Re: GPIO Character device driver

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

On Wed, Apr 24, 2019 at 3:10 PM gianluca <gianlucarenzi@xxxxxxxx> wrote:
> On 04/23/2019 07:51 PM, Linus Walleij wrote:
> >> Do I have to __DISABLE__ the sysfs support from kernel config?
> >
> > No.
>
> Ok, mainly because the kernel config has GPIOLIB enabled by default and
> I added a _NOT_MODULE_ sysfs driver i.e. it is monolithic inside the
> kernel...

The character device is created from inside the core GPIOLIB code
and cannot be disabled.

> No, I mean which is the 'entry point' for creating those
> /dev/gpiochip[X] devices? I suppose some system call has to be issued to
> tell udev to create those char drivers...

The character device for each gpiochip is created inside the
gpiochip_setup_dev() call in drivers/gpio/gpiolib.c

What happens next is that the kernel invokes the uevent helper.
This used to be /sbin/hotplug but udev+systemd systems
nowadays use a netlink socket to send the events to userspace
and I have no idea how that works, sorry.

> > /dev/gpiochipN even appears on my Intel laptop.
>
> I suppose you are using non OF_ but ACPI_ stuff. ;-)

It doesn't matter. The /dev/gpiochipN are the same no matter what
hardware description you use. The appear when the driver
call [devm_]gpiochip_add[_data](). This is the same for all
GPIO drivers.

> >> The documentation is lacking those information. It seems I can have
> >> sysfs and gpiolib at the same time...
> >
> > You can.
>
> Ok. Now this reveals a question: if I configure some pin using sysfs,
> those pins can be used in gpiolib chardev stuff? I suppose I have to
> unregister the driver with 'unexport'.

I want to forget how the sysfs works but it is protected AFAIK
so you cannot use the same pins with sysfs and the chardev.

exporting/unexporting just affects individual lines on a chip
it doesn't register/unregister the driver att all.

> P.S.: by the way, it is an HONOR to me, having a mail with the 2nd Linus
> of the Kernel! ;-P

I'm just some engineer, it's not that big deal.

Yours,
Linus Walleij