举例，背光设置，背光对应的代码如下：
char const*const LCD_FILE
= "/sys/class/leds/lcd-backlight/brightness";
static int
rgb_to_brightness(struct light_state_t const* state)
{
int color = state->color & 0x00ffffff;
return ((77*((color>>16)&0x00ff))
+ (150*((color>>8)&0x00ff)) + (29*(color&0x00ff))) >> 8;
}
static int
set_light_backlight(struct light_device_t* dev,
struct light_state_t const* state)
{
int err = 0;
int brightness = rgb_to_brightness(state);
pthread_mutex_lock(&g_lock);
g_backlight = brightness;
err = write_int(LCD_FILE, brightness);
if (g_haveTrackballLight) {
handle_trackball_light_locked(dev);
}
pthread_mutex_unlock(&g_lock);
return err;
}
也就是往文件/sys/class/leds/lcd-backlight/brightness写入亮度值，然后驱动会根据该文件更改背光的亮度。LCD_FILE的路径根据实际情况更改，同时需要在init.rc 修改其权限，使其可写rgb_to_brightness也根据实际更改，比如要直接亮度值控制，那只要获取r,g,b其中的一个值就行了，如：
static int
rgb_to_brightness(struct light_state_t const* state)
{
int color = state->color & 0x000000ff;
return color;
}
4，led类驱动
4.1，驱动创建leds类，系统启动时执行leds_init在目录/sys/class/创建子目录leds
kernel\drivers\leds\Led-class.c
static int __init leds_init(void)
{
leds_class = class_create(THIS_MODULE, "leds");
if (IS_ERR(leds_class))
return PTR_ERR(leds_class);
leds_class->suspend = led_suspend;
leds_class->resume = led_resume;
return 0;
}
4.2，led_classdev_register，调用这个函数就在目录/sys/class/leds创建子目录led_cdev->name和属性文件brightness
对brightness文件写就执行led_brightness_store，对brightness文件读就执行led_brightness_show，为下面的lcd，led注册做好准备
kernel\drivers\leds\Led-class.c
static ssize_t led_brightness_show(struct device *dev,
struct device_attribute *attr, char *buf)
{
struct led_classdev *led_cdev = dev_get_drvdata(dev);
/* no lock needed for this */
led_update_brightness(led_cdev);
return sprintf(buf, "%u\n", led_cdev->brightness);
}
static ssize_t led_brightness_store(struct device *dev,
struct device_attribute *attr, const char *buf, size_t size)
{
struct led_classdev *led_cdev = dev_get_drvdata(dev);
ssize_t ret = -EINVAL;
char *after;
unsigned long state = simple_strtoul(buf, &after, 10);
size_t count = after - buf;
if (*after && isspace(*after))
count++;
if (count == size) {
ret = count;
if (state == LED_OFF)
led_trigger_remove(led_cdev);
led_set_brightness(led_cdev, state);
}
return ret;
}
static DEVICE_ATTR(brightness, 0644, led_brightness_show, led_brightness_store);
/**
* led_classdev_register - register a new object of led_classdev class.
* @parent: The device to register.
* @led_cdev: the led_classdev structure for this device.
*/
int led_classdev_register(struct device *parent, struct led_classdev *led_cdev)
{
int rc;
led_cdev->dev = device_create(leds_class, parent, 0, led_cdev,
"%s", led_cdev->name);
if (IS_ERR(led_cdev->dev))
return PTR_ERR(led_cdev->dev);
/* register the attributes */
rc = device_create_file(led_cdev->dev, &dev_attr_brightness);
if (rc)
goto err_out;
#ifdef CONFIG_LEDS_TRIGGERS
init_rwsem(&led_cdev->trigger_lock);
#endif
/* add to the list of leds */
down_write(&leds_list_lock);
list_add_tail(&led_cdev->node, &leds_list);
up_write(&leds_list_lock);
led_update_brightness(led_cdev);
#ifdef CONFIG_LEDS_TRIGGERS
rc = device_create_file(led_cdev->dev, &dev_attr_trigger);
if (rc)
goto err_out_led_list;
led_trigger_set_default(led_cdev);
#endif
printk(KERN_INFO "Registered led device: %s\n",
led_cdev->name);
return 0;
#ifdef CONFIG_LEDS_TRIGGERS
err_out_led_list:
device_remove_file(led_cdev->dev, &dev_attr_brightness);
list_del(&led_cdev->node);
#endif
err_out:
device_unregister(led_cdev->dev);
return rc;
}
EXPORT_SYMBOL_GPL(led_classdev_register);
4.3，lcd驱动调用led_classdev_register，在目录/sys/class/leds创建子目录lcd-backlight和属性文件brightness
kernel\drivers\video\msm\Msm_fb.c
static int lcd_backlight_registered;
static void msm_fb_set_bl_brightness(struct led_classdev *led_cdev,
enum led_brightness value)
{
struct msm_fb_data_type *mfd = dev_get_drvdata(led_cdev->dev->parent);
int bl_lvl;
if (value > MAX_BACKLIGHT_BRIGHTNESS)
value = MAX_BACKLIGHT_BRIGHTNESS;
/* This maps Android backlight level 0 to 255 into
driver backlight level 0 to bl_max with rounding */
bl_lvl = (2 * value * mfd->panel_info.bl_max + MAX_BACKLIGHT_BRIGHTNESS)
/(2 * MAX_BACKLIGHT_BRIGHTNESS);
if (!bl_lvl && value)
bl_lvl = 1;
msm_fb_set_backlight(mfd, bl_lvl, 1);
}
static struct led_classdev backlight_led = {
.name = "lcd-backlight",
.brightness = MAX_BACKLIGHT_BRIGHTNESS,
.brightness_set = msm_fb_set_bl_brightness,
};
if (!lcd_backlight_registered) {
if (led_classdev_register(&pdev->dev, &backlight_led))
printk(KERN_ERR "led_classdev_register failed\n");
else
lcd_backlight_registered = 1;
}