Add lesson 22

Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>

2 files changed, 118 insertions, 0 deletions

diff --git a/Lesson_22/README.md b/Lesson_22/README.md
new file mode 100644
index 0000000..5280e72
--- /dev/null
+++ b/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.
+
diff --git a/README.md b/README.md
index 8ece2bd..2160c4f 100644
--- a/README.md
+++ b/README.md
@@ -24,6 +24,7 @@ Lessons description:
 - Lesson 19: Boot option modification with the help of `bcfg` command
 - Lesson 20: Intro to Platfrom Configuration Database (PCD). Declare and get simple fixed PCD
 - Lesson 21: Override order for PCD variables
+- Lesson 22: Feature flag PCD and its comparision to BOOLEAN fixed at build PCD
 
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