usb HID 報告描述符 + 自己的解釋

先記錄下來：

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 are627  * enumerated, fields are attached to these reports.628  */629 630 static struct hid_device *hid_parse_report(__u8 *start, unsigned size)

參考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 is248  * persistent for main-items（my comment : save env in current collection, purpose is to remove duplicated global item）. The global environment can be saved and249  * 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 };

在網上看了許多前輩的文章，就是無法理解報告描述符中的那些數字字元所表達的含義，幾經周折，總算大致瞭解了。

結合 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，網文多是如上的報告描述符，據說是用一個工具產生的，有一些還配上了中文的說明，但也多是描述性的，而不是拆分解釋這些字元所代表的含義，比如，數組第一行的 0x05, 0x01,                    // USAGE_PAGE (Generic Desktop) 

我們一眼就能看出雙斜線後面的文字，是對前面數位說明，即0x05,0x01所表達的是USAGE_PAGE (Generic Desktop) 的含義，但是，為何如此表達，則描述的不太清楚，對於熟悉的人而言，這自然不是問題，但對於新手，可就要費點功夫了。

 

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

分兩部分，0x05為一部分，表示首碼，0x01為資料部分。

0x05轉換成二進位，就是0000  01001，按照HID類協議5.3 generic item format的定義，這個位元組被分成3個部分，bit0~bit1代表的是這個首碼後面跟的資料長度，兩位可以表示最大4位元組的資料，即bsize;bit2~bit3代表的是這個首碼的類型，總共可以有三種類型：0=main，1=global，2=local，3=reserved；bit4~bit7代表首碼的tag，一般分為input(二進位的1000 00 nn,即bit4~bit7=1000,代表一個tag，bit2~bit3=00，代表main，bit0~bit1=nn，代表這個首碼後面還有nn所代表的資料)，output(二進位的
1001 00 nn)，feature(1011 00 nn)，collection(1010 00 nn)，end collection(1100 00 nn)，遵照這個原則，我們就可以解析0x05所表達的含義。

 

0x05轉換為二進位就是0000 0101,其高4位全為0，表示的tag為usage page  tag(協議45頁)，bit2~bit3=01，表示的是類型，從協議中可以知道，這是一個全域類型的item（協議36頁），bit0~bit1=01，表示的是這個首碼後面跟著的資料長度為1位元組，即0x05後面，有0x01作為這個首碼的資料部分，而0x01表示的是general desktop page(《universal serial bus HID usage table》第五頁，目錄)，因此，這兩個數字合起來就是USAGE_PAGE (Generic Desktop)的含義。

 

總之，要瞭解報告描述符，需要兩份資料：《device class definition for human interface device (HID)》, 《Universal Serial Bus HID Usage Tables》