Re: [libgpiod] Unexpected output of gpioinfo for claimed pins

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

Hi Stefan,

I'll try to answer as best I can :)

On Wed, Jan 2, 2019 at 4:59 PM Stefan Wahren <stefan.wahren@xxxxxxxx> wrote:

> during test of my latest BCM2835 DT patch series [1], i noticed a discrepancy between
> pinctrl debugfs and gpioinfo from libgpio on my Raspberry Pi 3 (linux-next, multi_v7_defconfig):
>
> 1. Pins configured as alternative function (e.g. pin 0 as alt0) are considered by gpioinfo as
> input even the get_direction callback of the pinctrl-bcm2835 returns -EINVAL in this case

We have no semantics for for "GPIO line is not there" in gpiolib.

In general, not all GPIOs in the world are even connected to pins,
some are internal or say connected to an FPGA fabric. They are
general purpose input/output but not necessarily associated with
a physical pin.

Also there are many GPIO drivers without pin control back-ends
and those are generally considered "there" even if the pin itself
is not connected to anything.

So that is the generic problem space.

So far, since the inception of the GPIO subsystem, the line has
been considered as existing if it has a register bit and some
electronic driver associated with it.

If you want to change this, you need to jit GPIO and pin control
closer together. As of today, many unused GPIOs are represented
in debugfs and also in lsgpio and friends.

> 2. Pins whose mux is claimed by a kernel driver
> (e.g. pin 0 by i2c) are considered by gpioinfo as unused

Since they are disconnected from the pin they are kind of unused.

This is a semantic question I guess it is in one sense unintuitive
but you can often set the bit in the register and once the pin is
muxed back to GPIO from whatever it was before, it is driven high,
so it still has a state and in that sense it is there.

> I think this is very confusing for users.

Yeah I guess it's like that. Generally I think the biggest problem for
user is that platforms have not named the gpio lines with names so
the user does not know what they are for. But this could be improved
too.

> Where does this needs to be fixed (pinctrl/gpio driver, gpiolib, libgpiod)?

All over the place I'm afraid. If it annoys you then start hacking :)

Yours,
Linus Walleij