7.6 Communicate with DXE Modules

There are three methods for PEIMs to communicate with each other: PPIs, HOBs and PCDs.

7.6.1 HOB

PEIMs can pass some information to the DXE Foundation and DXE modules, such as the information of a memory bank discovered in PEI phase, by using a Hand-Off Block (HOB).

In EDK II, the Hob Library provides a set of interfaces to help to build Hobs, such as BuildGuidHob(). For example:

EFI_MEMINIT_CONFIG_DATA *ConfigData;
EFI_PEI_HOB_POINTERS Hob;
UINTN BufferSize;
BufferSize = sizeof (EFI_MEMINIT_CONFIG_DATA);
Hob.Raw = BuildGuidHob (
            &gEfiMemoryConfigDataGuid,
            BufferSize
            );
ASSERT (Hob.Raw);
ConfigData = (EFI_MEMINIT_CONFIG_DATA *) Hob.Raw;
CopyMem (
  ConfigData->SpdData,
  SpdData,
  sizeof (MEMINIT_SPD_DATA) * MAX_SOCKETS
  );

In EDK II, the Hob Library also provides a set of APIs to locate HOBs for PEIMs and DXE drivers.

7.6.2 Variable

PEIMs can read variables previously assigned by DXE drivers. PEIMs cannot write variables.

PEIMs can use ReadOnlyVariable2 PPI to obtain variables. Follow these steps:

Locate ReadOnlyVariable2 PPI.
Invoke GetVariable() with size is 0, to get variable's actual size.
Allocate memory for variable.
Invoke GetVariable(), again with actual size to get the variable.

The following is one example of how to obtain the variable.

Status = PeiServicesLocatePpi (
           &gEfiPeiReadOnlyVariable2PpiGuid,
           0,
           NULL,
           (VOID **) &VariablePpi
           );
ASSERT_EFI_ERROR (Status);
Size = 0;
Status = VariablePpi->GetVariable (
                        VariablePpi,
                        VariableName,
                        (EFI_GUID *) VariableGuid,
                        NULL,
                        &Size,
                        NULL
                        );
if (Status == EFI_BUFFER_TOO_SMALL)
{
  Status = PeiServicesAllocatePool (Size, &Buffer);
  ASSERT_EFI_ERROR (Status);
  Status = VariablePpi->GetVariable (
                          VariablePpi,
                          (UINT16 *) VariableName,
                          (EFI_GUID *) VariableGuid,
                          NULL,
                          &Size,
                          Buffer
                          );
  ASSERT_EFI_ERROR (Status);
  *VariableSize = Size;
  *VariableData = Buffer;
}

7.6.3 PCD

PEIMs can communicate with DXE drivers through dynamic PCDs. As with variables, PEIMs can get dynamic PCDs values that were previously set by DXE drivers. The usage for obtaining PCDs is covered in Appendix A .

7.6 Communicate with DXE Modules