Re: [PATCH 1/6] dt-bindings: clock: imx8m: document nominal/overdrive properties

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Conor,

On 19.12.24 20:49, Conor Dooley wrote:
> On Thu, Dec 19, 2024 at 08:27:32AM +0100, Ahmad Fatoum wrote:
>> The imx8m-clock.yaml binding covers the clock controller inside all
>> of the i.MX8M Q/M/N/P SoCs. All of them have in common that they
>> support two operating modes: nominal and overdrive mode.
> 
> This implies that only the two modes you mention are possible, but you
> leave the option open to a dts author to use either. How come?
> 
> Makes it seem like we only need one of these, for whatever the
> non-default option is?

There is no real default. The mode is configured implicitly by the
bootloader setting VDD_SOC and then kernel needs to adhere to the
limits that imposes.

For i.MX8M Mini and Nano, the kernel SoC DTSIs has assigned-clock-rates
that are all achievable in nominal mode. For i.MX8MP, there are some
rates only validated for overdrive mode.

But even for the i.MX8M Mini/Nano boards, we don't know what rates they
may configure at runtime, so it's not possible to infer from just the
device tree what the mode is, which is why I need to allow for absence
of either property. I can make it a single property with two possible
values though if that's preferable.

Theoretically, we could infer mode at runtime from VDD_SOC voltage,
but we need to set up clocks to read out the PMIC and I want to
apply the constraints as early as possible as I don't want the SoC
to run outside of spec even for a short while.

Thanks,
Ahmad

> 
>>
>> While the overdrive mode allows for higher frequencies for many IPs,
>> the nominal mode needs a lower SoC voltage, thereby reducing
>> heat generation and power usage.
>>
>> In any case, software should respect the maximum clock rate limits
>> described in the datasheet for each of the two operating modes.
>>
>> To allow device tree consumers to enforce these limits, document two new
>> optional properties that can be used to sanity check the clock tree.
>>
>> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
>> ---
>>  Documentation/devicetree/bindings/clock/imx8m-clock.yaml | 14 ++++++++++++++
>>  1 file changed, 14 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/clock/imx8m-clock.yaml b/Documentation/devicetree/bindings/clock/imx8m-clock.yaml
>> index c643d4a81478..a6ae5257ef53 100644
>> --- a/Documentation/devicetree/bindings/clock/imx8m-clock.yaml
>> +++ b/Documentation/devicetree/bindings/clock/imx8m-clock.yaml
>> @@ -43,6 +43,14 @@ properties:
>>        ID in its "clocks" phandle cell. See include/dt-bindings/clock/imx8m-clock.h
>>        for the full list of i.MX8M clock IDs.
>>  
>> +  fsl,nominal-mode:
>> +    description: Set if SoC is operated in nominal mode
>> +    $ref: /schemas/types.yaml#/definitions/flag
>> +
>> +  fsl,overdrive-mode:
>> +    description: Set if SoC is operated in overdrive mode
>> +    $ref: /schemas/types.yaml#/definitions/flag
>> +
>>  required:
>>    - compatible
>>    - reg
>> @@ -95,6 +103,12 @@ allOf:
>>              - const: clk_ext2
>>              - const: clk_ext3
>>              - const: clk_ext4
>> +  - if:
>> +      required:
>> +        - fsl,overdrive-mode
>> +    then:
>> +      properties:
>> +        fsl,nominal-mode: false
>>  
>>  additionalProperties: false
>>  
>>
>> -- 
>> 2.39.5
>>


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