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diff --git a/Lessons/Lesson_20/README.md b/Lessons/Lesson_20/README.md new file mode 100644 index 0000000..46a4c5b --- /dev/null +++ b/Lessons/Lesson_20/README.md @@ -0,0 +1,108 @@ +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. |