Re: Replacing global GPIO numbers in sysfs with hardware offsets

[Marek Vasut <marex@denx.de>]

On 2/25/25 8:54 PM, Linus Walleij wrote:
> On Fri, Feb 21, 2025 at 8:41 PM Marek Vasut <marex@denx.de> wrote:
> 
>> I would very much like to avoid having to enable debugfs on production
>> systems to access GPIOs in early userspace (e.g. initramfs).
> 
> I didn't understand that. It was because Bartosz used the word "play":

Uh oh ... in short, the entire discussion between me and Bartosz at 
FOSDEM could be summarized to (please correct me if I'm wrong and I'm 
stuffing words into someone's mouth):

- Bartosz does not like fixed static GPIO number assignment, we agree
- Bartosz wants to write more userspace tools to operate GPIO chardev
   API, we do not agree, Marek already has all the tools built into shell
   (printf, echo, cat)

=> Some sysfs API "v2" which does not suffer from the defects of the
    current one would be great. The benefit would be:
    - Can be operated without extra tools, simple printf/echo/cat into
      sysfs attributes would suffice
    - Would work even in the simplest of userspaces (initramfs, remote
      access VMs, etc.), which is practical for board bring up and well,
      other limited deployments

That's really all this is about, get rid of the defects of the old sysfs 
API, but keep the tooling requirements to minimum.

Also note that API "v2" attribute layout could differ from API "v1" , 
that is not a problem.

>>> The gist of it is: some people need to play with GPIOs very early, for
>>> example in an initramfs
> 
> So I took that word literal: explore, play around. Not develop
> products.
> 
>> This was so
>> far possible via the sysfs API without tools, currently it is becoming
>> not easily possible. A sysfs API "v2" which makes that possible would be
>> very much appreciated.
> 
> I understand, I'm fine with sysfs if it needs to be a "support forever"
> ABI, as long as it's:
> 
> - Using the per-chip HW numberspace

This is no issue for me.

> - Doesn't need any echo NN > export to see the lines in
>    sysfs.

Can we really make do without export/unexport ?

Consider this scenario:

- User open()s and write()s /sys/bus/gpio/gpiochip0/gpio0/value
- User keeps FD to /sys/bus/gpio/gpiochip0/gpio0/value open
- Kernel module gets loaded, binds to DT node which references the same GPIO
- User write()s /sys/bus/gpio/gpiochip0/gpio0/value open again

I wonder if this could pose a problem ?

I suspect the kernel module loading should fail , right ?