Re: [PATCH v7 00/16] pinctrl: introduce the concept of a GPIO pin function category

[Andy Shevchenko <andriy.shevchenko@intel.com>]

On Tue, Sep 02, 2025 at 01:59:09PM +0200, Bartosz Golaszewski wrote:
> Problem: when pinctrl core binds pins to a consumer device and the
> pinmux ops of the underlying driver are marked as strict, the pin in
> question can no longer be requested as a GPIO using the GPIO descriptor
> API. It will result in the following error:
> 
> [    5.095688] sc8280xp-tlmm f100000.pinctrl: pin GPIO_25 already requested by regulator-edp-3p3; cannot claim for f100000.pinctrl:570
> [    5.107822] sc8280xp-tlmm f100000.pinctrl: error -EINVAL: pin-25 (f100000.pinctrl:570)
> 
> This typically makes sense except when the pins are muxed to a function
> that actually says "GPIO". Of course, the function name is just a string
> so it has no meaning to the pinctrl subsystem.
> 
> We have many Qualcomm SoCs (and I can imagine it's a common pattern in
> other platforms as well) where we mux a pin to "gpio" function using the
> `pinctrl-X` property in order to configure bias or drive-strength and
> then access it using the gpiod API. This makes it impossible to mark the
> pin controller module as "strict".
> 
> This series proposes to introduce a concept of a sub-category of
> pinfunctions: GPIO functions where the above is not true and the pin
> muxed as a GPIO can still be accessed via the GPIO consumer API even for
> strict pinmuxers.
> 
> To that end: we first clean up the drivers that use struct function_desc
> and make them use the smaller struct pinfunction instead - which is the
> correct structure for drivers to describe their pin functions with. We
> also rework pinmux core to not duplicate memory used to store the
> pinfunctions unless they're allocated dynamically.
> 
> First: provide the kmemdup_const() helper which only duplicates memory
> if it's not in the .rodata section. Then rework all pinctrl drivers that
> instantiate objects of type struct function_desc as they should only be
> created by pinmux core. Next constify the return value of the accessor
> used to expose these structures to users and finally convert the
> pinfunction object within struct function_desc to a pointer and use
> kmemdup_const() to assign it. With this done proceed to add
> infrastructure for the GPIO pin function category and use it in Qualcomm
> drivers. At the very end: make the Qualcomm pinmuxer strict.

I read all this and do not understand why we take all this way,
Esp. see my Q in patch 16. Can we rather limit this to the controller
driver to decide and have it handle all the possible configurations,
muxing, etc?

I think what we are trying to do here is to delegate part of the
driver's work pin mux / pin control core. While it sounds like right
direction the implementation (design wise) seems to me unscalable.

In any case first 12 patch (in case they are not regressing) are good
to go as soon as they can. I like the part of constification.

-- 
With Best Regards,
Andy Shevchenko