usb HID 报告描述符 + 自己的解释

先记录下来：
http://www.cs.fsu.edu/~baker/devices/lxr/http/source/linux/drivers/usb/input/hid-core.c?v=2.6.11.8#L640

625 /*
626  * Parse a report description into a hid_device structure. Reports are
627  * enumerated, fields are attached to these reports.
628  */
629 
630 static struct hid_device *hid_parse_report(__u8 *start, unsigned size)

参考HID_11 spec，APPENDIX D AND HID USAGE TABLE SPEC,  P134  for push/pop usage.

http://www.cs.fsu.edu/~baker/devices/lxr/http/source/linux/drivers/usb/input/hid.h?v=2.6.11.8
Push/Pop:

246 /*
247  * This is the global environment of the parser. This information is
248  * persistent for main-items（my comment : save env in current collection,
 purpose is to remove duplicated global item）. The global environment can be saved and
249  * restored with PUSH/POP statements.
250  */
251 
252 struct hid_global {
253         unsigned usage_page;
254         __s32    logical_minimum;
255         __s32    logical_maximum;
256         __s32    physical_minimum;
257         __s32    physical_maximum;
258         __s32    unit_exponent;
259         unsigned unit;
260         unsigned report_id;
261         unsigned report_size;
262         unsigned report_count;
263 };

在网上看了许多前辈的文章，就是无法理解报告描述符中的那些数字字符所表达的含义，几经周折，总算大致了解了。

结合 http://group.chinaaet.com/122/8005

code char MouseReportDescriptor[63] = {     
/*

Unsigned integer specifying the current Usage
Page. Since a usage are 32 bit values, Usage
Page items can be used to conserve space in a
report descriptor by setting the high order 16 bits
of a subsequent usages. Any usage that follows
which is defines 16 bits or less is interpreted as a
Usage ID and concatenated with the Usage Page
to form a 32 bit Usage.

*/

    0x05, 0x01,                    // USAGE_PAGE (Generic Desktop)     

   //  0x9 = 0000 10 01 -> type is local type, go to local items section in HID spec,

    //  0000 means USAGE tag, 01 means 1 byte data which is usage id, data is 6 meaning "Keyboard" in HID usage table spec.

    0x09, 0x06,                  // USAGE (Keyboard)  

   //0xa1 = 1010 00 01 -> type (00) is main type, go to main items section in HID spec,

    // 1010 means Collection tag, data 0x01 means "Application (mouse, keyboard)"

    0xa1, 0x01,                    // COLLECTION (Application)   

    // 0x5 = 0000 01 01, type is global item, 0000 means usage page, 0x07 means Keyboard/Keypad Page in HID usage table spec

    0x05, 0x07,                    //   USAGE_PAGE (Keyboard)     

   // 0x19 = 0001 10 01, type is local type, go to local items section in HID spec, 0001 means Usage Minimum, 0xe0   

   //means USAGE_MINIMUM in HID spec, in HID usage table spec, it means Keyboard LeftControl.

    0x19, 0xe0,                    //   USAGE_MINIMUM (Keyboard LeftControl)     
    0x29, 0xe7,                    //   USAGE_MAXIMUM (Keyboard Right GUI)     
    0x15, 0x00,                    //   LOGICAL_MINIMUM (0)     
    0x25, 0x01,                    //   LOGICAL_MAXIMUM (1)     
    0x75, 0x01,                    //   REPORT_SIZE (1)     
    0x95, 0x08,                    //   REPORT_COUNT (8)     
    0x81, 0x02,                    //   INPUT (Data,Var,Abs)  
    0x95, 0x01,                    //   REPORT_COUNT (1)     
    0x75, 0x08,                    //   REPORT_SIZE (8)     
    0x81, 0x03,                    //   INPUT (Cnst,Var,Abs) 

    0x95, 0x05,                    //   REPORT_COUNT (5)     
    0x75, 0x01,                    //   REPORT_SIZE (1)     
    0x05, 0x08,                    //   USAGE_PAGE (LEDs)     
    0x19, 0x01,                    //   USAGE_MINIMUM (Num Lock)     
    0x29, 0x05,                    //   USAGE_MAXIMUM (Kana)     
    0x91, 0x02,                    //   OUTPUT (Data,Var,Abs)   

    0x95, 0x01,                    //   REPORT_COUNT (1)     
    0x75, 0x03,                    //   REPORT_SIZE (3)     
    0x91, 0x03,                    //   OUTPUT (Cnst,Var,Abs) 

    0x95, 0x06,                    //   REPORT_COUNT (6)     
    0x75, 0x08,                    //   REPORT_SIZE (8)     
    0x15, 0x00,                    //   LOGICAL_MINIMUM (0)     
    0x25, 0xFF,                    //   LOGICAL_MAXIMUM (255)     
    0x05, 0x07,                    //   USAGE_PAGE (Keyboard)     
    0x19, 0x00,                    //   USAGE_MINIMUM (Reserved (no event indicated))     
    0x29, 0x65,                    //   USAGE_MAXIMUM (Keyboard Application)     
    0x81, 0x00,                    //   INPUT (Data,Ary,Abs)  

    0xc0                           // END_COLLECTION   

}; 

一般的，网文多是如上的报告描述符，据说是用一个工具产生的，有一些还配上了中文的说明，但也多是描述性的，而不是拆分解释这些字符所代表的含义，比如，数组第一行的 0x05, 0x01,                    // USAGE_PAGE (Generic Desktop) 

我们一眼就能看出双斜线后面的文字，是对前面数字的说明，即0x05,0x01所表达的是USAGE_PAGE (Generic Desktop) 的含义，但是，为何如此表达，则描述的不太清楚，对于熟悉的人而言，这自然不是问题，但对于新手，可就要费点功夫了。

 

0x05, 0x01,代表USAGE_PAGE (Generic Desktop) 的功能，是由《device class definition for human interface device (HID)》第24页的Report Descriptors 规定的。

分两部分，0x05为一部分，表示前缀，0x01为数据部分。

0x05转换成二进制，就是0000  01001，按照HID类协议5.3 generic item format的定义，这个字节被分成3个部分，bit0~bit1代表的是这个前缀后面跟的数据长度，两位可以表示最大4字节的数据，即bsize;bit2~bit3代表的是这个前缀的类型，总共可以有三种类型：0=main，1=global，2=local，3=reserved；bit4~bit7代表前缀的tag，一般分为input(二进制的1000 00 nn,即bit4~bit7=1000,代表一个tag，bit2~bit3=00，代表main，bit0~bit1=nn，代表这个前缀后面还有nn所代表的数据)，output(二进制的 1001 00 nn)，feature(1011 00 nn)，collection(1010 00 nn)，end collection(1100 00 nn)，遵照这个原则，我们就可以解析0x05所表达的含义。

 

0x05转换为二进制就是0000 0101,其高4位全为0，表示的tag为usage page  tag(协议45页)，bit2~bit3=01，表示的是类型，从协议中可以知道，这是一个全局类型的item（协议36页），bit0~bit1=01，表示的是这个前缀后面跟着的数据长度为1字节，即0x05后面，有0x01作为这个前缀的数据部分，而0x01表示的是general desktop page(《universal serial bus HID usage table》第五页，目录)，因此，这两个数字合起来就是USAGE_PAGE (Generic Desktop)的含义。

 

总之，要了解报告描述符，需要两份资料：《device class definition for human interface device (HID)》, 《Universal Serial Bus HID Usage Tables》