#115 Add support for /dev/gpiomem (rootless GPIO access)

[Luke Wren]

Currently, libraries such as RPi.GPIO are accessing /dev/mem to get to the GPIO registers directly, which works very well but has a number of issues, not the least of them the fact that you have to run as root whenever you touch GPIO.

As a first step away from this, we're adding a device, /dev/gpiomem, which gives you access to the GPIO registers without root permission. It will always map your pointer to the correct location of the registers, so there is no need to parse /proc/devicetree to try and figure out where they are.

See this pull request on raspberrypi/linux for more details.

I've attached a patch for c_gpio.c which will first check for /dev/gpiomem and then fall back to requiring root privileges if it doesn't exist. /dev/gpiomem will appear in the next Raspbian release (or sooner via rpi-update), at which point RPi.GPIO and similar tools will no longer require root.

[Ben Croston]

This hopefully be part of the next release - ETA w/c 24/8/2015.

[Luke Wren]

Awesome! Thanks.

[Ben Croston]

Coded it ready for testing... waiting for the new kernel with /dev/gpiomem to compile!

[Ben Croston]

Tested - passes unit test suite as root.

Does not work as a normal user - file permissions on /dev/gpiomem don't look right:
crw------- 1 root root 244, 0 Jan 1 1970 /dev/gpiomem

Did a sudo chmod 666 /dev/gpiomem and ran unit test suite again.
GPIO.setup / GPIO.input / GPIO.output work OK. Nothing works that uses /sys/class/gpio i.e. events.

On my system, the gpio group doesn't exist (never has done). It is a very old installation of Raspbian that has just been dist-upgraded for the last couple of years. In other words, the gpio group and udev rules for /sys/class/gpio don't exist on my system. These used to be created by installing the pifacecommon raspbian package. See http://piface.github.io/pifacecommon/installation.html . I can't find out where or how it is set up now - it has now moved somewhere else! Has anyone any ideas?

[Anonymous]

I had no gpio group and wrong permission on my ArchLinux system. I solved by

groupadd -r gpio

and the following udev rules, which I copied from this thread post

SUBSYSTEM=="bcm2835-gpiomem", KERNEL=="gpiomem", GROUP="gpio", MODE="0660"
SUBSYSTEM=="gpio", KERNEL=="gpiochip*", ACTION=="add", PROGRAM="/bin/sh -c 'chown root:gpio /sys/class/gpio/export /sys/class/gpio/unexport ; chmod 220 /sys/class/gpio/export /sys/class/gpio/unexport'"
SUBSYSTEM=="gpio", KERNEL=="gpio*", ACTION=="add", PROGRAM="/bin/sh -c 'chown root:gpio /sys%p/active_low /sys%p/direction /sys%p/edge /sys%p/value ; chmod 660 /sys%p/active_low /sys%p/direction /sys%p/edge /sys%p/value'"

Hope this helps.

[Anonymous]

After fixing permission, I'm still unable to use events: I suspect that this is a timing issue with the /sys/class/gpio udev rule. Permission are changed right after the relevant files (/sys/class/gpio/gpioXX/edge ...) are created, but too late for access by the python code.

[Ben Croston]

Have found the following in spindle (this is how it gets in the Foundation Raspbian image):
https://github.com/RPi-Distro/spindle/blob/master/wheezy-stage3

make_udev_com_rule() {
  onvm_chroot sh -l -e - <<EOF
printf "SUBSYSTEM==\"gpio*\", PROGRAM=\"/bin/sh -c 'chown -R root:gpio /sys/class/gpio && chmod -R 770 /sys/class/gpio; chown -R root:gpio /sys/devices/virtual/gpio && chmod -R 770 /sys/devices/virtual/gpio'\"\n" > /etc/udev/rules.d/99-com.rules
printf 'SUBSYSTEM=="i2c-dev", GROUP="i2c", MODE="0660"\n' >> /etc/udev/rules.d/99-com.rules
printf 'SUBSYSTEM=="spidev", GROUP="spi", MODE="0660"\n' >> /etc/udev/rules.d/99-com.rules
EOF
}

Edit: the /sys/class/gpio/gpioXX/* files do not have the correct owner/permissions with the above (default in latest Raspbian)

Also found in spindle:

groupadd -f -r spi
groupadd -f -r i2c
groupadd -f -r gpio
for GRP in gpio spi i2c; do
  adduser pi $GRP
done

[Ben Croston]

Created 0.6.0a2 - should work around the timing issues discovered by Stefano Miccoli above.

Have also added create_gpio_user_permissions.py that creates the gpio group and udev rules that work (thanks again to Stefano Miccoli). Note that the udev settings in the Foundation Raspbian Wheezy do not work properly. I will patch spindle an create a pull request. In the meantime, I will enhance create_gpio_user_permissions.py so that it detects and replaces incorrect udev settings. Should this be run automatically on installation (by pip and apt-get)?

[Anonymous]

While create_gpio_user_permissions.py is a good idea for helping inexperienced users and therefore could be included in the distribution by a

scripts = ['create_gpio_user_permissions.py']

clause in setup.py, I think that it should not be run automatically at install time.

With pip usally one installs in an isolated virtual environment (created via virtualenv or pyvenv), therefore pip should not tamper with system files, or require superuser privileges for running.

As what regards Debian, the gpio group should be created by the postinst script . You can run

grep -El 'groupadd|addgroup' /var/lib/dpkg/info/*postinst

to have a list of examples.

The /etc/udev/rules.d/99-gpio.rules should be handled by dpkg via the conffiles control file. See also https://www.debian.org/doc/debian-policy/ch-files.html#s-config-files

[Randy Marsh]

Any update on when this new version will be released?

[Ben Croston]

Raspbian Jessie .deb packages of 0.6.0a3 have been produced/tested and will hopefully be in the next Foundation Raspbian release (due out shortly I believe). For other Linux distributions, testing is still ongoing.

[Ben Croston]

Release 0.6.1 has been made for Jessie and Wheezy. You do not need sudo under Jessie.

In Wheezy, you do not need sudo once user groups and udev settings have been modified. At the moment this has to be done manually.

Other distributions will need the gpio group, udev settings and correct kernel that provides /dev/gpiomem adding for sudoless operation.