Messages in this thread | | | From | Linus Walleij <> | Date | Thu, 18 Aug 2022 14:25:39 +0200 | Subject | Re: [PATCH] gpio: Allow user to customise maximum number of GPIOs |
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On Thu, Aug 18, 2022 at 1:33 PM Arnd Bergmann <arnd@arndb.de> wrote: > On Thu, Aug 18, 2022 at 1:13 PM Linus Walleij <linus.walleij@linaro.org> wrote:
> > static inline bool gpio_is_valid(int number) > > { > > return number >= 0 && number < ARCH_NR_GPIOS; > > } > > > > ? > > > > If using GPIO descriptors, any descriptor != NULL is valid, > > this one is just used with legacy GPIOs. Maybe we should just > > delete gpio_is_valid() everywhere and then drop the cap? > > I think it makes sense to keep gpio_is_valid() for as long as we > support the numbers.
Hmmm....
> > I think there may be systems and users that still depend on GPIO base > > numbers being assigned from ARCH_NR_GPIOS and > > downwards (userspace GPIO numbers in sysfs will also change...) > > otherwise we could assign from 0 and up. > > Is it possible to find in-kernel users that depend on well-known > numbers for dynamically assigned gpios? I would argue > that those are always broken.
Most in-kernel users hard-code the base to something like 0 etc it's only the ones that code -1 into .base that are in trouble because that will activate dynamic assignment for the base.
git grep 'base = -1' yields these suspects:
arch/arm/common/sa1111.c: sachip->gc.base = -1; arch/arm/common/scoop.c: devptr->gpio.base = -1; arch/powerpc/platforms/52xx/mpc52xx_gpt.c: gpt->gc.base = -1; arch/powerpc/platforms/83xx/mcu_mpc8349emitx.c: gc->base = -1;
That's all! We could just calculate these to 512-ngpios and hardcode that instead.
> Even for the sysfs interface, it is questionable to rely on > specific numbers because at least in an arm multiplatform > kernel the top number changes based on kernel configuration.
Yeah :/ still these users tend to angrily report any breakage due to expected (fragile) behaviour.
> > Right now the safest would be: > > Assign from 512 and downwards until we hit 0 then assign > > from something high, like U32_MAX and downward. > > > > That requires dropping gpio_is_valid() everywhere. > > > > If we wanna be bold, just delete gpio_is_valid() and assign > > bases from 0 and see what happens. But I think that will > > lead to regressions. > > I'm still unsure how removing gpio_is_valid() would help.
If we allow GPIO base all the way to U32_MAX this function becomes:
static inline bool gpio_is_valid(int number) { return number >= 0 && number < U32_MAX; }
and we can then just
#define gpio_is_valid true
and in that case it is better to delete the use of this function altogether since it can not fail.
> What I could imagine as a next step would be to mark all > consumer drivers and the sysfs interface that use gpio > numbers as 'depends on GPIO_LEGACY' and then only > provide the corresponding drivers if that option is set.
Hm I wonder what Bartosz and Alexandre Courbot and thinks about a GPIO_LEGACY symbol to phase out the global GPIO numberspace. I kind of like the idea.
I made the sysfs depend on CONFIG_EXPERT to at least make it less accessible and not provide users with guns to shoot themselves in the foot.
Yours, Linus Walleij
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