HID报告描述符的GLOBAL ITEM

HID报告描述符的GLOBAL ITEM相当于报告描述符的全部变量，只能不更新，以前的值就永远有意义。

ITEM_PREFIX的BIT2,BIT3为01，bit4-7的值为：
GLOBAL ITEM包括的内容有：

#define ITEM_TAG_GLOBAL_USAGE_PAGE            0x0
#define ITEM_TAG_GLOBAL_LOGICAL_MINIMUM        0x1
#define ITEM_TAG_GLOBAL_LOGICAL_MAXIMUM        0x2
#define ITEM_TAG_GLOBAL_PHYSICAL_MINIMUM    0x3
#define ITEM_TAG_GLOBAL_PHYSICAL_MAXIMUM    0x4
#define ITEM_TAG_GLOBAL_UNIT_EXPONENT        0x5
#define ITEM_TAG_GLOBAL_UNIT                0x6
#define ITEM_TAG_GLOBAL_REPORT_SIZE            0x7
#define ITEM_TAG_GLOBAL_REPORT_ID            0x8
#define ITEM_TAG_GLOBAL_REPORT_COUNT        0x9
#define ITEM_TAG_GLOBAL_PUSH                0xa
#define ITEM_TAG_GLOBAL_POP                    0xb

全局项的功能比较简单，就是更新GLOBAL_ITEM_STATE GlobalItemState相关的参数。

typedef struct
{
...
    //
    // global item state
    //
    GLOBAL_ITEM_STATE GlobalItemState;
...
}HID_PARSER_CONTEXT, *PHID_PARSER_CONTEXT;

GLOBAL_ITEM_STATE的定义如下：

typedef struct __GLOBAL_ITEM_STATE_
{
    USHORT UsagePage;
    ULONG  LogicalMinimum;
    ULONG  LogicialMaximum;
    ULONG  PhysicalMinimum;
    ULONG  PhysicalMaximum;
    UCHAR  UnitExponent;
    UCHAR  Unit;
    ULONG  ReportSize;
    ULONG  ReportCount;
    UCHAR  ReportId;
    struct __GLOBAL_ITEM_STATE__ * Next;
}GLOBAL_ITEM_STATE, *PGLOBAL_ITEM_STATE;

分析过程如下：

 case ITEM_TYPE_GLOBAL:
 {
     switch (CurrentItem->Tag) {
         case ITEM_TAG_GLOBAL_USAGE_PAGE:
             Parser->Debug("[HIDPARSE] ITEM_TAG_GLOBAL_USAGE_PAGE %x\n", Data);
             ParserContext->GlobalItemState.UsagePage = Data;
             break;
         case ITEM_TAG_GLOBAL_LOGICAL_MINIMUM:
             Parser->Debug("[HIDPARSE] ITEM_TAG_GLOBAL_LOGICAL_MINIMUM %x\n", Data);
             ParserContext->GlobalItemState.LogicalMinimum = Data;
             break;
         case ITEM_TAG_GLOBAL_LOGICAL_MAXIMUM:
             Parser->Debug("[HIDPARSE] ITEM_TAG_GLOBAL_LOCAL_MAXIMUM %x\n", Data);
             ParserContext->GlobalItemState.LogicialMaximum = Data;
             break;
         case ITEM_TAG_GLOBAL_PHYSICAL_MINIMUM:
             Parser->Debug("[HIDPARSE] ITEM_TAG_GLOBAL_PHYSICAL_MINIMUM %x\n", Data);
             ParserContext->GlobalItemState.PhysicalMinimum = Data;
             break;
         case ITEM_TAG_GLOBAL_PHYSICAL_MAXIMUM:
             Parser->Debug("[HIDPARSE] ITEM_TAG_GLOBAL_PHYSICAL_MAXIMUM %x\n", Data);
             ParserContext->GlobalItemState.PhysicalMaximum = Data;
             break;
         case ITEM_TAG_GLOBAL_UNIT_EXPONENT:
             Parser->Debug("[HIDPARSE] ITEM_TAG_GLOBAL_REPORT_UNIT_EXPONENT %x\n", Data);
             ParserContext->GlobalItemState.UnitExponent = Data;
             break;
         case ITEM_TAG_GLOBAL_UNIT:
             Parser->Debug("[HIDPARSE] ITEM_TAG_GLOBAL_REPORT_UNIT %x\n", Data);
             ParserContext->GlobalItemState.Unit = Data;
             break;
         case ITEM_TAG_GLOBAL_REPORT_SIZE:
             Parser->Debug("[HIDPARSE] ITEM_TAG_GLOBAL_REPORT_SIZE %x\n", Data);
             ParserContext->GlobalItemState.ReportSize = Data;
             break;
         case ITEM_TAG_GLOBAL_REPORT_ID:
             Parser->Debug("[HIDPARSE] ITEM_TAG_GLOBAL_REPORT_ID %x\n", Data);
             ParserContext->GlobalItemState.ReportId = Data;
             ParserContext->UseReportIDs = TRUE;
             break;
         case ITEM_TAG_GLOBAL_REPORT_COUNT:
             Parser->Debug("[HIDPARSE] ITEM_TAG_GLOBAL_REPORT_COUNT %x\n", Data);
             ParserContext->GlobalItemState.ReportCount = Data;
             break;
         case ITEM_TAG_GLOBAL_PUSH:
         {
             Parser->Debug("[HIDPARSE] ITEM_TAG_GLOBAL_PUSH\n");
             //
             // allocate global item state
             //
             LinkedGlobalItemState = (PGLOBAL_ITEM_STATE)Parser->Alloc(sizeof(GLOBAL_ITEM_STATE));
             ASSERT(LinkedGlobalItemState);
             //
             // copy global item state
             //
             Parser->Copy(LinkedGlobalItemState, &ParserContext->GlobalItemState, sizeof(GLOBAL_ITEM_STATE));
             //
             // store pushed item in link member
             //
             ParserContext->GlobalItemState.Next = (struct __GLOBAL_ITEM_STATE__*)LinkedGlobalItemState;
             break;
         }
         case ITEM_TAG_GLOBAL_POP:
         {
             Parser->Debug("[HIDPARSE] ITEM_TAG_GLOBAL_POP\n");
             if (ParserContext->GlobalItemState.Next == NULL) 
             {
                 //
                 // pop without push
                 //
                 ASSERT(FALSE);
                 break;
             }
             //
             // get link
             //
             LinkedGlobalItemState = (PGLOBAL_ITEM_STATE)ParserContext->GlobalItemState.Next;
             //
             // replace current item with linked one
             //
             Parser->Copy(&ParserContext->GlobalItemState, LinkedGlobalItemState, sizeof(GLOBAL_ITEM_STATE));
             //
             // free item
             //
             Parser->Free(LinkedGlobalItemState);
             break;
         }
         default:
             //
             // unknown  / unsupported tag
             //
             ASSERT(FALSE);
             break;
     }
     break;
 }