An Example of Linux Kernel Module using GPIO Subsystem

README.md

An Example of Linux Kernel Module using GPIO Subsystem

This is a kernel module, with DT Overlay, to illustrate usage of GPIO subsystem in Linux kernel.

Hardware Envoronment

1. Raspberry Pi 3 Model B Rev 1.2
2. Arduino UNO

Software Environment

Raspberry Pi OS (32-bit) Lite (2020-08-20 release)

Hardware Configuration

Connect GPIO17 on Raspberry Pi Model B, through proper logic level shifter, to pin 7 on Arduino UNO.

Device Tree Overlay

First compile device tree:

dtc -@ -I dts -O dtb -o arduino-gpio.dtbo arduino-gpio.dts

Then move it to /boot/overlays/ folder:

sudo cp arduino-gpio.dtbo /boot/overlays/

After that, add the following lines to /boot/config.txt

dtoverlay=arduino-gpio

Reboot your Raspberry Pi. You may check current device tree after reboot with:

dtc -I fs /proc/device-tree | less

If everything goes well, you should be able to find somethings like this under gpio@7e200000 nodes:

...
gpio@7e200000 {
      ...
			arduino_gpio_conf {
				brcm,pins = < 0x11 >;
				phandle = < 0x86 >;
				brcm,pull = < 0x01 >;
				brcm,function = < 0x01 >;
			};
      ...
...

Also, below root node:

/{
        ...
        arduino_gpio {
                gpios = < 0x10 0x11 0x00 >;
                compatible = "arduino";
                status = "ok";
                phandle = < 0x87 >;
                pinctrl-0 = < 0x86 >;
                pinctrl-names = "default";
        };
        ...

Install

To build the module, simply use make:

make

To check if the module works, connect Arduino UNO with USB, then upload gpio_read.ino to Arduino UNO. Open serial console before install module.

sudo insmod arduino_gpio_module.ko

You will see Raspberry Pi alter GPIO level for 5 times (once per second).

arduino-gpio.dts

/dts-v1/;
/plugin/;
/ {
    compatible="brcm,brcm2835";
    fragment@0 {
        target = <&gpio>;
        __overlay__ {
            arduino_gpio: arduino_gpio_conf {
                brcm,pins = <0x11>;
                brcm,function = <0x1>;
                brcm,pull = <0x1>;
            };
        };
    };
    fragment@1 {
        target-path = "/";
        __overlay__ {
            arduino: arduino_gpio {
                gpios = <&gpio 0x11 0x0>;
                compatible = "arduino";
                status = "ok";
                pinctrl-0 = <&arduino_gpio>;
                pinctrl-names = "default";
            };
        };
    };
};

arduino_gpio_module.c

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/platform_device.h>
#include <linux/of.h>
#include <linux/gpio/consumer.h>
#include <linux/workqueue.h>
#include <linux/delay.h>

struct job_instance {
    int delay;
    struct work_struct job;
} jobs[5];

struct arduino {
    struct gpio_desc *gpio;
    int gpio_state;
} arduino;

void invert_output(struct work_struct *work)
{
    struct job_instance *w = container_of(work, struct job_instance, job);
    msleep((w->delay) * 1000);
    arduino.gpio_state = !gpiod_get_value(arduino.gpio);
    pr_info("Inverting GPIO outut to %d.\n", arduino.gpio_state);
    gpiod_set_value(arduino.gpio, arduino.gpio_state);
}

static int arduino_probe(struct platform_device *pdev)
{
    int i = 0;
    pr_info("Arduino gpio consumer being probed.\n");
    arduino.gpio = devm_gpiod_get(&(pdev->dev), NULL, GPIOD_OUT_LOW);
    if (IS_ERR(arduino.gpio)) {
        pr_err("Error when assigning GPIO.\n");
	return -22;
    }
    for (i = 0; i < 5; i++) {
	jobs[i].delay = i + 1;
	INIT_WORK(&(jobs[i].job), invert_output);
	schedule_work(&(jobs[i].job));
    }
    return 0;
}

static int arduino_remove(struct platform_device *pdev)
{
    pr_info("Arduino gpio consumer being removed.\n");
    gpiod_set_value(arduino.gpio, 0);
    devm_gpiod_put(&(pdev->dev), arduino.gpio);
    return 0;
}

static struct of_device_id arduino_id_tables[] = {
    {.compatible="arduino", },
    {}
};
MODULE_DEVICE_TABLE(of, arduino_id_tables);

static struct platform_driver arduino_drv = {
    .probe = arduino_probe,
    .remove = arduino_remove,
    .driver = {
        .name = "Arduino GPIO Consumer",
	.owner = THIS_MODULE,
	.of_match_table = arduino_id_tables,
    },
};

module_platform_driver(arduino_drv);
MODULE_LICENSE("GPL");

gpio_read.ino

#define RPI_DATA_PIN 7
#define DELAY_INTERVAL 250

void setup() {
    Serial.begin(9600);
    pinMode(RPI_DATA_PIN, INPUT);
}

void loop() {
    Serial.println(digitalRead(RPI_DATA_PIN));
    delay(DELAY_INTERVAL);
}

makefile

PWD := $(shell pwd)
KVERSION := $(shell uname -r)
KERNEL_DIR := /lib/modules/$(shell uname -r)/build

MODULE_NAME = arduino_gpio_module
obj-m := $(MODULE_NAME).o

all:
	make -C $(KERNEL_DIR) M=$(PWD) modules
clean:
	make -C $(KERNEL_DIR) M=$(PWD) clean

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