On a linux stm32mp157 system can gpio with pullup or pull-down resistors be configured in user space ? If yes how?

Rc.1 · ‎2022-04-13

I just spent a day trying to configure a gpio with internal pullup using gpiolib. Could not get it configured. Then spent 2 days with device drivers and device trees, and got it working. Seems insane to me that configuring gpios should be this complex. Is there an easier way?

PatrickF · ‎2022-04-13

Hi @Rc.1

I agree that according to this https://wiki.st.com/stm32mpu/wiki/How_to_control_a_GPIO_in_userspace

there is no way to control pull-up/down directly.

You could write directly in GPIO register using something like devmem2 tool, but this is not recommended outside debug as too many risk of interaction with kernel enabling/disabling the GPIO bus clocks or other read/modify/write collisions.

I succeeded to use pull-up with python with something like:

from gpiod import chip, line_request
import time
 
def gpio_status(gpio_name):
        gpio_bank = 'gpiochip' + "{}".format(ord(gpio_name[1:2]) - ord('A'))
        gpio_number = int(gpio_name[2:])
        bank = chip(gpio_bank)
        gpio = bank.get_line(int(gpio_number))
        gpio_config = line_request()
        gpio_config.consumer = "MyGPIOConsumer"
        gpio_config.request_type = line_request.DIRECTION_INPUT
        gpio_config.flags = line_request.FLAG_BIAS_PULL_UP
        gpio.request(gpio_config)
        time.sleep(0.01)  # wait a bit to be sure input is taken into register
        gpio_value = gpio.get_value()
        gpio.release()
        time.sleep(0.01)
        return gpio_value
 
gpio_status("PA13")

Regards.

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Rc.1 · ‎2022-04-13

Thanks Patrick!

I am in the process of trying this - if it works then it IS possible from userspace using the gpiod library and https://wiki.st.com/stm32mpu/wiki/How_to_control_a_GPIO_in_userspace is incorrect.

UPDATE:

I installed python3 and pip and gpiod and ran the sample code. Got

File "/root/.local/lib/python3.9/site-packages/gpiod/libgpiod/__init__.py", line 403, in _line_request_values

status = ioctl(fd, GPIO_GET_LINEHANDLE_IOCTL, req)

OSError: [Errno 22] Invalid argument

If I remove the line

gpio_config.flags = line_request.FLAG_BIAS_PULL_UP

the script executes OK. My conclusion is that ioctl is capable of changing the bias on inputs.

PatrickF · ‎2022-04-13

The provided python example work on my Discovery board with ecosystem v3.1 starter package (after installed as you mention python3, pip and gpiod).

libgpiod also support pull-up/down directly from command line:

# gpioget --bias=pull-up gpiochip4 1
1
# gpioget --bias=pull-down gpiochip4 1
0

Please check :

gpioget --help
gpioset --help

Please details how you think wiki is incorrect ? I agree it is maybe incomplete.

Regards,

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Rc.1 · ‎2022-04-14

# gpioinfo gpiochip3

gpiochip3 - 16 lines:

...

line 5: unnamed kernel input active-high [used]

line 6: unnamed unused input active-high

line 7: unnamed kernel input active-high [used]

line 8: unnamed kernel input active-high [used]

...

# gpioget --bias=pullup gpiochip3 6

gpioget: invalid bias: pullup

# gpioget -v

gpioget (libgpiod) v1.6.2

License: LGPLv2.1

This is free software: you are free to change and redistribute it.

There is NO WARRANTY, to the extent permitted by law.

# uname -a

Linux buildroot 5.4.31 #1 SMP PREEMPT Wed Apr 13 17:02:17 CST 2022 armv7l GNU/Linux

Maybe my kernel is older or misconfigured?

PatrickF · ‎2022-04-14

All my test above were done on latest Ecosystem v3.1.0 Starter Package (openstlinux-5.10-dunfell-mp1-21-11-17) .

Kernel is v5.10.61

https://wiki.st.com/stm32mpu/wiki/STM32MP15_ecosystem_release_note

Your are probably using an old ecosystem.

In Ecosystem v2.1.0 (Openstlinux-5.4-dunfell-mp1-20-11-12) , it is Kernel is v5.4.56

Regards.

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rhaberkorn · ‎2022-10-16

@Rc.1 How did you manage to configure the pullup via the device tree? I have the problem that my GPIO muxings and additional settings are not that easily picked up and applied by the kernel. The only workaround I found so far involves writing a dummy driver, only so you can reference your blocks via `pinctrl-0`.