linux-kernel - Re: [PATCH v2 2/7] dt-bindings: clock: meson: a1: pll: introduce new syspll bindings

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Message-ID: <1jjzjxzy4d.fsf@starbuckisacylon.baylibre.com>
Date: Mon, 13 May 2024 14:04:41 +0200
From: Jerome Brunet <jbrunet@...libre.com>
To: Conor Dooley <conor@...nel.org>
Cc: Dmitry Rokosov <ddrokosov@...utedevices.com>, neil.armstrong@...aro.org,
 jbrunet@...libre.com, mturquette@...libre.com, sboyd@...nel.org,
 robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
 khilman@...libre.com, martin.blumenstingl@...glemail.com,
 jian.hu@...ogic.com, kernel@...rdevices.ru, rockosov@...il.com,
 linux-amlogic@...ts.infradead.org, linux-clk@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 2/7] dt-bindings: clock: meson: a1: pll: introduce
 new syspll bindings


On Sat 11 May 2024 at 14:08, Conor Dooley <conor@...nel.org> wrote:

> [[PGP Signed Part:Undecided]]
> On Fri, May 10, 2024 at 12:08:54PM +0300, Dmitry Rokosov wrote:
>> The 'syspll' PLL is a general-purpose PLL designed specifically for the
>> CPU clock. It is capable of producing output frequencies within the
>> range of 768MHz to 1536MHz.
>> 
>> The clock source sys_pll_div16, being one of the GEN clock parents,
>> plays a crucial role and cannot be tagged as "optional". Unfortunately,
>> it was not implemented earlier due to the cpu clock ctrl driver's
>> pending status on the TODO list.
>
> It's fine to not mark it optional in the binding, but it should be
> optional in the driver as otherwise backwards compatibility will be
> broken. Given this is an integral clock driver, sounds like it would
> quite likely break booting on these devices if the driver doesn't treat
> syspll_in as optional.
> A lesson perhaps in describing the hardware entirely, even if the
> drivers don't make use of all the information yet?

That is nice but it is only possible if/when we have perfect knowledge
of the HW being implemented. I don't know about you, but I rarely get
perfect documentation for HW, let alone a public one.

Those things are bound to happen as we implement support for the HW and
discover how it works, not to mention the mistakes humans will
inevitably do. If Linux was only supporting perfectly documented HW, it
would not be supporting much of them I suspect.

Stable API is already hard with ioctl but there, both sides are
perfectly known. That is a fundamental difference with the 'DT ABI'

Getting it right on day 1, every time - because things are set in stone
afterwards - is unrealistic. As a maintainer, I do spend a
disproportionate amount of time checking the bindings submission because
I know how painful it gets to fix things up down the line.

Unless I missed the simple solution to this problem, we can expect the
problem keep happening again and again, no matter the number of lessons
learned.

>
> Cheers,
> Conor.
>
>> 
>> Signed-off-by: Dmitry Rokosov <ddrokosov@...utedevices.com>
>> ---
>>  .../devicetree/bindings/clock/amlogic,a1-pll-clkc.yaml     | 7 +++++--
>>  include/dt-bindings/clock/amlogic,a1-pll-clkc.h            | 2 ++
>>  2 files changed, 7 insertions(+), 2 deletions(-)
>> 
>> diff --git a/Documentation/devicetree/bindings/clock/amlogic,a1-pll-clkc.yaml b/Documentation/devicetree/bindings/clock/amlogic,a1-pll-clkc.yaml
>> index a59b188a8bf5..fbba57031278 100644
>> --- a/Documentation/devicetree/bindings/clock/amlogic,a1-pll-clkc.yaml
>> +++ b/Documentation/devicetree/bindings/clock/amlogic,a1-pll-clkc.yaml
>> @@ -26,11 +26,13 @@ properties:
>>      items:
>>        - description: input fixpll_in
>>        - description: input hifipll_in
>> +      - description: input syspll_in
>>  
>>    clock-names:
>>      items:
>>        - const: fixpll_in
>>        - const: hifipll_in
>> +      - const: syspll_in
>>  
>>  required:
>>    - compatible
>> @@ -53,7 +55,8 @@ examples:
>>              reg = <0 0x7c80 0 0x18c>;
>>              #clock-cells = <1>;
>>              clocks = <&clkc_periphs CLKID_FIXPLL_IN>,
>> -                     <&clkc_periphs CLKID_HIFIPLL_IN>;
>> -            clock-names = "fixpll_in", "hifipll_in";
>> +                     <&clkc_periphs CLKID_HIFIPLL_IN>,
>> +                     <&clkc_periphs CLKID_SYSPLL_IN>;
>> +            clock-names = "fixpll_in", "hifipll_in", "syspll_in";
>>          };
>>      };
>> diff --git a/include/dt-bindings/clock/amlogic,a1-pll-clkc.h b/include/dt-bindings/clock/amlogic,a1-pll-clkc.h
>> index 2b660c0f2c9f..a702d610589c 100644
>> --- a/include/dt-bindings/clock/amlogic,a1-pll-clkc.h
>> +++ b/include/dt-bindings/clock/amlogic,a1-pll-clkc.h
>> @@ -21,5 +21,7 @@
>>  #define CLKID_FCLK_DIV5		8
>>  #define CLKID_FCLK_DIV7		9
>>  #define CLKID_HIFI_PLL		10
>> +#define CLKID_SYS_PLL		11
>> +#define CLKID_SYS_PLL_DIV16	12
>>  
>>  #endif /* __A1_PLL_CLKC_H */
>> -- 
>> 2.43.0
>> 
>> 
>
> [[End of PGP Signed Part]]


-- 
Jerome