linux-kernel - Re: [PATCH 2/3] dt-bindings: pwm: adi,axi-pwmgen: add external clock

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Message-ID: <1424236a-eee7-4343-b73f-42cd416b594c@baylibre.com>
Date: Wed, 21 May 2025 08:50:12 -0500
From: David Lechner <dlechner@...libre.com>
To: Krzysztof Kozlowski <krzk@...nel.org>
Cc: Michael Hennerich <michael.hennerich@...log.com>,
 Nuno Sá <nuno.sa@...log.com>,
 Trevor Gamblin <tgamblin@...libre.com>,
 Uwe Kleine-König <ukleinek@...nel.org>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, linux-pwm@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] dt-bindings: pwm: adi,axi-pwmgen: add external clock

On 5/21/25 8:28 AM, Krzysztof Kozlowski wrote:
> On 21/05/2025 15:14, David Lechner wrote:
>> On 5/21/25 5:09 AM, Krzysztof Kozlowski wrote:
>>> On Tue, May 20, 2025 at 04:00:45PM GMT, David Lechner wrote:
>>>> Add external clock to the schema.
>>>>
>>>> The AXI PWMGEN IP block has a compile option ASYNC_CLK_EN that allows
>>>> the use of an external clock for the PWM output separate from the AXI
>>>> clock that runs the peripheral.
>>>>
>>>> In these cases, we should specify both clocks in the device tree. The
>>>> intention here is that if you specify both clocks, then you include the
>>>> clock-names property and if you don't have an external clock, then you
>>>> omit the clock-names property.
>>>>
>>>> There can't be more than one allOf: in the top level of the schema, so
>>>> it is stolen from $ref since it isn't needed there and used for the
>>>> more typical case of the if statement (even though technically it isn't
>>>> needed there either at this time).
>>>>
>>>> Signed-off-by: David Lechner <dlechner@...libre.com>
>>>> ---
>>>>  .../devicetree/bindings/pwm/adi,axi-pwmgen.yaml    | 26 ++++++++++++++++++----
>>>>  1 file changed, 22 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/pwm/adi,axi-pwmgen.yaml b/Documentation/devicetree/bindings/pwm/adi,axi-pwmgen.yaml
>>>> index bc44381692054f647a160a6573dae4cff2ee3f31..90f702a5cd80bd7d62e2436b2eed44314ab4fd53 100644
>>>> --- a/Documentation/devicetree/bindings/pwm/adi,axi-pwmgen.yaml
>>>> +++ b/Documentation/devicetree/bindings/pwm/adi,axi-pwmgen.yaml
>>>> @@ -16,8 +16,7 @@ description:
>>>>  
>>>>    https://analogdevicesinc.github.io/hdl/library/axi_pwm_gen/index.html
>>>>  
>>>> -allOf:
>>>> -  - $ref: pwm.yaml#
>>>> +$ref: pwm.yaml#
>>>>  
>>>>  properties:
>>>>    compatible:
>>>> @@ -30,7 +29,13 @@ properties:
>>>>      const: 3
>>>>  
>>>>    clocks:
>>>> -    maxItems: 1
>>>> +    minItems: 1
>>>> +    maxItems: 2
>>>> +
>>>> +  clock-names:
>>>> +    items:
>>>> +      - const: axi
>>>> +      - const: ext
>>>>  
>>>>  required:
>>>>    - reg
>>>> @@ -38,11 +43,24 @@ required:
>>>>  
>>>>  unevaluatedProperties: false
>>>>  
>>>> +allOf:
>>>> +  - if:
>>>> +      required: [clock-names]
>>>
>>>
>>> No, don't do that. If you want clock-names, just add them for both
>>> cases. Otherwise, just describe items in clocks and no need for
>>> clock-names.
>>
>> Would it be OK then to make clock-names required and just let the
>> driver still handle one clocks, no clock-names for backwards compatibility?
> 
> So just don't make it required.
> 
>>
>>>
>>>
>>>
>>>> +    then:
>>>> +      properties:
>>>> +        clocks:
>>>> +          minItems: 2
>>>> +    else:
>>>> +      properties:
>>>> +        clocks:
>>>> +          maxItems: 1
>>>> +
>>>>  examples:
>>>>    - |
>>>>      pwm@...00000 {
>>>>          compatible = "adi,axi-pwmgen-2.00.a";
>>>>          reg = <0x44b00000 0x1000>;
>>>> -        clocks = <&spi_clk>;
>>>> +        clocks = <&fpga_clk>, <&spi_clk>;
>>>
>>> What was the clock[0] before? Axi, right, so SPI_CLK. Now FPGA is the
>>> AXI_CLK? This feels like clock order reversed.
>>
>> The problem being fixed here is that since there was only one clock in
>> the binding, existing .dts files have either have the spi_clock or
>> the FPGA/AXI clock. So the one clock could be either and there are
>> existing .dtbs out in the world with both cases.
> 
> No problem like that was explained in commit msg. Nevertheless driver

You are right. I explained it in the cover letter, but failed to repeat
that in this commit message.

> assumed the first clock is the SPI, didn't it? So that's your ABI, even
> if binding was not conclusive here.

Not quite. The driver (before this series) assumes that the clock
is the SPI clock ("ext") if the HDL was compiled with ASYNC_CLK_EN=1
or the AXI clock if the HDL was compiled with ASYNC_CLK_EN=0 (in this
case, there is no "ext"/SPI clock).

>>
>> But we could consider reversing this so that if someone uses the new
>> bindings with an old kernel, then it would still work.
> 
> You cannot use new bindings with old kernel. How would that work? Put
> YAML file there? Nothing would change.
> 
> Binding is supposed to be complete for exactly this reason. You cannot
> change it afterwards without breaking users.
> 
> Best regards,
> Krzysztof