Now let's explore `PatchableInModule` PCD. Add PCD to DEC file UefiLessonsPkg/UefiLessonsPkg.dec: ``` [PcdsPatchableInModule] gUefiLessonsPkgTokenSpaceGuid.PcdMyPatchableVar32|0x31313131|UINT32|0x00000004 ``` Populate it to INF UefiLessonsPkg/PCDLesson/PCDLesson.inf: ``` [PatchPcd] gUefiLessonsPkgTokenSpaceGuid.PcdMyPatchableVar32 ``` To get a value of PCD in a *.c file either `PatchPcdGet` or generic `PcdGet` should be used. Let's test both methods: ``` Print(L"PcdMyPatchableVar32=%d\n", PatchPcdGet32(PcdMyPatchableVar32)); Print(L"PcdMyPatchableVar32=%d\n", PcdGet32(PcdMyPatchableVar32)); ``` Now build and look to AutoGen files: Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/AutoGen.h ``` #define _PCD_TOKEN_PcdMyPatchableVar32 0U #define _PCD_PATCHABLE_VALUE_PcdMyPatchableVar32 ((UINT32)0x31313131U) extern volatile UINT32 _gPcd_BinaryPatch_PcdMyPatchableVar32; #define _PCD_GET_MODE_32_PcdMyPatchableVar32 _gPcd_BinaryPatch_PcdMyPatchableVar32 #define _PCD_PATCHABLE_PcdMyPatchableVar32_SIZE 4 #define _PCD_GET_MODE_SIZE_PcdMyPatchableVar32 _gPcd_BinaryPatch_Size_PcdMyPatchableVar32 extern UINTN _gPcd_BinaryPatch_Size_PcdMyPatchableVar32; #define _PCD_SET_MODE_32_PcdMyPatchableVar32(Value) (_gPcd_BinaryPatch_PcdMyPatchableVar32 = (Value)) #define _PCD_SET_MODE_32_S_PcdMyPatchableVar32(Value) ((_gPcd_BinaryPatch_PcdMyPatchableVar32 = (Value)), RETURN_SUCCESS) ``` Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/AutoGen.c ``` volatile UINT32 _gPcd_BinaryPatch_PcdMyPatchableVar32 = _PCD_PATCHABLE_VALUE_PcdMyPatchableVar32; GLOBAL_REMOVE_IF_UNREFERENCED UINTN _gPcd_BinaryPatch_Size_PcdMyPatchableVar32 = 4; ``` So in this case our call to `PcdGet32(PcdMyPatchableVar32)` would be expanded to: ``` PcdGet32(PcdMyPatchableVar32) --> _PCD_GET_MODE_32_PcdMyPatchableVar32 --> _gPcd_BinaryPatch_PcdMyPatchableVar32 ``` This variable would be assigned in `AutoGen.c`: ``` volatile UINT32 _gPcd_BinaryPatch_PcdMyPatchableVar32 = ((UINT32)0x31313131U); ``` So the main difference from the FixedAtBuild and FeatureFlag PCDs is that variable defined as `volatile` and set functions aren't blocked. Let's try to set our PCD then. As with get, there are two possibilities. You can use either `PatchPcdSet` or generic `PcdSetS`. Keep in mind that `PcdSet32S` unravels to a macro returning a value. https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Library/PcdLib.h: ``` #define PcdSet32S(TokenName, Value) _PCD_SET_MODE_32_S_##TokenName ((Value)) ``` Therefore: ``` PcdSet32S(PcdMyPatchableVar32, 44) --> _PCD_SET_MODE_32_S_PcdMyPatchableVar32 ((Value)) --> ((_gPcd_BinaryPatch_PcdMyPatchableVar32 = (Value)), RETURN_SUCCESS) ``` So if you don't want such error: ``` /home/kostr/tiano/edk2/Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/AutoGen.h:108:106: error: right-hand operand of comma expression has no effect [-Werror=unused-value] 108 | #define _PCD_SET_MODE_32_S_PcdMyPatchableVar32(Value) ((_gPcd_BinaryPatch_PcdMyPatchableVar32 = (Value)), RETURN_SUCCESS) | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~. ``` You should use something like this: ``` RETURN_STATUS PcdStatus = PcdSet32S(PcdMyPatchableVar32, 44); Print(L"PcdStatus=%r\n", PcdStatus); ``` Alternatively you can use `PatchPcdSet32` which don't require such thing as it is defined as follows: ``` #define PatchPcdSet32(TokenName, Value) (_gPcd_BinaryPatch_##TokenName = (Value)) ``` Test both set methods in app code: ``` Print(L"PcdMyPatchableVar32=0x%x\n", PcdGet32(PcdMyPatchableVar32)); RETURN_STATUS PcdStatus = PcdSet32S(PcdMyPatchableVar32, 44); Print(L"PcdStatus=%r\n", PcdStatus); Print(L"PcdMyPatchableVar32=%d\n", PcdGet32(PcdMyPatchableVar32)); PatchPcdSet32(PcdMyPatchableVar32, 45); Print(L"PcdMyPatchableVar32=%d\n", PatchPcdGet32(PcdMyPatchableVar32)); ``` Now if you execute our app under OVMF prints for patchable PCDs would output: ``` PcdMyPatchableVar32=0x31313131 PcdStatus=Success PcdMyPatchableVar32=44 PcdMyPatchableVar32=45 ``` _________________________________ In case you've wondered why I've assigned a hex value for PCD default value or why this PCD type is named `PatchableInModule` this section is for you. This PCD type is named like that because the value of this PCD can be changed in a binary PE/COFF image (*.efi file). To do this two utilities are used: - `GenPatchPcdTable` - this tool is used to get the patchable PCD offset for the EFI image by parsing the map file - `PatchPcdValue` - this tool is used to actually patch PCD value You can download rtf manuals for these utilities from edk2 repo: -https://github.com/tianocore/edk2/blob/master/BaseTools/UserManuals/GenPatchPcdTable_Utility_Man_Page.rtf -https://github.com/tianocore/edk2/blob/master/BaseTools/UserManuals/PatchPcdValue_Utility_Man_Page.rtf Let's start with `GenPatchPcdTable`. First check out help for this tool: ``` $ ./BaseTools/BinWrappers/PosixLike/GenPatchPcdTable -h Usage: GenPatchPcdTable.py -m -e -o Copyright (c) 2008 - 2018, Intel Corporation. All rights reserved. Options: --version show program's version number and exit -h, --help show this help message and exit -m MAPFILE, --mapfile=MAPFILE Absolute path of module map file. -e EFIFILE, --efifile=EFIFILE Absolute path of EFI binary file. -o OUTFILE, --outputfile=OUTFILE Absolute path of output file to store the got patchable PCD table. ``` Now let's create a `PatchPcdTable` for our app: For the *.efi file we can use one of: ``` Build/UefiLessonsPkg/RELEASE_GCC5/X64/PCDLesson.efi Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/OUTPUT/PCDLesson.efi Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/PCDLesson.efi ``` For the *.map file we can use one of: ``` Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/PCDLesson.map Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/OUTPUT/PCDLesson.map ``` In case you wonder how our PCD can be found in a map file, execute: ``` $ grep MyPatchableVar32 Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/PCDLesson.map -A2 .data._gPcd_BinaryPatch_PcdMyPatchableVar32 0x0000000000001920 0x4 /tmp/PCDLesson.dll.m6t8YL.ltrans0.ltrans.o *fill* 0x0000000000001924 0xc ``` So as you can see the default value for our PCD is placed under 0x1920 offset. Now let's execute `GenPatchPcdTable` and create a file `PCDLessonPatchPcdTable` in app Build DEBUG folder: ``` ./BaseTools/BinWrappers/PosixLike/GenPatchPcdTable \ -m Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/PCDLesson.map \ -e Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/PCDLesson.efi \ -o Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/PCDLessonPatchPcdTable ``` Checkout the created `PCDLessonPatchPcdTable` file. ``` $ cat Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/PCDLessonPatchPcdTable PCD Name Offset Section Name PcdMyPatchableVar32 0x1920 .data ``` As you can see it has the same offset that we've seen in a map file. Now let's get a final look at our default PCD value in an *.elf file: ``` hexdump -s 0x1920 Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/PCDLesson.efi -n 4 0001920 3131 3131 0001924 ``` Checkout help for the `PatchPcdValue` tool: ``` $ ./BaseTools/BinWrappers/PosixLike/PatchPcdValue -h Usage: PatchPcdValue.py -f Offset -u Value -t Type [-s MaxSize] Copyright (c) 2010 - 2018, Intel Corporation. All rights reserved. Options: -f PCDOFFSET, --offset=PCDOFFSET Start offset to the image is used to store PCD value. -u PCDVALUE, --value=PCDVALUE PCD value will be updated into the image. -t PCDTYPENAME, --type=PCDTYPENAME The name of PCD data type may be one of VOID*,BOOLEAN, UINT8, UINT16, UINT32, UINT64. -s PCDMAXSIZE, --maxsize=PCDMAXSIZE Max size of data buffer is taken by PCD value.It must be set when PCD type is VOID*. -v, --verbose Run verbosely -d LOGLEVEL, --debug=LOGLEVEL Run with debug information -q, --quiet Run quietly -? show this help message and exit --version show program's version number and exit -h, --help show this help message and exit ``` Now use it to patch our PCD in an *.elf file: ``` ./BaseTools/BinWrappers/PosixLike/PatchPcdValue \ --offset=0x1920 \ --value=0xDEADDEAD \ --type=UINT32 \ Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/PCDLesson.efi ``` Look at hexdump again: ``` $ hexdump -s 0x1920 Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/PCDLesson.efi -n 4 0001920 dead dead 0001924 ``` We've successfully changed our PCD in a binary! Now if you execute our app under OVMF prints for patchable PCDs would output: ``` PcdMyPatchableVar32=0xDEADDEAD PcdStatus=Success PcdMyPatchableVar32=44 PcdMyPatchableVar32=45 ```