Intro: `sysfs` GPIO¶

Warning

The sysfs GPIO userspace interface is deprecated, and will be removed from the kernel eventually. However it’s use is really simple, and it is used here as an introduction to the style how hardware access usually looks like in Linux.

`sysfs`?¶

Virtual filesystem
- Exposes kernel information as ordinary file system entries
- Files and directories, mainly
Usually mounted on /sys
Exposes connected hardware to userspace
Dedicated subtrees …
- /sys/bus/pci
- /sys/bus/spi
- /sys/bus/i2c
- /sys/class/pwm
- /sys/class/gpio
- …

General Purpose IO (GPIO)¶

IO pins, accessible by userspace
Rooted at /sys/class/gpio

$ ls -l /sys/class/gpio/
total 0
--w--w---- 1 root gpio 4096 Feb  1 12:31 export
lrwxrwxrwx 1 root gpio    0 Jan 31 16:16 gpiochip0 -> ../../devices/platform/soc/fe200000.gpio/gpio/gpiochip0
lrwxrwxrwx 1 root gpio    0 Jan 31 16:16 gpiochip488 -> ../../devices/platform/soc/fe804000.i2c/i2c-1/1-0040/gpio/gpiochip488
lrwxrwxrwx 1 root gpio    0 Jan 31 16:16 gpiochip504 -> ../../devices/platform/soc/soc:firmware/soc:firmware:gpio/gpio/gpiochip504
--w--w---- 1 root gpio 4096 Feb  1 13:10 unexport

Very simple access: reading and writing files
⟶ no programming - shell only

Requesting A GPIO Pin ¶

By default, GPIO pins are only used by kernel drivers
⟶ explicit export

Requesting GPIO #26¶

$ echo 26 > /sys/class/gpio/export

⟶ New directory gpio26 in /sys/class/gpio

$ ls -l /sys/class/gpio/gpio26/
total 0
-rw-rw---- 1 root gpio 4096 Feb  1 13:15 active_low
lrwxrwxrwx 1 root gpio    0 Feb  1 13:15 device -> ../../../gpiochip0
-rw-rw---- 1 root gpio 4096 Feb  1 13:15 direction
-rw-rw---- 1 root gpio 4096 Feb  1 13:15 edge
drwxrwxr-x 2 root gpio    0 Feb  1 13:15 power
lrwxrwxrwx 1 root gpio    0 Feb  1 13:15 subsystem -> ../../../../../../../class/gpio
-rw-rw-r-- 1 root gpio 4096 Feb  1 13:15 uevent
-rw-rw---- 1 root gpio 4096 Feb  1 13:15 value

GPIO Attributes As Files ¶

File	Meaning
`direction`	Configured as input (`in`) or output (`out`)
`value`	`0` or `1`
`edge`	Interrupt configuration: `none`, `rising`, `falling`, `both` ⟶ Events from the `value` file, via `select()`, `poll()`, or `epoll()`
`active_low`	Inverts the meaning of `value` and `edge`

Intro: sysfs GPIO¶

sysfs?¶