On Thu, Aug 18, 2022 at 2:46 PM Arnd Bergmann [off-list ref] wrote:

On Thu, Aug 18, 2022 at 2:25 PM Linus Walleij [off-list ref] wrote:

quoted

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.

How do the consumers find the numbers for these four?

For SA1111 the chip gets named "sa1111" and some consumers actually
use proper machine descriptions, maybe all?

arch/arm/mach-sa1100/jornada720.c:              GPIO_LOOKUP("sa1111",
0, "s0-power", GPIO_ACTIVE_HIGH),
arch/arm/mach-sa1100/jornada720.c:              GPIO_LOOKUP("sa1111",
1, "s1-power", GPIO_ACTIVE_HIGH),
(...)

For Scoop it is conditionally overridden in the code. I guess always
overridden.

For powerpc these seem to be using (old but working) device tree
lookups, so should not be an issue.

Sadly I'm not 100% sure that there are no random hard-coded
GPIO numbers referring to whatever the framework gave them
at the time the code was written :(

Another reason the base is assigned from above (usually
from 512 and downward) is that the primary SoC GPIO usually
want to be at base 0 and there is no guarantee that it will
get probed first. So hard-coded GPIO bases go from 0 -> n
and dynamically allocateed GPIO bases from n <- 512.

Then we hope they don't meet and overlap in the middle...

quoted

and in that case it is better to delete the use of this function
altogether since it can not fail.

S32_MAX might be a better upper bound. That allows to
just have no number assigned to a gpio chip. Any driver
code calling desc_to_gpio() could then get back -1
or a negative error code.

Making the ones that are invalid today valid sounds like
a step backwards to me if the goal is to stop using
gpio numbers and most consumers no longer need them.

OK I get it...

Now: who wants to write this patch? :)

Christophe? Will you take a stab at it?

Yours,
Linus Walleij

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