On 03/06/2015 08:11 PM, Lee Crocker wrote:
> All of these suggestions are terrible. The right question to ask is "How do I
> give a non-root user permission to set GPIOs?" If this can't be done, then
> this distribution of Linux is broken.
>
> I can add my user to the "kmem" group, then change the group ownership of
> /sys/class/gpio/export to kmem, the chmod g+w, and then my user can export
> the GPIOs. But when they appear, "direction" is still owned root:root perms
> 644, so I can't do anything with the GPIO I just created, even though, as a
> member of kmem, I could do the equivalent memory-mapped IO.
>
> It is not acceptable to require a program to run as root. I not quite
> Linux-savvy enough to fix this.
The Linux way to fix this is with udev.
1) define a user group that should be able to access gpio, such as "gpio"
2) add a udev rule that runs a script on the necessary udev events. Here's my
ubuntu rule (your distro might configure udev differently; good luck with
that):
peter@black:~$ cat /etc/udev/rules.d/50-gpio.rules
ACTION!="add", GOTO="gpio_end"
KERNEL=="gpio[0-9]*", SUBSYSTEM=="gpio", ACTION=="add",
PROGRAM="/lib/udev/gpio_pin %p"
ENV{OF_NAME}=="gpio", SUBSYSTEM=="platform", ACTION=="add",
PROGRAM="/lib/udev/gpio_pin enable"
LABEL="gpio_end"
3) udev has the appropriate permissions to run the script (below) which
changes the group/permissions:
peter@black:~$ cat /lib/udev/gpio_pin
#!/bin/sh
PATH='/sbin:/bin'
basedir="/sys/class/gpio"
devpath=$1
if [ "$1" = "enable" ] ; then
for f in export unexport ; do
chmod g+w $basedir/$f
chown :gpio $basedir/$f
done
else
for f in active_low direction edge value ; do
chmod g+w /sys${devpath}/$f
chown :gpio /sys${devpath}/$f
done
fi
exit 0
** NOTE **
Naturally there is a small delay between when the pin sysfs
inode is added and when the permissions have been changed, but
it's relatively simple to use a short timeout/delay loop when
opening a just-exported gpio pin.
** NOTE(2) **
When a user group is added, the user must re-login for the group
to become effective in their session.
** NOTE (3) **
udev can be triggered to re-play events after new rules are added
but it can be difficult to do that right, so it's easier just
to reboot.
Regards,
Peter Hurley
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