usb协议是一个复杂的协议，目前涉及到的版本就有usb1.0， usb2.0， usb3.0。大家如果打开kernel usb host目录，就会发现下面包含了ohci，uhci，ehci，xhci，whci等多种形式的控制器驱动。那么，对于我们这些不是很了解usb的开发人员，如何了解usb的代码结构呢？
1、代码分布
drivers/usb目录下面，host目录包括了host驱动代码，core目录包含了主要的api接口代码，而其他目录则主要是device驱动代码。
2、device驱动怎么看
device驱动大多数和上层协议有关，不涉及到具体的寄存器读写。示例代码可以参考usb-skeleton.c
3、host驱动怎么看
a，不妨以s3c2410的host作为范例进行分析，首先找到Makefile，
| obj-$(CONFIG_USB_OHCI_HCD_S3C2410) += ohci-s3c2410.o |
b，再查看一下Kconfig，
| config USB_OHCI_HCD_S3C2410 tristate "OHCI support for Samsung S3C24xx/S3C64xx SoC series" depends on USB_OHCI_HCD && (ARCH_S3C24XX || ARCH_S3C64XX) default y ---help--- Enables support for the on-chip OHCI controller on S3C24xx/S3C64xx chips. |
c，通过Makefile和Kconfig发现，s3c2410依赖于USB_OHCI_HCD_S3C2410 和 USB_OHCI_HCD，那USB_OHCI_HCD呢？
| config USB_OHCI_HCD tristate "OHCI HCD (USB 1.1) support" depends on HAS_DMA && HAS_IOMEM ---help--- The Open Host Controller Interface (OHCI) is a standard for accessing USB 1.1 host controller hardware. It does more in hardware than Intel's UHCI specification. If your USB host controller follows the OHCI spec, say Y. On most non-x86 systems, and on x86 hardware that's not using a USB controller from Intel or VIA, this is appropriate. If your host controller doesn't use PCI, this is probably appropriate. For a PCI based system where you're not sure, the "lspci -v" entry will list the right "prog-if" for your USB controller(s): EHCI, OHCI, or UHCI. To compile this driver as a module, choose M here: the module will be called ohci-hcd. |
d，USB_OHCI_HCD只依赖于DMA和IOMEM。继续回到Makefile，判断USB_OHCI_HCD会编译哪些文件
| obj-$(CONFIG_USB_OHCI_HCD) += ohci-hcd.o |
e，看到这里，我们明白要打开s3c2410的host功能，只需要编译ohci-hcd.c和ohci-s3c2410.c两个文件就好了
f，通过观察，发现ohci-hcd.c和ohci-s3c2410.c的代码都很少，这原因是什么？下面这段代码来自于ohci-hcd.c。
