Realize LED driver under Linux without typing a line of code!

Preface

If you want to implement a device driver, you don’t need to write a single line of driver code. This sounds like Isn't it a fantasy?

But this is not impossible, because kernel developers all over the world are very enthusiastic. As long as they can write drivers, they have basically written them.

Today, we stand on the shoulders of giants and use drivers that have been written by kernel developers to achieve the functions we want. This article discusses LED drivers.

LED driver

We take the onboard LED of the imx6ull pro development board as an example. It has an onboard Controllable Led2, the schematic diagram is as follows:

LED2 has a pull-up resistor, and the other pin is connected to GPIO5_3, so GPIO5_3Output low level to light up the LED. Here's how to control this LED.

Kernel configuration:

 Device Drivers  --->
  [*] LED Support  --->
   <*>   LED Class Support
   <*>   LED Support for GPIO connected LEDs
   [*]   LED Trigger support  --->

Our LED driver is based on GPIO, so we need to turn on the support of the kernel LED driver.

The kernel has two corresponding drivers, namely GPIO driver and LED driver. The GPIO-based LED driver calls the functions exported by the GPIO driver. .

Please refer to: drivers/leds/leds-gpio.c for the LED driver implementation code. It implements a leds class to control LEDs through the sysfs interface.

设备树：

leds{
 compatible = "gpio-leds";
 
        led2{
            label = "led2";
            gpios = <&gpio5 3 GPIO_ACTIVE_LOW>;//GPIO_ACTIVE_LOW，代表低电平点亮LED
            default-state = "on";
        };
}

在设备树中创建一个名为leds的节点，compatible为"gpio-leds"，这样可以匹配到leds-gpio.c的驱动。

然后创建一个子节点，名为led2。需要填三个属性：label、gpios和default-state。

label：lable是出现在sys目录下的名字，即生成/sys/class/leds/led2

gpios: The first two values ​​specify the GPIO to which the LED is connected. The third value can be filled in GPIO_ACTIVE_HIGH or GPIO_ACTIVE_LOW. GPIO_ACTIVE_HIGH means high level lights up the LED, GPIO_ACTIVE_LOW means low level lights up the LED.

Note here that the third parameter of the gpios attribute represents whether the gpio needs a high level or a low level to light up the LED. Pay attention to whether it is lighting the LED, please take a closer look

default-state: on means that the default LED is on, off means that the default LED is off

Note again here that when default-state is on , in fact, the level output by gpio is the level specified in the gpios attribute when the LED is turned on

After the device tree is configured, compile and replace the dtb, and then restart the development board. You can see the /sys/class/leds/led2 directory:

/sys/class/leds/led2/目录下有一个brightnes文件，可以通过echo cat的方式查看和修改LED的亮度。因为LED连接在GPIO上，所以亮度只有0和1，在本文示例的led2中，0表示点亮，1表示熄灭。

点亮LED：

echo 0 > /sys/class/leds/led2/brightness

熄灭LED：

echo 1 > /sys/class/leds/led2/brightness

应用层控制

除了可以在shell中通过echo、cat的方式控制Led，我们也可以在写一个应用层程序来操作/sys/class/leds/下的节点，应用层代码：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <string.h>

#include <sys/stat.h>
#include <sys/types.h>

#define LED_DEV_PATH "/sys/class/leds/led%d/brightness"
#define ON
#define OFF

int fs4412_set_led(unsigned int lednum, unsigned int mode)
{
 int fd;
    int ret;
    char devpath[128];
    char *on = "1\n";
    char *off = "0\n";
    char *m = NULL;
    
    snprintf(devpath, sizeof(devpath), LED_DEV_PATH, lednum);
    fd = open(devpath, O_WRONLY);
    if (fd == -1) {
  perror("fsled->open");
        return -1;
 }
    
    if (mode == ON)
         m = on;
    else
         m = off;
    
    ret = write(fd, m, strlen(m));
    if (ret == -1) {
  perror("fsled->wrtie");
        close(fd);
        return -1;
    }
    
    close(fd);
    return 0;

}

int main(int argc, char *argv[])
{
 unsigned int lednum = 2;
    
    while(1){
  fs4412_set_led(lednum, on);
        usleep(500000);
        fs4412_set_led(lednum, OFF);
  usleep(500000);
        
        lednum++;
        if (lednum > 5)
             lednum = 2;
 }

 return 0;
}

上述应用层代码执行后，led2会闪烁。

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