7.5 Communicate between PEIM Modules

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

7.5.1 PPI

PEIM modules can communicate with each other using a structure called a PEIM-toPEIM Interface (PPI). Each PPI has one GUID. The Pei Service Table provides some Pei services to access the PPI database.

In EDK II, one PEIM module can invoke PeiServicesInstallPpi() to publish its PPI services into the PPI database by GUID. Another PEIM module can invoke PeiServicesLocatePpi()to locate PPI services from the PPI database by GUID.

7.5.1.1 Installing a PPI

For example, if Module A wants to publish one Template PPI service (including three APIs: Interface2, Interface2 and Interface3), it can install the Template PPI by using PeiServicesInstallPpi. For example:

//
// Template PPI
//
EFI_PEI_TEMPLATE_PPI gEfiTemplatePpi = {
  Interface1,
  Interface2,
  Interface3
};
EFI_PEI_PPI_DESCRIPTOR gPpiListTemplatePpi = {
  (EFI_PEI_PPI_DESCRIPTOR_PPI | EFI_PEI_PPI_DESCRIPTOR_TERMINATE_LIST),
  &gEfiTemplateGuid,
  &gEfiTemplatePpi
};
EFI_STATUS
PeimEntryPoint (
  IN EFI_FFS_FILE_HEADER  *FfsHeader,
  IN EFI_PEI_SERVICES     **PeiServices
  )
{
  EFI_STATUS  Status;
  //
  // Publish Template PPI.
  //
  Status = PeiServicesInstallPpi (&gPpiListTemplatePpi);
  return Status;
}

7.5.1.2 Locating a PPI

If Module B needs to invoke Interface2() provided by Template PPI, it can locate Template PPI by using the following code:

//
// Get Template PPI
//
Status = PeiServicesLocatePpi (
           &gEfiTemplateGuid,
           0,
           NULL,
           (VOID **)&TemplatePpi
           );
ASSERT_EFI_ERROR (Status);
//
// Invoke Interface2()
//
Status = TemplatePpi->Interface2 (...);
ASSERT_EFI_ERROR (Status);

7.5.2 HOB

PEIM modules can build a Hand-Off Block (HOB) to pass some information to the DXE Foundation and DXE modules. In addition, other PEIMs can obtain similar information from a HOB by using the HOB services in the Pei Service Table.

In EDK II, the Hob Library provides the generic interfaces to access HOBs for PEIMs and DXE drivers.

7.5.3 PCD

A PEIM can communicate with other PEIMs through dynamic PCDs. As with HOBs, only PEIMs can obtain dynamic PCDs values, which were previously set by DXE drivers. The usage of getting PCDs is introduced in Appendix A, Dynamic PCD.

7.5 Communicate between PEIM Modules