the device driver.
>>> 
>>>>
>>>> Best regards,
>>>> Krzysztof
>>> 
>>> 


-- 
Jerome