Re: Replacing global GPIO numbers in sysfs with hardware offsets

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

On Sun, Feb 2, 2025 at 1:46 PM Bartosz Golaszewski <brgl@xxxxxxxx> wrote:

> I think it was Ahmad or Marek who suggested that users aren't really
> attached to the global numbering but to the ease of use of sysfs.
>
> I floated an idea of introducing a backward compatible change to sysfs
> that would allow users to identify GPIOs by the label of their parent
> chip and the hardware offset of the line within that chip (...)
>
> We could then encourage users to switch to using the chip-local
> exports and eventually at least remove the global export/unexport pair
> if we cannot make the entire sysfs class go away.
>
> Please let me know what you think about it?

Yes, I think it is mostly equivalent to what I say in drivers/gpio/TODO,
my only point being that when we add something like this, we
put it in debugfs where it belongs, and as illustrated by your
example, it is indeed used for debugging/exploring the
system:

----------------8<----------------------------8<------------------------
Debugfs in place of sysfs

The old sysfs code that enables simple uses of GPIOs from the
command line is still popular despite the existance of the proper
character device. The reason is that it is simple to use on
root filesystems where you only have a minimal set of tools such
as "cat", "echo" etc.

The old sysfs still need to be strongly deprecated and removed
as it relies on the global GPIO numberspace that assume a strict
order of global GPIO numbers that do not change between boots
and is independent of probe order.

To solve this and provide an ABI that people can use for hacks
and development, implement a debugfs interface to manipulate
GPIO lines that can do everything that sysfs can do today: one
directory per gpiochip and one file entry per line:

/sys/kernel/debug/gpiochip/gpiochip0
/sys/kernel/debug/gpiochip/gpiochip0/gpio0
/sys/kernel/debug/gpiochip/gpiochip0/gpio1
/sys/kernel/debug/gpiochip/gpiochip0/gpio2
/sys/kernel/debug/gpiochip/gpiochip0/gpio3
...
/sys/kernel/debug/gpiochip/gpiochip1
/sys/kernel/debug/gpiochip/gpiochip1/gpio0
/sys/kernel/debug/gpiochip/gpiochip1/gpio1
...

The exact files and design of the debugfs interface can be
discussed but the idea is to provide a low-level access point
for debugging and hacking and to expose all lines without the
need of any exporting. Also provide ample ammunition to shoot
oneself in the foot, because this is debugfs after all.
----------------8<----------------------------8<------------------------

Yours,
Linus Walleij