该函数创建了一个名为 “platform”的设备，后续platform的设备都会以此为parent。在sysfs中表示为：所有platform类型的设备都会添加在 platform_bus所代表的目录下，即 /sys/devices/platform下面。
-sh-3.1# ls /sys/devices/platform/
Fixed MDIO bus.0 fsl-i2c.0 serial8250
fsl-ehci.0 fsl-i2c.1 serial8250.0
fsl-gianfar.0 mpc83xx_spi.0 uevent
fsl-gianfar.1 mpc83xx_wdt.0
fsl-gianfar_mdio.-5 power
-sh-3.1# ls /sys/
block/ class/ firmware/ kernel/ power/
bus/ devices/ fs/ module/
-sh-3.1# ls /sys/bus/
i2c/ of_platform/ pci_express/ scsi/ usb/
mdio_bus/ pci/ platform/ spi/
-sh-3.1# ls /sys/bus/i2c/
devices/ drivers_autoprobe uevent
drivers/ drivers_probe
-sh-3.1# ls /sys/bus/platform/devices/
Fixed MDIO bus.0/ fsl-gianfar_mdio.-5/ mpc83xx_wdt.0/
fsl-ehci.0/ fsl-i2c.0/ serial8250/
fsl-gianfar.0/ fsl-i2c.1/ serial8250.0/
fsl-gianfar.1/ mpc83xx_spi.0/
-sh-3.1# ls /sys/bus/platform/drivers
drivers/ drivers_autoprobe drivers_probe
-sh-3.1# ls /sys/bus/platform/drivers/
fsl-ehci/ fsl-gianfar_mdio/ mpc83xx_spi/ serial8250/
fsl-gianfar/ fsl-i2c/ mpc83xx_wdt/
platform_bus必须在系统注册任何platform driver和platform device之前初始化，那么这是如何实现的呢？
14/**
15 * driver_init - initialize driver model.
16 *
17 * Call the driver model init functions to initialize their
18 * subsystems. Called early from init/main.c.
19 */
20void __init driver_init(void)
21{
22 /* These are the core pieces */
23 devices_init();
24 buses_init();
25 classes_init();
26 firmware_init();
27 hypervisor_init();
28
29 /* These are also core pieces, but must come after the
30 * core core pieces.
31 */
32 platform_bus_init();
33 system_bus_init();
34 cpu_dev_init();
35 memory_dev_init();
36}
init/main.c
start_kernel 》 rest_init 》 kernel_init 》 do_basic_setup》driver_init 》platform_bus_init
724/*
725 * Ok, the machine is now initialized. None of the devices
726 * have been touched yet, but the CPU subsystem is up and
727 * running, and memory and process management works.
728 *
729 * Now we can finally start doing some real work..
730 */
731static void __init do_basic_setup(void)
732{
733 /* drivers will send hotplug events */
734 init_workqueues();
735 usermodehelper_init();
736 driver_init();
737 init_irq_proc();
738 do_initcalls();
739}
platform driver和platform device的初始化是在do_initcalls中进行的。
8.2 定义platform_device
中定义了系统的资源，是一个高度可移植的文件，大部分板级资源都在这里集中定义。