Access to GPIO and PWM as non-root user not functioning

[nicnicknicky]

Hello, I encountered several issues when trying to use the GPIO and PWM as a non-root user. The solutions I have seen in other similar issues have not used any concrete solutions and most were for kernels 3.xx.

Right before posting this issue, I managed to make a scrappy solution enable the GPIOs with PWM to work when my ownership is changed to non-root user. This solution was based off reading various threads and I pieced together my own which happened to work.
This was my main reference.
Essentially my contribution was that I went to look at how gpio and pwm were called in the different libraries and implemented permission changes on those files.

My kernel version:

#uname -a
Linux BeagleBoneBlack 4.4.36-ti-r72 #1 SMP Wed Dec 7 22:29:53 UTC 2016 armv7l GNU/Linux

I am still new to quite new to linux so can I verify if I am actually doing the right thing to get it to work?

I have a user nick which is part of the group digital

Create custom udev rule in /etc/udev/rules.d/80-gpio-permissions.rules

KERNEL=="gpio*", SUBSYSTEM=="gpio", ACTION=="add", PROGRAM="/usr/local/bin/udev-gpio-permissions.sh"

Create script in /usr/local/bin/udev-gpio-permissions.sh

  #!/bin/bash
  
  # Result: chown nick:digital /sys/class/gpio
  
  chown -R nick:digital /sys/devices/gpio
  chown -R nick:digital /sys/class/gpio
  chown -R nick:digital /sys/devices/platform/ocp
  chown -R nick:digital /sys/class/pwm
  
  #GPIO ( active_low device direction edge power subsystem uevent value )
  find /sys/class/gpio -type d -exec chmod 2770 {} \;
  find /sys/class/gpio -name "direction" -exec chmod 0660 {} \;
  find /sys/class/gpio -name "edge" -exec chmod 0660 {} \;
  find /sys/class/gpio -name "value" -exec chmod 0660 {} \;
  find /sys/class/gpio -name "active_low" -exec chmod 0660 {} \;
  find /sys/class/gpio -name "device" -exec chmod 0660 {} \;
  find /sys/class/gpio -name "power" -exec chmod 0660 {} \;
  find /sys/class/gpio -name "subsystem" -exec chmod 0660 {} \;
  
  #PWM ( duty_cycle enable period polarity power uevent )
  find /sys/devices/platform/ocp -name "duty_cycle" -exec chmod 0660 {} \;
  find /sys/devices/platform/ocp -name "enable" -exec chmod 0660 {} \;
  find /sys/devices/platform/ocp -name "period" -exec chmod 0660 {} \;
  find /sys/devices/platform/ocp -name "polarity" -exec chmod 0660 {} \;
  find /sys/devices/platform/ocp -name "power" -exec chmod 0660 {} \;
  find /sys/class/pwm -name "duty_cycle" -exec chmod 0660 {} \;
  find /sys/class/pwm -name "enable" -exec chmod 0660 {} \;
  find /sys/class/pwm -name "period" -exec chmod 0660 {} \;
  find /sys/class/pwm -name "polarity" -exec chmod 0660 {} \;
  find /sys/class/pwm -name "power" -exec chmod 0660 {} \;
  
  chmod 0220 /sys/class/gpio/export
  chmod 0220 /sys/class/gpio/unexport
  chmod 0220 /sys/class/pwm/export
  chmod 0220 /sys/class/pwm/unexport
  
  find /sys/class/gpio -name "uevent" -exec chmod 0660 {} \;
  find /sys/class/gpio -name "autosuspend_delay_ms" -exec chmod 0660 {} \;
  find /sys/class/gpio -name "control" -exec chmod 0660 {} \;
  find /sys/class/pwm -name "uevent" -exec chmod 0660 {} \;
  find /sys/devices/platform/ocp -name "uevent" -exec chmod 0660 {} \;
  
  # Additional code for getting AIN pins set up
  ain_activator=/sys/devices/platform/bone_capemgr/slots
  chown -R nick:digital $ain_activator/
  chmod -R 2770 $ain_activator/

Ensure the above file is executable

ls -al /usr/local/bin/udev-gpio-permissions.sh
-rwxr--r-- 1 root staff 896 Jan 31 11:23 /usr/local/bin/udev-gpio-permissions.sh

Run udevadm control --reload-rules

Reboot and test!

[pdp7]

@nicnicknicky thanks for posting. I've just returned from a conference this past week and will check this out

[pdp7]

@nicnicknicky thank you, I've tested your udev rule and script and added to this repo in a new udev directory

[troywweber7]

I think the real solution is actually a part of this issue: beagleboard/linux#124

[dplanella]

This is a closed issue, but for those landing here and thinking the workaround works, I found out that at least for a 4.4.x kernel it doesn't for PWM. It seems that the PWM device tree attributes (duty_cycle & co.) are only created as files on demand. Thus the commands along these lines at boot time:

find /sys/devices/platform/ocp -name "duty_cycle"

return no results, as the duty_cycle file is not there. I don't actually know what creates it, but I noticed that it only gets created after an app using the PWM module is run.

The previous comment refers to the kernel issue as the proper fix. The last comment on that issue refers to the landing of the patch being blocked on fixes required on adafruit-beaglebone-io-python (and bonescript). However, I don't see any related open issue here, perhaps it's fallen off the radar?

[RobertCNelson]

it's created after you export 0 (or 1) in the pwm system node. only v4.11.x+ has the udev patch for permissions. (didn't want to break older userspace, we still need to fix bonescript before i backport that patch to v4.4.x/v4.9.x)

Regards,

[dplanella]

Aha, thanks @RobertCNelson! So I guess in the meantime, for <4.11 kernels the bit to export 0 should be added to the workaround script for it to work.

[dplanella]

@RobertCNelson would a similar patch be required for the quadrature encoder inputs? I wrote a udev rule based on the existing 81-gpio-noroot.rules, but seems to have had no effect:

# /etc/udev/rules.d/83-eqep-noroot.rules
#
# Corrects sys eQEP permissions on the Beaglebone so non-root users in the eqep group can set up 
# the quadrature encoder inputs
#
# Change group to eqep
SUBSYSTEM=="eqep", PROGRAM="/bin/sh -c '/bin/chown -R root:eqep /sys/devices/platform/ocp/*.epwmss/*.eqep'"
# Change user permissions to ensure user and group have read/write permissions
SUBSYSTEM=="eqep", PROGRAM="/bin/sh -c '/bin/chmod -R ug+rw /sys/devices/platform/ocp/*.epwmss/*.eqep'"

I created a new group, eqep, then reloaded the udev rules:

$ sudo udevadm control --reload

And I finally rebooted, but the ownership/permissions didn't get changed after reboot.

That's on a 4.4.x kernel:

$ sudo /opt/scripts/tools/version.sh
git:/opt/scripts/:[a5d5f437baeaa7f8e14d5ae037268abf7a67e88e]
eeprom:[A335BNLT00C03614BBBK0298]
model:[TI_AM335x_BeagleBone_Black]
dogtag:[BeagleBoard.org Debian Image 2017-10-10]
bootloader:[eMMC-(default)]:[/dev/mmcblk1]:[U-Boot 2017.09-00002-g0f3f1c7907]
kernel:[4.4.91-ti-r133]
nodejs:[v6.12.0]
uboot_overlay_options:[enable_uboot_overlays=1]
uboot_overlay_options:[uboot_overlay_pru=/lib/firmware/AM335X-PRU-RPROC-4-4-TI-00A0.dtbo]
uboot_overlay_options:[enable_uboot_cape_universal=1]
pkg:[bb-cape-overlays]:[4.4.20171107.0-0rcnee1~stretch+20171107]
pkg:[bb-wl18xx-firmware]:[1.20170829-0rcnee1~stretch+20170829]
pkg:[firmware-ti-connectivity]:[20170823-1rcnee0~stretch+20170830]