On Tue, Sep 2, 2025 at 4:46 PM Andy Shevchenko
<andriy.shevchenko@intel.com> wrote:
>
> 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.
>
I'm not sure how to rephrase it. Strict pinmuxers are already a thing,
but on many platforms it's impossible to use them BECAUSE pinctrl
doesn't care about what a function does semantically. It just so
happens that some functions are GPIOs and as such can also be used by
GPIOLIB. Except that if the pinmuxer is "strict", any gpiod_get() call
will fail BECAUSE pinctrl does not know that a function called "gpio"
is actually a GPIO and will say NO if anything tries to request a
muxed pin. This (the function name) is just a string, it could as well
be called "andy" for all pinctrl cares. This is why we're doing it at
the pinctrl core level - because it will benefit many other platforms
as Linus mentioned elsewhere - he has some other platforms lined up
for a similar conversion. And also because it cannot be done at the
driver level at the moment, it's the pinctrl core that says "NO" to
GPIOLIB. I think you missed the entire point of this series.
Bartosz