linux-kernel - Re: [PATCH 1/4] gpiolib: add opt-out for existing drivers with static GPIO base

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Message-ID: <449b4618-7d06-49c8-b521-5fdd02a2ba1b@pengutronix.de>
Date: Tue, 21 Jan 2025 11:26:33 +0100
From: Ahmad Fatoum <a.fatoum@...gutronix.de>
To: Kent Gibson <warthog618@...il.com>
Cc: Andy Shevchenko <andy.shevchenko@...il.com>,
 Linus Walleij <linus.walleij@...aro.org>, Bartosz Golaszewski
 <brgl@...ev.pl>, Andy Whitcroft <apw@...onical.com>,
 Joe Perches <joe@...ches.com>, Dwaipayan Ray <dwaipayanray1@...il.com>,
 Lukas Bulwahn <lukas.bulwahn@...il.com>, Fabio Estevam <festevam@...il.com>,
 Shawn Guo <shawnguo@...nel.org>, Sascha Hauer <s.hauer@...gutronix.de>,
 Pengutronix Kernel Team <kernel@...gutronix.de>,
 Dario Binacchi <dario.binacchi@...rulasolutions.com>,
 Haibo Chen <haibo.chen@....com>,
 Catalin Popescu <catalin.popescu@...ca-geosystems.com>,
 linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org,
 imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/4] gpiolib: add opt-out for existing drivers with static
 GPIO base

Hi Kent,

On 15.01.25 14:04, Kent Gibson wrote:
> On Wed, Jan 15, 2025 at 08:07:38AM +0100, Ahmad Fatoum wrote:
>> On 14.01.25 20:38, Andy Shevchenko wrote:
>>> On Tue, Jan 14, 2025 at 12:06 PM Ahmad Fatoum <a.fatoum@...gutronix.de> wrote:
>>>> On 14.01.25 10:49, Andy Shevchenko wrote:
>>>>> On Tue, Jan 14, 2025 at 12:20 AM Ahmad Fatoum <a.fatoum@...gutronix.de> wrote:
>>>>>>
>>>>>> Some drivers have had deterministic GPIO numbering for most of
>>>>>> their existence, e.g. the i.MX GPIO since commit 7e6086d9e54a
>>>>>> ("gpio/mxc: specify gpio base for device tree probe"), more than
>>>>>> 12 years ago.
>>>>>>
>>>>>> Reverting this to dynamically numbered will break existing setups in
>>>>>> the worst manner possible: The build will succeed, the kernel will not
>>>>>> print warnings, but users will find their devices essentially toggling
>>>>>> GPIOs at random with the potential of permanent damage.
>>>>>>
>>>>>> As these concerns won't go away until the sysfs interface is removed,
>>>>>> let's add a new struct gpio_chip::legacy_static_base member that can be
>>>>>> used by existing drivers that have been grandfathered in to suppress
>>>>>> the warning currently being printed:
>>>>>>
>>>>>>   gpio gpiochip0: Static allocation of GPIO base is deprecated,
>>>>>>   use dynamic allocation.
>>>>>
>>>>> Warning is harmless and still a good reminder for the stuff that needs
>>>>> more love.
>>>>> NAK.
>>>>
>>>> A warning is a call-to-action and it's counterproductive to keep tricking
>>>> people into removing the static base and breaking other users' scripts.
>>>
>>> Are you prepared to say the same when the entire GPIO SYSFS will be
>>> removed? Because that's exactly what I referred to in the reply to the
>>> cover letter as an impediment to move forward.
>>
>> No. But this gives me an idea: We could make the warning dependent
>> on CONFIG_GPIO_SYSFS and add a comment to the i.MX code suggesting
>> users do that instead. What do you think?
>>
> 
> AIUI, the purpose of the warning is to remind driver authors, not end users,
> to update their drivers, as the old behaviour is deprecated. That is
> independent of GPIO SYSFS - that just happens to be something that makes the
> change visible to userspace.
> 
> Rather than making the warning conditional, how about making the fix for the
> warning in your driver, so switching to dynamic allocation, conditional on
> CONFIG_GPIO_SYSFS not being set?
> That would provide a path forward for users that want to dispense with
> the warning - as long as they dispense with GPIO SYSFS.

That could work for gpio-mxc, provided that SysFS is the only user for which
the static base matters. I assume that's the case, but I am not sure.

An argument for suppressing the warning selectively in the GPIO core is that
this doesn't only affect gpio-mxc, but also e.g. gpio-zynq or gpio-mxs.

Cheers,
Ahmad



> 
> Cheers,
> Kent.
> 


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