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diff --git a/Lesson_20/README.md b/Lesson_20/README.md deleted file mode 100644 index 46a4c5b..0000000 --- a/Lesson_20/README.md +++ /dev/null @@ -1,108 +0,0 @@ -The Platform Configuration Database (PCD) is a database that contains a variety of current platform settings or directives that can be accessed by a driver or application. - -You can checkout edk2 specification https://edk2-docs.gitbook.io/edk-ii-pcd-specification/ or https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/edkii-platform-config-database-entries-paper.pdf for more explanation on PCD. - -PCD entry is also called PCD, so we will use this term further. - -The PCD entry is defined in a DEC file in a format: -``` -<TokenSpaceGuidCName>.<PcdCName>|<DefaultValue>|<DatumType>|<Token> -``` -`<TokenSpaceGuidCName>` is a GUID value, `<Token>` is a 32-bit value. Together they are used to uniqely identify PCD. - -First let's declare a Token Space that would contain all our PCDs. -Usually in is defined as a `g<PackageName>TokenSpaceGuid`, so add this to our `UefiLessonsPkg/UefiLessonsPkg.dec`: -``` -[Guids] - ... - gUefiLessonsPkgTokenSpaceGuid = {0x150cab53, 0xad47, 0x4385, {0xb5, 0xdd, 0xbc, 0xfc, 0x76, 0xba, 0xca, 0xf0}} -``` - -Now we can define our PCDs in the same *.dec file. Let's start with `UINT32 PcdMyVar32 = 42`: -``` -[PcdsFixedAtBuild] - gEfiUefiLessonsPkgTokenSpaceGuid.PcdMyVar32|42|UINT32|0x00000001 -``` - -Now create an app `PCDLesson` with the following code in its entry point function: -``` -Print(L"PcdMyVar32=%d\n", FixedPcdGet32(PcdMyVar32)); -``` -To use `FixedPcdGet32` in our code we need to add the necessary include: -``` -#include <Library/PcdLib.h> -``` -If you check out this file (https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Library/PcdLib.h) you'll see that `FixedPcdGet32` is simply a define statement: -``` -#define FixedPcdGet32(TokenName) _PCD_VALUE_##TokenName -``` - -If we try to build our app now, build will fail, as we don't have such define in our app: -``` -/home/kostr/tiano/edk2/MdePkg/Include/Library/PcdLib.h:97:45: error: ‘_PCD_VALUE_PcdMyVar32’ undeclared (first use in this function) - 97 | #define FixedPcdGet32(TokenName) _PCD_VALUE_##TokenName - | ^~~~~~~~~~~ -``` - -To fix this we need to add this PCD to our app `*.inf` file: -``` -[FixedPcd] - gUefiLessonsPkgTokenSpaceGuid.PcdMyVar32 -``` - -Also we need to include "dec" file that defines this PCD: -``` -[Packages] - ... - UefiLessonsPkg/UefiLessonsPkg.dec -``` - -Now compilation would succeed. - -If you check out the content of autogenerated files `AutoGen.h`/`AutoGen.c`, you'll see, that our PCD is there. - -Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/AutoGen.h -``` -// Definition of PCDs used in this module - -#define _PCD_TOKEN_PcdMyVar32 0U -#define _PCD_SIZE_PcdMyVar32 4 -#define _PCD_GET_MODE_SIZE_PcdMyVar32 _PCD_SIZE_PcdMyVar32 -#define _PCD_VALUE_PcdMyVar32 42U -extern const UINT32 _gPcd_FixedAtBuild_PcdMyVar32; -#define _PCD_GET_MODE_32_PcdMyVar32 _gPcd_FixedAtBuild_PcdMyVar32 -//#define _PCD_SET_MODE_32_PcdMyVar32 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 -``` -// Definition of PCDs used in this module -GLOBAL_REMOVE_IF_UNREFERENCED const UINT32 _gPcd_FixedAtBuild_PcdMyVar32 = _PCD_VALUE_PcdMyVar32; -``` - -So in our case preprocessor expands code like this: -``` -FixedPcdGet32(PcdMyVar32) -> _PCD_VALUE_PcdMyVar32 -> 42U -``` - -If you execute app code under OVMF: -``` -FS0:\> PCDLesson.efi -PcdMyVar32=42 -``` - -There are multiple types of PCDs. `FixedAtBuild` PCD is only one of them. In our code we've used `FixedPcdGet` call to get PCD value, this call would only work if PCD is `FixedAtBuild`. However there is a generic `PcdGet` call that can be used to get a value of PCD regardless its type. -https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Library/PcdLib.h: -``` -#define PcdGet32(TokenName) _PCD_GET_MODE_32_##TokenName -``` - -In our case this would expand to: -``` -PcdGet32(PcdMyVar32) -> _PCD_GET_MODE_32_PcdMyVar32 -> _gPcd_FixedAtBuild_PcdMyVar32 -``` -The latter one is a variable that is defined in a `AutoGen.c` file: -``` - GLOBAL_REMOVE_IF_UNREFERENCED const UINT32 _gPcd_FixedAtBuild_PcdMyVar32 = 42U; -``` -So as you can see result would be the same. The difference is that `PcdGet` would work with other PCD types, that we would cover in the next lessons. |