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

[Conor Dooley <conor@kernel.org>]

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On Thu, Dec 19, 2024 at 09:14:10PM +0100, Ahmad Fatoum wrote:
> 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.

Apologies for the delay responding to you, doing it today cos I feel
guilty! I think what you've explained here is fine, but could you add a
bit more of that info to the commit message, explaining why one cannot
be default? With that,
Acked-by: Conor Dooley <conor.dooley@microchip.com>

Cheers,
Conor.

> 
> 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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