To reduce the size of the firmware and also to speed up the boot process (it is sometimes faster to decompress a compressed file than to copy the original file from a block storage), it is often common to compress the DXE & BDS modules.

To speed up the boot process on platform with slow CPU (eg: FPGA platform), it might help to disable the decompression and copy the image directly into DRAM.
This section will use ArmPlatformPkg/ArmVExpressPkg/ArmVExpress-RTSM-AEMv8Ax4.fdf as an example.

At the end of the build process you will see these three FVs (Firmware Volume):

FV Space InformationFVMAIN_SEC [14%Full] 524288 total, 77504 used, 446784 freeFVMAIN_COMPACT [45%Full] 2621440 total, 1184312 used, 1437128 freeFVMAIN [99%Full] 4004224 total, 4004168 used, 56 free

FVMAIN_SEC is the secure part of the firmware that should live into Trusted Memory.
FVMAIN is the compressed image. This FV image is added in FVMAIN_COMPACT.

The compression is driven by the Guided Section in the FDF file. See the section '[FV.FVMAIN_COMPACT]':

  FILE FV_IMAGE = 9E21FD93-9C72-4c15-8C4B-E77F1DB2D792 {    SECTION GUIDED EE4E5898-3914-4259-9D6E-DC7BD79403CF PROCESSING_REQUIRED = TRUE {      SECTION FV_IMAGE = FVMAIN    }  }

The GUID 'EE4E5898-3914-4259-9D6E-DC7BD79403CF' corresponds to the LZMA decompression (defined in IntelFrameworkModulePkg/IntelFrameworkModulePkg.dec).
This section says it will compress the FV image named 'FVMAIN' into a file named 'FV_IMAGE'.

The FVMAIN FV image is defined above by the section '[FV.FvMain]'.

1. Flatten the FDF file: Move the content of FVMAIN into FVMAIN_COMPACT:

- remove FV_IMAGE file from FVMAIN_COMPACT

- move all the files from FVMAIN to FVMAIN_COMPACT

If you build the UEFI firmware after this step, you might see your firmware does not fit anymore in the size defined by the FDF file:

Generating FVMAIN_COMPACT FV###########################################################################################################################Return Value = 2GenFv: ERROR 3000: Invalid  the required fv image size 0x46ede8 exceeds the set fv image size 0x280000

2. Increase the size of the FDF file

From:

[FD.RTSM_VE_AEMv8_EFI]BaseAddress   = 0x00000000|gArmTokenSpaceGuid.PcdFdBaseAddress  # The base address of the Firmware in NOR Flash.Size          = 0x00300000|gArmTokenSpaceGuid.PcdFdSize         # The size in bytes of the FLASH Device(...)BlockSize     = 0x00001000NumBlocks     = 0x300(...)0x00080000|0x00280000gArmTokenSpaceGuid.PcdFvBaseAddress|gArmTokenSpaceGuid.PcdFvSize

To:

[FD.RTSM_VE_AEMv8_EFI]BaseAddress   = 0x00000000|gArmTokenSpaceGuid.PcdFdBaseAddress  # The base address of the Firmware in NOR Flash.Size          = 0x00580000|gArmTokenSpaceGuid.PcdFdSize         # The size in bytes of the FLASH Device(...)BlockSize     = 0x00001000NumBlocks     = 0x580(...)0x00080000|0x00500000gArmTokenSpaceGuid.PcdFvBaseAddress|gArmTokenSpaceGuid.PcdFvSize

3. Build the new firmware:

FV Space InformationFVMAIN_SEC [14%Full] 524288 total, 77504 used, 446784 freeFVMAIN_COMPACT [88%Full] 5242880 total, 4648424 used, 594456 free