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GPIO

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From the version 4.8, the Linux kernel introduces a new user space API based on character devices for managing and controlling GPIOs (General-Purpose Input/Output).

The gpiod package provides commands to access the gpio, and also a C library to handle gpio devices.

The following guide shows how to use the new userspace char device driver and API (gpiod).The new character device and API makes use of /dev/gpiochip devices.

ls /dev/gpiochip*
/dev/gpiochip0
/dev/gpiochip1
/dev/gpiochip2
/dev/gpiochip3
/dev/gpiochip4

/dev/gpiochipX corresponds to a GPIO controller device, a controller of a bank of GPIOs.

gpiodetect

To discover gpio controller devices and how many GPIOs are attached.

# on a imx8mm board 
gpiodetect
gpiochip0 [30200000.gpio] (32 lines)
gpiochip1 [30210000.gpio] (32 lines)
gpiochip2 [30220000.gpio] (32 lines)
gpiochip3 [30230000.gpio] (32 lines)
gpiochip4 [30240000.gpio] (32 lines)

gpioinfo

To list the information for the gpio lines per bank.

# to get line info of gpiochip0 controller
gpioinfo -c gpiochip0
gpiochip0 - 32 lines:
    line   0:  "SODIMM_43"         "cd"   input  active-low [used]
    line   1:  "SODIMM_45"    "Wake-Up"   input  active-high [used]
    line   2:  "SODIMM_135"      unused   input  active-high 
    line   3:  "SODIMM_22"       unused   input  active-high
gpioinfo 5
line 0: "BTN1" unused          output active-high
line 1: "LED1" unused          output active-high
line 2: "LED2" unused          output active-high

gpiofind 

# to find the controller and the line index of the gpio line name “SODIMM_135”
gpiofind SODIMM_135
gpiochip0 2

gpioset

The gpioset command can be used to set a GPIO line. In the example below LED2 will turn on.

gpioset 5 2=1

gpioget

The gpioget command can be used to read the value of a GPIO line. In the example below the value of BTN1 will be read.

gpioget 5 0

gpiomon

Wait for events on GPIO line

gpiomon 0 13
event: FALLING EDGE offset: 13 timestamp: [1570281706.661390750]
event: FALLING EDGE offset: 13 timestamp: [1570281706.661435750]
event: RISING  EDGE offset: 13 timestamp: [1570281706.661604000]
event: RISING  EDGE offset: 13 timestamp: [1570281706.916220125]
event: FALLING EDGE offset: 13 timestamp: [1570281706.918247625]

Find gpio number

To use a GPIO line the gpio kernel number needs to be known. If we know the bank and the gpio line index, the kernel’s number can be calculated using the following formula on imx8 (arm64)

N = (BANK – 1) * 32 + IO

The kernel GPIO number for GPIO3_IO09 is
N = (3 – 1) * 32 + 9 = 73