# echo 1 > /sys/class/leds/green/brightness // 點亮 LED
# echo 0 > /sys/class/leds/green/brightness // 熄滅 LED
# echo heartbeat > /sys/class/leds/green/trigger // 讓 LED 像心跳一樣閃爍

# echo mmc0 > /sys/class/leds/green/trigger
# cat /sys/class/leds/green/trigger
none rc-feedback kbd-scrolllock kbd-numlock kbd-capslock kbd-kanalock 
kbd-shiftlock kbd-altgrlock kbd-ctrllock kbd-altlock kbd-shiftllock 
kbd-shiftrlock kbd-ctrlllock kbd-ctrlrlock 
[mmc0] heartbeat default-on ir-power-click axp20x-usb-online

# touch aa | sync

	struct led_classdev *cdev;
	int ret;
	cdev = kzalloc(sizeof(*cdev), GFP_KERNEL);
	if (!cdev)
		return -ENOMEM;
	cdev->name = "aaa";
	// cdev->brightness_set = ebsa110_led_set;
	// cdev->brightness_get = ebsa110_led_get;
	// cdev->default_trigger = "heartbeat";
	ret = led_classdev_register(NULL, cdev);
	if (ret	< 0) {
		kfree(cdev);
		return ret;
	}

led_classdev_register()
	of_led_classdev_register() // register a new object(對象) of led_classdev class.
		led_classdev_next_name()
		device_create_with_groups()
		led_add_brightness_hw_changed()
		list_add_tail() // add to the list of leds
		led_update_brightness()
		//led_trigger_set_default()

subsys_initcall(leds_init);
leds_init()	// 創(chuàng)建 leds 類，即產(chǎn)生 /sys/class/leds 目錄
	class_create()
		__class_create()
			__class_register()
				kset_register()

# ls /sys/class/
ata_device    extcon        mdio_bus      ptp           sound
ata_link      gpio          mem           pwm           spi_master
ata_port      graphics      misc          rc            thermal
bdi           hwmon         mmc_host      regulator     tty
block         i2c-adapter   net           rtc           udc
bsg           i2c-dev       phy           scsi_device   vc
dma           input         power_supply  scsi_disk     vtconsole
drm           leds          pps           scsi_host     watchdog

# ls /sys/class/leds/aaa/
brightness      power           trigger
max_brightness  subsystem       uevent

classes_init()
	kset_create_and_add("class", NULL, NULL); // create a struct kset dynamically and add it to sysfs
		kset_create()
			kobject_set_name()
		kset_register()
			kset_init()
			kobject_add_internal()
				kobject_get()
				kobj_kset_join()
					kset_get()
					list_add_tail()
						__list_add()
						{
							next->prev = new;
							new->next = next;
							new->prev = prev;
						}
				create_dir() // 創(chuàng)建目錄

/* kernel */
start_kernel()
	rest_init() // Do the rest non-__init'ed, we're now alive
		kernel_thread(kernel_init, NULL, CLONE_FS);
		kernel_init()
			kernel_init_freeable()
			/*
			 * Ok, the machine is now initialized. None of the devices
			 * have been touched yet, but the CPU subsystem is up and
			 * running, and memory and process management works.
			 *
			 * Now we can finally start doing some real work..
			 */
			do_basic_setup()
			driver_init() // to initialize their subsystems.
				devtmpfs_init()
				devices_init()
				buses_init()
				classes_init() // 剛剛的 classes_init()
				firmware_init()
				hypervisor_init()
				platform_bus_init()
				cpu_dev_init()
				memory_dev_init()
				container_dev_init()
				of_core_init()

boot_jump_linux()
	announce_and_cleanup()
		printf("\nStarting kernel ...%s\n", fake ? "(fake run for tracing)" : "");

/* uboot */
boot_jump_linux()
	announce_and_cleanup()
		printf("\nStarting kernel ...%s\n"); // printf() 
		bootstage_mark_name(BOOTSTAGE_ID_BOOTM_HANDOFF, "start_kernel");
		cleanup_before_linux()
	kernel_entry(0, machid, r2);
/* kernel */
start_kernel()
	rest_init() // Do the rest non-__init'ed, we're now alive
		kernel_thread(kernel_init, NULL, CLONE_FS);
		kernel_init()
			kernel_init_freeable()
			/*
			 * Ok, the machine is now initialized. None of the devices
			 * have been touched yet, but the CPU subsystem is up and
			 * running, and memory and process management works.
			 *
			 * Now we can finally start doing some real work..
			 */
			do_basic_setup()
			driver_init() // to initialize their subsystems.
				devtmpfs_init()
				devices_init()
				buses_init()
				classes_init()
					kset_create_and_add("class", NULL, NULL); // create a struct kset dynamically and add it to sysfs
						kset_create()
							kobject_set_name()
						kset_register()
							kset_init()
							kobject_add_internal()
								kobject_get()
								kobj_kset_join()
									kset_get()
									list_add_tail()
										__list_add()
										{
											next->prev = new;
											new->next = next;
											new->prev = prev;
										}
								create_dir()
				firmware_init()
				hypervisor_init()
				platform_bus_init()
				cpu_dev_init()
				memory_dev_init()
				container_dev_init()
				of_core_init()

subsys_initcall(leds_init);
leds_init()	// 創(chuàng)建 leds 類，即 /sys/class/leds 目錄
	class_create()
		__class_create()
			__class_register()
				kset_register()

led_classdev_register()
	of_led_classdev_register() // register a new object(對象) of led_classdev class.
		led_classdev_next_name()
		device_create_with_groups()
		led_add_brightness_hw_changed()
		list_add_tail() // add to the list of leds
		led_update_brightness()
		//led_trigger_set_default()