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

[Jan Dakinevich <jan.dakinevich@xxxxxxxxxxxxxxxxx>]

On 3/18/24 15:19, Jerome Brunet wrote:
> 
> 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.
> 

The point is to use fclk_div3 clock as source for PDM's sysclock and
keep hiff_pll clock free for TDM. Because, I can get 256MHz from any
hifi_pll and fclk_div3, but only hifi_pll is able to provide accurate
48kHz (after several divider).

> 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.
> 

Thank you for the explanation.

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

-- 
Best regards
Jan Dakinevich