Issue
I am trying to write a small kernel module that can control some LEDs connected to the GPIO pins on my RaspberryPi 4 Model B. My Pi is running Raspbian GNU/Linux 11 (bullseye) and Linux kernel v6.1.21-v7l+.
I am currently trying to figure out how the DeviceTree works with my kernel module.
This is my DTS file
/dts-v1/;
/plugin/;
/ {
fragment@0 {
target-path = "/";
__overlay__ {
led {
compatible = "led";
status = "okay";
label = "led";
led-gpio = <&gpio 17 0>,
<&gpio 27 0>;
};
};
};
};
My kernel module code is
#include <linux/module.h>
#include <linux/init.h>
#include <linux/mod_devicetable.h>
#include <linux/property.h>
#include <linux/platform_device.h>
#include <linux/of_device.h>
#include <linux/gpio/consumer.h>
#include <linux/proc_fs.h>
MODULE_AUTHOR("****");
MODULE_LICENSE("GPL v2");
static int dt_probe(struct platform_device *pdev);
static int dt_remove(struct platform_device *pdev);
static struct of_device_id led_ids[] = {
{
.compatible = "led",
}, { /* sentinel */ }
};
MODULE_DEVICE_TABLE(of, led_ids);
static struct platform_driver led_driver = {
.probe = dt_probe,
.remove = dt_remove,
.driver = {
.name = "led",
.of_match_table = led_ids,
},
};
static struct gpio_desc *blue = NULL, *red = NULL;
static struct proc_dir_entry *proc_file = NULL;
static ssize_t led_write(struct file *filp, const char *user_buffer, size_t count, loff_t *offset) {
int value;
char command[2];
if (copy_from_user(command, user_buffer, 1)) {
pr_err("[led] Failed to read command from user\n");
return -EFAULT;
}
command[1] = 0; // null terminate the string so that kstrtoint doesn't fail
if (kstrtoint(command, 10, &value)) {
pr_err("[led] Invalid command from user\n");
return -EINVAL;
}
gpiod_set_value(blue, !!value);
return count;
}
static ssize_t led_read(struct file *filp, char __user *user_buffer, size_t count, loff_t *offset) {
int value;
char led_value_str;
if (*offset)
return 0;
value = gpiod_get_raw_value(blue);
switch (value) {
case 0:
led_value_str = '0';
break;
case 1:
led_value_str = '1';
break;
default:
break;
}
if (copy_to_user(user_buffer, &led_value_str, 1)) {
pr_err("[led] Failed to copy led value to user\n");
return -EFAULT;
}
(*offset)++;
return 1;
};
static struct proc_ops fops = {
.proc_write = led_write,
.proc_read = led_read,
};
static int dt_probe(struct platform_device *pdev) {
struct device *dev = &pdev->dev;
const char *label;
int ret;
/* Check for device properties */
if (!device_property_present(dev, "label")) {
pr_err("[led] Device property 'label' not found!\n");
return -1;
}
/* Read device properties */
ret = device_property_read_string(dev, "label", &label);
if (ret) {
pr_err("[led] Failed to read 'label'\n");
return -1;
}
/* Init GPIO */
blue = gpiod_get_index(dev, "led", 0, GPIOD_OUT_LOW);
if (IS_ERR(blue)) {
pr_err("[led] Failed to setup blue LED\n");
return PTR_ERR(blue);
}
red = gpiod_get_index(dev, "led", 1, GPIOD_OUT_LOW);
if (IS_ERR(red)) {
printk("[led] Failed to setup red LED\n");
gpiod_put(blue);
return PTR_ERR(red);
}
/* Creating procfs file */
proc_file = proc_create("led", 0666, NULL, &fops);
if (proc_file == NULL) {
pr_err("[led] Failed to create /proc/led\n");
gpiod_put(blue);
gpiod_put(red);
return -ENOMEM;
}
pr_info("[led] Successfully setup GPIO driver\n");
return 0;
}
static int dt_remove(struct platform_device *pdev) {
pr_info("[led] Removing device from tree\n");
gpiod_put(blue);
proc_remove(proc_file);
return 0;
}
static int __init led_init(void) {
pr_info("[led] Loading LED driver\n");
if (platform_driver_register(&led_driver)) {
pr_err("[led] Failed to register driver\n");
return -1;
}
return 0;
};
static void __exit led_cleanup(void) {
pr_info("[led] Cleaning up LED driver\n");
platform_driver_unregister(&led_driver);
};
module_init(led_init);
module_exit(led_cleanup);
My Makefile
obj-m += led.o
KDIR = /lib/modules/$(shell uname -r)/build
all: module dt
echo Built Device Tree Overlay and kernel module
module:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
dt: ledoverlay.dts
dtc -@ -I dts -O dtb -o ledoverlay.dtbo ledoverlay.dts
clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
rm -rf ledoverlay.dtbo
Loading the DTBO object works fine, but when I try to load my module the following gets printed to dmsg -
[ 9573.984173] [led] Loading LED driver
[ 9573.984509] [led] Failed to setup blue LED
[ 9573.984518] led: probe of led failed with error -2
Sources I looked at -
- https://github.com/Johannes4Linux/Linux_Driver_Tutorial/blob/main/21_dt_gpio/dt_gpio.c
- https://github.com/PacktPublishing/Linux-Device-Driver-Development-Second-Edition/blob/main/Chapter16/gpio/gpio-descriptor-module.c
Does anyone have any insight on why the call to gpiod_get_index() fails? I feel like I'm misunderstanding something or doing something wrong.
Edit:
If I change the DTS file and split the GPIOs this
...
led {
compatible = "led";
status = "okay";
label = "led";
led-blue-gpio = <&gpio 17 0>,;
led-red-gpio = <&gpio 27 0>;
};
And the module code to use gpiod_get() instead of gpiod_get_index() like so
...
blue = gpiod_get(dev, "led-blue", GPIOD_OUT_LOW);
...
red = gpiod_get(dev, "led-red", GPIOD_OUT_LOW);
Everything seems to work fine.
Solution
So it turns out, as @IanAbbott mentioned, the issue was with the name of the led-gpio property. I renamed it to led-gpios and it worked with gpiod_get_index().
Answered By - CoolSteve Answer Checked By - Cary Denson (WPSolving Admin)