参考书：《linux device drivers》、《usb 2.0规范》 《usb3.1规范》《usb白皮书》
以linux为例来说明usb系统。
先看一下usb蓝牙适配器的详细信息：
$ lsusb | grep Cambridge Bus 001 Device 006: ID 0a12:0001 Cambridge Silicon Radio, Ltd Bluetooth Dongle (HCI mode) $ sudo lsusb -s 001:006 -v
[sudo] password for host:
Bus 001 Device 006: ID 0a12:0001 Cambridge Silicon Radio, Ltd Bluetooth Dongle (HCI mode)
Device Descriptor:
bLength 18
bDescriptorType 1
bcdUSB 2.00
bDeviceClass 224 Wireless
bDeviceSubClass 1 Radio Frequency
bDeviceProtocol 1 Bluetooth
bMaxPacketSize0 64
idVendor 0x0a12 Cambridge Silicon Radio, Ltd
idProduct 0x0001 Bluetooth Dongle (HCI mode)
bcdDevice 88.91
iManufacturer 0
iProduct 2 CSR8510 A10
iSerial 0
bNumConfigurations 1
Configuration Descriptor:
bLength 9
bDescriptorType 2
wTotalLength 177
bNumInterfaces 2
bConfigurationValue 1
iConfiguration 0
bmAttributes 0xe0
Self Powered
Remote Wakeup
MaxPower 100mA
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 0
bAlternateSetting 0
bNumEndpoints 3
bInterfaceClass 224 Wireless
bInterfaceSubClass 1 Radio Frequency
bInterfaceProtocol 1 Bluetooth
iInterface 0
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x81 EP 1 IN
bmAttributes 3
Transfer Type Interrupt
Synch Type None
Usage Type Data
wMaxPacketSize 0x0010 1x 16 bytes
bInterval 1
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x02 EP 2 OUT
bmAttributes 2
Transfer Type Bulk
Synch Type None
Usage Type Data
wMaxPacketSize 0x0040 1x 64 bytes
bInterval 1
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x82 EP 2 IN
bmAttributes 2
Transfer Type Bulk
Synch Type None
Usage Type Data
wMaxPacketSize 0x0040 1x 64 bytes
bInterval 1
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 1
bAlternateSetting 0
bNumEndpoints 2
bInterfaceClass 224 Wireless
bInterfaceSubClass 1 Radio Frequency
bInterfaceProtocol 1 Bluetooth
iInterface 0
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x03 EP 3 OUT
bmAttributes 1
Transfer Type Isochronous
Synch Type None
Usage Type Data
wMaxPacketSize 0x0000 1x 0 bytes
bInterval 1
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x83 EP 3 IN
bmAttributes 1
Transfer Type Isochronous
Synch Type None
Usage Type Data
wMaxPacketSize 0x0000 1x 0 bytes
bInterval 1
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 1
bAlternateSetting 1
bNumEndpoints 2
bInterfaceClass 224 Wireless
bInterfaceSubClass 1 Radio Frequency
bInterfaceProtocol 1 Bluetooth
iInterface 0
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x03 EP 3 OUT
bmAttributes 1
Transfer Type Isochronous
Synch Type None
Usage Type Data
wMaxPacketSize 0x0009 1x 9 bytes
bInterval 1
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x83 EP 3 IN
bmAttributes 1
Transfer Type Isochronous
Synch Type None
Usage Type Data
wMaxPacketSize 0x0009 1x 9 bytes
bInterval 1
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 1
bAlternateSetting 2
bNumEndpoints 2
bInterfaceClass 224 Wireless
bInterfaceSubClass 1 Radio Frequency
bInterfaceProtocol 1 Bluetooth
iInterface 0
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x03 EP 3 OUT
bmAttributes 1
Transfer Type Isochronous
Synch Type None
Usage Type Data
wMaxPacketSize 0x0011 1x 17 bytes
bInterval 1
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x83 EP 3 IN
bmAttributes 1
Transfer Type Isochronous
Synch Type None
Usage Type Data
wMaxPacketSize 0x0011 1x 17 bytes
bInterval 1
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 1
bAlternateSetting 3
bNumEndpoints 2
bInterfaceClass 224 Wireless
bInterfaceSubClass 1 Radio Frequency
bInterfaceProtocol 1 Bluetooth
iInterface 0
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x03 EP 3 OUT
bmAttributes 1
Transfer Type Isochronous
Synch Type None
Usage Type Data
wMaxPacketSize 0x0019 1x 25 bytes
bInterval 1
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x83 EP 3 IN
bmAttributes 1
Transfer Type Isochronous
Synch Type None
Usage Type Data
wMaxPacketSize 0x0019 1x 25 bytes
bInterval 1
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 1
bAlternateSetting 4
bNumEndpoints 2
bInterfaceClass 224 Wireless
bInterfaceSubClass 1 Radio Frequency
bInterfaceProtocol 1 Bluetooth
iInterface 0
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x03 EP 3 OUT
bmAttributes 1
Transfer Type Isochronous
Synch Type None
Usage Type Data
wMaxPacketSize 0x0021 1x 33 bytes
bInterval 1
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x83 EP 3 IN
bmAttributes 1
Transfer Type Isochronous
Synch Type None
Usage Type Data
wMaxPacketSize 0x0021 1x 33 bytes
bInterval 1
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 1
bAlternateSetting 5
bNumEndpoints 2
bInterfaceClass 224 Wireless
bInterfaceSubClass 1 Radio Frequency
bInterfaceProtocol 1 Bluetooth
iInterface 0
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x03 EP 3 OUT
bmAttributes 1
Transfer Type Isochronous
Synch Type None
Usage Type Data
wMaxPacketSize 0x0031 1x 49 bytes
bInterval 1
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x83 EP 3 IN
bmAttributes 1
Transfer Type Isochronous
Synch Type None
Usage Type Data
wMaxPacketSize 0x0031 1x 49 bytes
bInterval 1
Device Status: 0x0001
Self Powered
根据《linux device drivers》中的说明，一个设备可以有一个或者多个configuration（通常只有一个）,
一个configuration可以有一个或者多个interface,一个interface中可以有一个或者多个endpoint.
上面的蓝牙适配器有1个configuration,这个configuration有7个interface,每个interface有2个或者3个endpoint.
在linux驱动中usb设备驱动与interface相对应。
usb之间最基本的通信是通过endpoint完成的，endpoint之间只能向一个方向发送数据（主机到设备(即OUT endpoint)或者设备到主机(即IN endpoint))。
endpoint传输数据可以是下面四种类型之一：
(1) control endpoint: 用于存取usb设备的不通部分，可以配置设备、向设备发命令或者获取设备状态信息。
(2)interrupt endpoint: 主机向设备请求数据量较少时使用，例如键盘和鼠标
(3)bulk endpoint:用于大量数据传送。 如果传送数据过大，则可能分成多个块传送
(4)isochronous endpoint:可以传送大量数据，通常用于能处理数据丢失以及固定速率传送数据的设备。
usb数据传输实际是通过packet来进行传输的，每个packet由SYNC(1个字节）、PID(1个字节）、CONTENT（不同PID可能有不同长度）和CRC(不同PID可能有不同长度）
详细的packet定义可以查看usb规范。
interface用于处理一种类型的usb连接，例如鼠标或者键盘等。前面的蓝牙适配器有多个interface,可以用来支持多个蓝牙设备。
interface还可能有alternate setting, 可以用来选择不同的interface参数值。
查看pci设备信息：
$ lspci 00:00.0 Host bridge: Intel Corporation 82G33/G31/P35/P31 Express DRAM Controller (rev 10) 00:02.0 VGA compatible controller: Intel Corporation 82G33/G31 Express Integrated Graphics Controller (rev 10) 00:1b.0 Audio device: Intel Corporation NM10/ICH7 Family High Definition Audio Controller (rev 01) 00:1c.0 PCI bridge: Intel Corporation NM10/ICH7 Family PCI Express Port 1 (rev 01) 00:1c.1 PCI bridge: Intel Corporation NM10/ICH7 Family PCI Express Port 2 (rev 01) 00:1c.2 PCI bridge: Intel Corporation NM10/ICH7 Family PCI Express Port 3 (rev 01) 00:1c.3 PCI bridge: Intel Corporation NM10/ICH7 Family PCI Express Port 4 (rev 01) 00:1d.0 USB controller: Intel Corporation NM10/ICH7 Family USB UHCI Controller #1 (rev 01) 00:1d.1 USB controller: Intel Corporation NM10/ICH7 Family USB UHCI Controller #2 (rev 01) 00:1d.2 USB controller: Intel Corporation NM10/ICH7 Family USB UHCI Controller #3 (rev 01) 00:1d.3 USB controller: Intel Corporation NM10/ICH7 Family USB UHCI Controller #4 (rev 01) 00:1d.7 USB controller: Intel Corporation NM10/ICH7 Family USB2 EHCI Controller (rev 01) 00:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev e1) 00:1f.0 ISA bridge: Intel Corporation 82801GB/GR (ICH7 Family) LPC Interface Bridge (rev 01) 00:1f.1 IDE interface: Intel Corporation 82801G (ICH7 Family) IDE Controller (rev 01) 00:1f.2 IDE interface: Intel Corporation NM10/ICH7 Family SATA Controller [IDE mode] (rev 01) 00:1f.3 SMBus: Intel Corporation NM10/ICH7 Family SMBus Controller (rev 01) 04:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8101E/RTL8102E PCI Express Fast Ethernet controller (rev 02)
安装tree:
sudo apt-get install tree
查看sysfs中所有usb设备文件：
$ tree /sys/bus/usb/devices/ /sys/bus/usb/devices/ ├── 1-0:1.0 -> ../../../devices/pci0000:00/0000:00:1d.7/usb1/1-0:1.0 ├── 1-1 -> ../../../devices/pci0000:00/0000:00:1d.7/usb1/1-1 ├── 1-1:1.0 -> ../../../devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1:1.0 ├── 1-1.2 -> ../../../devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1.2 ├── 1-1.2:1.0 -> ../../../devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1.2/1-1.2:1.0 ├── 1-1.2:1.1 -> ../../../devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1.2/1-1.2:1.1 ├── 1-1.3 -> ../../../devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1.3 ├── 1-1.3:1.0 -> ../../../devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1.3/1-1.3:1.0 ├── 1-1.3:1.1 -> ../../../devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1.3/1-1.3:1.1 ├── 1-4 -> ../../../devices/pci0000:00/0000:00:1d.7/usb1/1-4 ├── 1-4:1.0 -> ../../../devices/pci0000:00/0000:00:1d.7/usb1/1-4/1-4:1.0 ├── 2-0:1.0 -> ../../../devices/pci0000:00/0000:00:1d.0/usb2/2-0:1.0 ├── 2-2 -> ../../../devices/pci0000:00/0000:00:1d.0/usb2/2-2 ├── 2-2:1.0 -> ../../../devices/pci0000:00/0000:00:1d.0/usb2/2-2/2-2:1.0 ├── 3-0:1.0 -> ../../../devices/pci0000:00/0000:00:1d.1/usb3/3-0:1.0 ├── 3-1 -> ../../../devices/pci0000:00/0000:00:1d.1/usb3/3-1 ├── 3-1:1.0 -> ../../../devices/pci0000:00/0000:00:1d.1/usb3/3-1/3-1:1.0 ├── 4-0:1.0 -> ../../../devices/pci0000:00/0000:00:1d.2/usb4/4-0:1.0 ├── 5-0:1.0 -> ../../../devices/pci0000:00/0000:00:1d.3/usb5/5-0:1.0 ├── usb1 -> ../../../devices/pci0000:00/0000:00:1d.7/usb1 ├── usb2 -> ../../../devices/pci0000:00/0000:00:1d.0/usb2 ├── usb3 -> ../../../devices/pci0000:00/0000:00:1d.1/usb3 ├── usb4 -> ../../../devices/pci0000:00/0000:00:1d.2/usb4 └── usb5 -> ../../../devices/pci0000:00/0000:00:1d.3/usb5
本机蓝牙适配器连接到usb集线器，usb集线器连接到pc的一个usb口上。并且usb集线器还连接了其他的一些设备。
从上面的输出可以推测，1-1开头部分应该是usb集线器上连接的设备。
/sys/devices/pci0000:00/0000:00:1d.7/usb1应该对应host controller设备，1-1对应struct usb_device.
1-1.2表示蓝牙适配器所在hub口，1-1.2:1.0和1-1.2:1.1可能分别表示蓝牙适配器的接口。
查看这两个接口的endpoint：
$ ls -d /sys/devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1.2/1-1.2:1.0/ep* /sys/devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1.2/1-1.2:1.0/ep_02 /sys/devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1.2/1-1.2:1.0/ep_82 /sys/devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1.2/1-1.2:1.0/ep_81
$ ls -d /sys/devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1.2/1-1.2:1.1/ep*
/sys/devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1.2/1-1.2:1.1/ep_03 /sys/devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1.2/1-1.2:1.1/ep_83
1-1.2:1.0下有三个endpoint,对应与lsusb输出信息中的第一个interface中的3个endpoint.
1-1.2:1.1下有两个endpoint,对应于lsusb输出信息中的其他interface中的2个endpoint。
但是lsusb中总共有7个interface,最后6个interface只用一个interface就能表示，似乎看起来很奇怪。
仔细看上面的输出信息可以知道,最后6个interface确实是一个interface，只是使用了alterate setting,其bAlternateSetting的值分别是0-5，其它组成部分的值一模一样。