linux-kernel - Re: [PATCH v3 1/2] regulator: dt-bindings: pca9450: add PMIC_RST

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Message-ID: <46fe43d0-28be-4acb-b0d4-dacd84fef8e5@kernel.org>
Date: Sat, 13 Apr 2024 23:40:18 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Fabio Aiuto <fabio.aiuto@...icam.com>
Cc: Mark Brown <broonie@...nel.org>, Liam Girdwood <lgirdwood@...il.com>,
 Rob Herring <robh@...nel.org>, Conor Dooley <conor+dt@...nel.org>,
 Robin Gong <yibin.gong@....com>, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, Matteo Lisi <matteo.lisi@...icam.com>,
 Mirko Ardinghi <mirko.ardinghi@...icam.com>
Subject: Re: [PATCH v3 1/2] regulator: dt-bindings: pca9450: add PMIC_RST_B
 warm reset property

On 13/04/2024 19:10, Fabio Aiuto wrote:
> Dear Krzysztof,
> 
> Il Sat, Apr 13, 2024 at 12:58:35PM +0200, Krzysztof Kozlowski ha scritto:
>> On 12/04/2024 09:21, Fabio Aiuto wrote:
>>> Dear Krzysztof,
>>>
>>> Il Thu, Apr 11, 2024 at 09:52:12PM +0200, Krzysztof Kozlowski ha scritto:
>>>> On 11/04/2024 18:58, Fabio Aiuto wrote:
>>>>> Add property to trigger warm reset on PMIC_RST_B assertion
>>>>>
>>>>
>>>> That's rather vague and does not tell me much why this is supposed to be
>>>> board level configuration. It sounds more like a debugging feature:
>>>> during development you want to retain memory contents for pstore etc.
>>>> Then I could imagine this should be turned runtime, e.g. via
>>>> sysfs/debugfs, because for example you want to start inspecting a
>>>> customer's device.
>>>
>>> thanks, I spent too few time writing this commit log and I apologize
>>> for that. I was thinking about something like:
>>>
>>>     The default configuration of the PMIC behavior makes the PMIC
>>>     power cycle most regulators on PMIC_RST_B assertion. This power
>>>     cycling causes the memory contents of OCRAM to be lost.
>>>     Some systems needs some memory that survives reset and
>>>     reboot, therefore add a property to tell PMIC_RST_B is
>>>     wired.
>>>
>>> The actual configuration is made at probe time, anyway we need
>>> to override the default behavior of the pmic to get a warm reset
>>> everytime the PMIC_RST_B pin is asserted and this property tells
>>> us that "something is wired to that pin" and "it has to behave
>>> that way on pin assertion". Our use cases do not meet the need
>>> of further runtime configuration change.
>>
>> What is the use case?
> 
> I just have an external power button connected to that pin, it works
> either with warm reset and cold-reset-except-ldo12. Moreover the default behavior
> is cold reset and not reset-disabled. Anyway I thought it was useful for other
> people to add a property selecting behavior for that pin too as was done for
> WDOG_B. That's why I mainly duplicated the logic. If there is a pin adding a
> reset source it's a good point to provide a way to access the register bits
> related to this signal.

I don't understand what is the use case. You wrote runtime does not
solve your use case. What is the use case?

> 
>>
>> Sorry, you did not bring any further argument why this is board
>> specific. And please don't explain how probing works, but address the
>> problem here: why type of reset is specific to board design. To me it is
>> OS policy.
>>
> 
> Why reset type is specific to board design? I'm sorry but I don't know
> what you mean, as said my intention was to enlarge the number of configurable
> bits in pca9450 register space hoping this would be useful for someone.
> 
> All I can say is that is specific to board design for the same reason the
> wdog_b- reset type was specific to board design.

Specific to board design means different boards have somehow different
configuration/schematics/layout/hardware meaning they need this property
to configure device differently.

I already said it implicitly, but let's reiterate: Devicetree is for
hardware properties, not OS policies.

I also said, so repeating the same argument, the choice how you want to
reboot the system based on button press, sounds like debugging choice
thus runtime suits better.

Unless you want to say there are two signals and you want to configure
them differently? But that's your job to explain it, not mine.

Best regards,
Krzysztof