Need a sleep/timeout after exporting pin?

[martinsp]

If I run blinky example with pi user then I receive error:

pi@raspberrypi /sys/class/gpio $ id
uid=1000(pi) gid=1000(pi) groups=1000(pi),4(adm),20(dialout),24(cdrom),27(sudo),29(audio),44(video),46(plugdev),60(games),100(users),105(netdev),110(i2c),999(input),1002(spi),1003(gpio)
pi@raspberrypi ~ $ ./blinky 8 800 400
We have a blinking problem: Permission denied (os error 13)

If I run it with root user, everything is working correctly.

By trial and error I have found that after exporting a pin at least a 80ms sleep is required before exported pin can be manipulated by users in group gpio.

If I change blink_my_led function in blinky.rs to

my_led.with_exported(|| {
        sleep_ms(80);
        ...

then everything runs successfully with a pi user.

I have found that when pin is exported for short period of time the owner of the exported pin is root:root. Later the owner is changed to root:gpio. During the period when owner is root:root users in gpio group can't operate the pin.

pi@raspberrypi /sys/class/gpio $ echo 8 > unexport
pi@raspberrypi /sys/class/gpio $ echo 8 > export; ls -l gpio8/*
-rw-r--r-- 1 root root 4096 Aug 10 10:56 gpio8/active_low
lrwxrwxrwx 1 root root    0 Aug 10 10:56 gpio8/device -> ../../../20200000.gpio
-rw-r--r-- 1 root root 4096 Aug 10 10:56 gpio8/direction
-rw-r--r-- 1 root root 4096 Aug 10 10:56 gpio8/edge
lrwxrwxrwx 1 root root    0 Aug 10 10:56 gpio8/subsystem -> ../../../../../class/gpio
-rw-r--r-- 1 root root 4096 Aug 10 10:56 gpio8/uevent
-rw-r--r-- 1 root root 4096 Aug 10 10:56 gpio8/value
pi@raspberrypi /sys/class/gpio $ ls -l gpio8/*
-rwxrwx--- 1 root gpio 4096 Aug 10 10:56 gpio8/active_low
lrwxrwxrwx 1 root gpio    0 Aug 10 10:56 gpio8/device -> ../../../20200000.gpio
-rwxrwx--- 1 root gpio 4096 Aug 10 10:56 gpio8/direction
-rwxrwx--- 1 root gpio 4096 Aug 10 10:56 gpio8/edge
lrwxrwxrwx 1 root gpio    0 Aug 10 10:56 gpio8/subsystem -> ../../../../../class/gpio
-rwxrwx--- 1 root gpio 4096 Aug 10 10:56 gpio8/uevent
-rwxrwx--- 1 root gpio 4096 Aug 10 10:56 gpio8/value

I'm not sure if this issue is specific to my Raspbery Pi / Raspbian version or this is how gpio sysfs works on Raspberry Pi. Maybe it should be documented somewhere that a timeout is required after exporting pin?

Raspberry Pi model 1 B+

pi@raspberrypi ~ $ uname -a
Linux raspberrypi 3.18.11+ #781 PREEMPT Tue Apr 21 18:02:18 BST 2015 armv6l GNU/Linux
pi@raspberrypi ~ $ cat /etc/debian_version
7.8

[posborne]

Thanks for filing the issue. I tested mostly on the BBB and RPi 1 and may have been running as root (I don't recall). My expectation is that the export would be a synchronous operation, but you clearly have some evidence to the contrary. Maybe changing of permissions is handled outside of the kernel by something like udev? I'll have to research a bit more.

[martinsp]

You are right, permissions are changed by udev, the rules are in /lib/udev/rules.d/60-python3-pifacecommon.rules and /lib/udev/rules.d/60-python-pifacecommon.rules files.

[posborne]

Ok, given that this is udev, I think I will update the example for greater compatibility and add a comment to the code stating that you may need a delay (or to poll for appropriate stat on some file). I don't think a change to the library core to add a delay would be appropriate as many systems would not be setup in the same way.

[dbrgn]

I agree that this library should not need to handle delays caused by udev.