Linux那些事儿之我是U盘(10)我是谁的他?

probe,disconnect,id_table,这三个咚咚中首先要登场亮相的是id_table,它是干嘛用的呢?

我们说过,一个device只能绑定一个driver,但driver并非只能支持一种设备,道理很简单,比如我有两块U盘,那么我可以一起都插入,但是我只需要加载一个模块,usb-storage,没听说过插入两块U盘就得加载两次驱动程序的,除非这两块U盘本身就得使用不同的驱动程序.也正是因为一个模块可以被多个设备共用,才会有模块计数这么一个说法.

ok,既然一个driver可以支持多个device,那么当发现一个device的时候,如何知道哪个driver才是她的Mr.Right呢?这就是id_table的用处,让每一个struct usb_driver准备一张表,里边注明该driver支持哪些设备,这总可以了吧.如果你这个设备属于这张表里的,那么ok,绑定吧,如果不属于这张表里的,那么不好意思,您请便.哪凉快上哪去.

来自struct usb_driver中的id_table,

const struct usb_device_id *id_table;

实际上是一个指针,一个struct usb_device_id结构体的指针,当然赋了值以后就是代表一个数组名了,正如我们在定义struct usb_driver usb_storage_driver中所赋的值那样,.id_table=storage_usb_ids,那好,我们来看一下usb_device_id这究竟是怎样一个结构体.

struct usb_device_id来自include/linux/mod_devicetable.h,

40 /*
41 * Device table entry for "new style" table-driven USB drivers.
42 * User mode code can read these tables to choose which modules to load.
43 * Declare the table as a MODULE_DEVICE_TABLE.
44 *
45 * A probe() parameter will point to a matching entry from this table.
46 * Use the driver_info field for each match to hold information tied
47 * to that match: device quirks, etc.
48 *
49 * Terminate the driver's table with an all-zeroes entry.
50 * Use the flag values to control which fields are compared.
51 */
52
53 /**
54 * struct usb_device_id - identifies USB devices for probing and hotplugging
55 * @match_flags: Bit mask controlling of the other fields are used to match
56 * against new devices. Any field except for driver_info may be used,
57 * although some only make sense in conjunction with other fields.
58 * This is usually set by a USB_DEVICE_*() macro, which sets all
59 * other fields in this structure except for driver_info.
60 * @idVendor: USB vendor ID for a device; numbers are assigned
61 * by the USB forum to its members.
62 * @idProduct: Vendor-assigned product ID.
63 * @bcdDevice_lo: Low end of range of vendor-assigned product version numbers.
64 * This is also used to identify individual product versions, for
65 * a range consisting of a single device.
66 * @bcdDevice_hi: High end of version number range. The range of product
67 * versions is inclusive.
68 * @bDeviceClass: Class of device; numbers are assigned
69 * by the USB forum. Products may choose to implement classes,
70 * or be vendor-specific. Device classes specify behavior of all
71 * the interfaces on a devices.
72 * @bDeviceSubClass: Subclass of device; associated with bDeviceClass.
73 * @bDeviceProtocol: Protocol of device; associated with bDeviceClass.
74 * @bInterfaceClass: Class of interface; numbers are assigned
75 * by the USB forum. Products may choose to implement classes,
76 * or be vendor-specific. Interface classes specify behavior only
77 * of a given interface; other interfaces may support other classes.
78 * @bInterfaceSubClass: Subclass of interface; associated with bInterfaceClass.
79 * @bInterfaceProtocol: Protocol of interface; associated with bInterfaceClass.
80 * @driver_info: Holds information used by the driver. Usually it holds
81 * a pointer to a descriptor understood by the driver, or perhaps
82 * device flags.
83 *
84 * In most cases, drivers will create a table of device IDs by using
85 * USB_DEVICE(), or similar macros designed for that purpose.
86 * They will then export it to userspace using MODULE_DEVICE_TABLE(),
87 * and provide it to the USB core through their usb_driver structure.
88 *
89 * See the usb_match_id() function for information about how matches are
90 * performed. Briefly, you will normally use one of several macros to help
91 * construct these entries. Each entry you provide will either identify
92 * one or more specific products, or will identify a class of products
93 * which have agreed to behave the same. You should put the more specific
94 * matches towards the beginning of your table, so that driver_info can
95 * record quirks of specific products.
96 */
97 struct usb_device_id {
98 /* which fields to match against? */
99 __u16 match_flags;
100
101 /* Used for product specific matches; range is inclusive */
102 __u16 idVendor;
103 __u16 idProduct;
104 __u16 bcdDevice_lo;
105 __u16 bcdDevice_hi;
106
107 /* Used for device class matches */
108 __u8 bDeviceClass;
109 __u8 bDeviceSubClass;
110 __u8 bDeviceProtocol;
111
112 /* Used for interface class matches */
113 __u8 bInterfaceClass;
114 __u8 bInterfaceSubClass;
115 __u8 bInterfaceProtocol;
116
117 /* not matched against */
118 kernel_ulong_t driver_info;
119 };
实际上这个结构体对每一个usb设备来说,就相当于是她的身份证,记录了她的一些基本信息,通常我们的身份证上会记录我们的姓名,性别,出生年月,户口地址等等,而usb设备她也有她需要记录的信息,以区分她和别的usb设备,比如Vendor-厂家,Product-产品,以及其他一些比如产品编号,产品的类别,遵循的协议,这些都会在usb的规范里边找到对应的冬冬.暂且先不细说.

于是我们知道,一个usb_driver会把它的这张id表去和每一个usb设备的实际情况进行比较,如果该设备的实际情况和这张表里的某一个id相同,准确地说,只有这许多特征都吻合,才能够把一个usb device和这个usb driver进行绑定,这些特征哪怕差一点也不行.就像我们每个人都是一道弧,都在不停寻找能让彼此嵌成完整的圆的另一道弧,事实却是,每个人对∏(PI)的理解不尽相同,而圆心能否重合,或许只有痛过才知道.差之毫厘,失之交臂.

那么usb设备的实际情况是什么时候建立起来的?嗯,在介绍.probe指针之前有必要先谈一谈另一个数据结构了,她就是struct usb_device.