系统行动执行msm_pmic_led_init(void)
调用 platform_driver_register(&msm_pmic_led_driver);
调用msm_pmic_led_probe
调用 led_classdev_register(&pdev->dev, &msm_kp_bl_led);
就在目录/sys/class/leds创建子目录 keyboard-backlight和属性文件brightness
当按键时，上层对属性文件/sys/class/leds/keyboard-backlight/brightness写入背光的亮度数值就
调用led_brightness_store
调用simple_strtoul(buf, &after, 10);将输入的字符串转换为10进制的数字
执行led_set_brightness
执行led_cdev->brightness_set(led_cdev, value
调用msm_keypad_bl_led_set ，因为 .brightness_set = msm_keypad_bl_led_set,
调用 ret = pmic_set_led_intensity(LED_KEYPAD, value / MAX_KEYPAD_BL_LEVEL);
最终改变LED驱动电路的设置，调节LED的亮度