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diff --git a/Lessons/Lesson_22/README.md b/Lessons/Lesson_22/README.md new file mode 100644 index 0000000..5280e72 --- /dev/null +++ b/Lessons/Lesson_22/README.md @@ -0,0 +1,117 @@ +Let's explore another PCD type - feature flag PCD. + +Add PCD to DEC file UefiLessonsPkg/UefiLessonsPkg.dec: +``` +[PcdsFeatureFlag] + gUefiLessonsPkgTokenSpaceGuid.PcdMyFeatureFlagVar|FALSE|BOOLEAN|0x20000001 +``` + +Populate it to INF UefiLessonsPkg/PCDLesson/PCDLesson.inf: +``` +[FeaturePcd] + gUefiLessonsPkgTokenSpaceGuid.PcdMyFeatureFlagVar +``` + +To get PCD value in a *.c file either `FeaturePcdGet` or `PcdGetBool` can be used which are practically the same https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Library/PcdLib.h: +``` +#define FeaturePcdGet(TokenName) _PCD_GET_MODE_BOOL_##TokenName +#define PcdGetBool(TokenName) _PCD_GET_MODE_BOOL_##TokenName +``` +Let's add both to our UefiLessonsPkg/PCDLesson/PCDLesson.c +``` +Print(L"PcdMyFeatureFlagVar=%d\n", FeaturePcdGet(PcdMyFeatureFlagVar)); +Print(L"PcdMyFeatureFlagVar=%d\n", PcdGetBool(PcdMyFeatureFlagVar)); +``` +Ok, we are ready to build. Build your module and check out AutoGen files. + +Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/AutoGen.h +``` +#define _PCD_TOKEN_PcdMyFeatureFlagVar 0U +#define _PCD_SIZE_PcdMyFeatureFlagVar 1 +#define _PCD_GET_MODE_SIZE_PcdMyFeatureFlagVar _PCD_SIZE_PcdMyFeatureFlagVar +#define _PCD_VALUE_PcdMyFeatureFlagVar ((BOOLEAN)0U) +extern const BOOLEAN _gPcd_FixedAtBuild_PcdMyFeatureFlagVar; +#define _PCD_GET_MODE_BOOL_PcdMyFeatureFlagVar _gPcd_FixedAtBuild_PcdMyFeatureFlagVar +//#define _PCD_SET_MODE_BOOL_PcdMyFeatureFlagVar ASSERT(FALSE) // It is not allowed to set value for a FIXED_AT_BUILD PCD +``` + +Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/AutoGen.c +``` +GLOBAL_REMOVE_IF_UNREFERENCED const BOOLEAN _gPcd_FixedAtBuild_PcdMyFeatureFlagVar = _PCD_VALUE_PcdMyFeatureFlagVar; +``` + +So both calls for our PCD would translate to: +``` +FeaturePcdGet/PcdGetBool(PcdMyFeatureFlagVar) --> _PCD_GET_MODE_BOOL_PcdMyFeatureFlagVar --> _gPcd_FixedAtBuild_PcdMyFeatureFlagVar +``` +And this variable is declared in AutoGen.c file: +``` +GLOBAL_REMOVE_IF_UNREFERENCED const BOOLEAN _gPcd_FixedAtBuild_PcdMyFeatureFlagVar = ((BOOLEAN)0U); +``` + +If you execute it under OVMF these print statements would get you: +``` +PcdMyFeatureFlagVar=0 +PcdMyFeatureFlagVar=0 +``` + +________________________ + +So as you can see a feature PCD is similar to a BOOLEAN PCD defined under `PcdsFixedAtBuild` section + +Let's create a fixed bool PCD and compare results: + +UefiLessonsPkg/UefiLessonsPkg.dec: +``` +[PcdsFixedAtBuild] + ... + gUefiLessonsPkgTokenSpaceGuid.PcdMyVarBool|FALSE|BOOLEAN|0x00000004 + +[PcdsFeatureFlag] + gUefiLessonsPkgTokenSpaceGuid.PcdMyFeatureFlagVar|FALSE|BOOLEAN|0x20000001 +``` +UefiLessonsPkg/PCDLesson/PCDLesson.inf: +``` +[FixedPcd] + ... + gUefiLessonsPkgTokenSpaceGuid.PcdMyVarBool + +[FeaturePcd] + gUefiLessonsPkgTokenSpaceGuid.PcdMyFeatureFlagVar +``` +UefiLessonsPkg/PCDLesson/PCDLesson.c: +``` +Print(L"PcdMyFeatureFlagVar=%d\n", FeaturePcdGet(PcdMyFeatureFlagVar)); +Print(L"PcdMyFeatureFlagVar=%d\n", PcdGetBool(PcdMyFeatureFlagVar)); +Print(L"PcdMyVarBool=%d\n", FixedPcdGetBool(PcdMyVarBool)); +Print(L"PcdMyVarBool=%d\n", PcdGetBool(PcdMyVarBool)); +``` + +Ok, now let's build and look at AutoGen files: + +AutoGen.c +``` +GLOBAL_REMOVE_IF_UNREFERENCED const BOOLEAN _gPcd_FixedAtBuild_PcdMyVarBool = _PCD_VALUE_PcdMyVarBool; +GLOBAL_REMOVE_IF_UNREFERENCED const BOOLEAN _gPcd_FixedAtBuild_PcdMyFeatureFlagVar = _PCD_VALUE_PcdMyFeatureFlagVar; +``` +AutoGen.h +``` +#define _PCD_TOKEN_PcdMyVarBool 0U +#define _PCD_SIZE_PcdMyVarBool 1 +#define _PCD_GET_MODE_SIZE_PcdMyVarBool _PCD_SIZE_PcdMyVarBool +#define _PCD_VALUE_PcdMyVarBool 0U +extern const BOOLEAN _gPcd_FixedAtBuild_PcdMyVarBool; +#define _PCD_GET_MODE_BOOL_PcdMyVarBool _gPcd_FixedAtBuild_PcdMyVarBool +//#define _PCD_SET_MODE_BOOL_PcdMyVarBool ASSERT(FALSE) // It is not allowed to set value for a FIXED_AT_BUILD PCD + +#define _PCD_TOKEN_PcdMyFeatureFlagVar 0U +#define _PCD_SIZE_PcdMyFeatureFlagVar 1 +#define _PCD_GET_MODE_SIZE_PcdMyFeatureFlagVar _PCD_SIZE_PcdMyFeatureFlagVar +#define _PCD_VALUE_PcdMyFeatureFlagVar ((BOOLEAN)0U) +extern const BOOLEAN _gPcd_FixedAtBuild_PcdMyFeatureFlagVar; +#define _PCD_GET_MODE_BOOL_PcdMyFeatureFlagVar _gPcd_FixedAtBuild_PcdMyFeatureFlagVar +//#define _PCD_SET_MODE_BOOL_PcdMyFeatureFlagVar ASSERT(FALSE) // It is not allowed to set value for a FIXED_AT_BUILD PCD +``` + +As you see the differences are subtle. `FeaturePcd` is simply a syntactic sugar for BOOLEAN `FixedAtBuild` PCD. + |