This chapter covers the entry point for the different categories of UEFI drivers and their optional features impacting the implementation of the driver entry point. The most common category of UEFI driver is one that follows the UEFI driver model. This category of driver is discussed first, followed by the other major types of drivers and the optional features those drivers may choose to implement. The following categories of UEFI drivers are discussed:

• UEFI Driver that follows the UEFI Driver Model

• UEFI Runtime Driver

• Initializing driver

• Root bridge driver

• Service driver

The driver entry point is the function called when a UEFI driver is started with the StartImage() service. At this point the driver has already been loaded into memory with the LoadImage() service.

UEFI drivers use the PE/COFF image format that is defined in the Microsoft Portable Executable and Common Object File Format Specification. This format supports a single entry point in the code section of the image. The StartImage() service transfers control to the UEFI driver at this entry point.

The example below shows the entry point to a UEFI driver called AbcDriverEntryPoint(). This example is expanded upon as each of UEFI driver categories and features are discussed. The entry point to a UEFI driver is identical to the standard UEFI image entry point that is discussed in the UEFI Image Entry Point section of the UEFI Specification. The image handle of the UEFI driver and a pointer to the UEFI system table are passed into every UEFI driver. The image handle allows the UEFI driver to discover information about itself, and the pointer to the UEFI system table allows the UEFI driver to make UEFI Boot Service and UEFI Runtime Service calls.

The UEFI driver can use the UEFI boot services to access the protocol interfaces that are installed in the handle database, which allows the UEFI driver to use the services that are provided through the various protocol interfaces.

Example 86-UEFI Driver Entry Point

#include <Uefi.h>
​
/**
  This is the declaration of an EFI image entry point. This entry point
  Is the same for UEFI Applications, UEFI OS Loaders, and UEFI Drivers including both device drivers and bus drivers.
  @param  ImageHandle  The firmware allocated handle for the UEFI image. @param SystemTable A pointer to the EFI System Table.
  @retval  EFI_SUCCESS  The operation completed successfully. @retval Others An unexpected error occurred.
**/
EFI_STATUS
EFIAPI
AbcDriverEntryPoint (
  IN EFI_HANDLE        ImageHandle,
  IN EFI_SYSTEM_TABLE  *SystemTable
  )
{
  return EFI_SUCCESS;
}

The name of the driver entry point function must be declared in the [Defines] section of the INF file associated with the UEFI Driver. The example below shows the INF file that defines ENTRY_POINT to the AbcDriverEntryPoint() function shown in the previous example. Where applicable, this INF file example is expanded upon as each of UEFI driver categories and features are discussed. See Section 30.3 for more details on UEFI Driver INF files and Appendix A for a complete template of an INF file for a UEFI Driver.

Example 87-UEFI Driver INF File

[Defines]
  INF_VERSION    = 0x00010005
  BASE_NAME      = AbcDriverMinimum
  FILE_GUID      = DA87D340-15C0-4824-9BF3-D52286674BEF
  MODULE_TYPE    = UEFI_DRIVER
  VERSION_STRING = 1.0
  ENTRY_POINT    = AbcDriverEntryPoint
​
[Sources]
  Abc.c
​
[Packages]
  MdePkg/MdePkg.dec
​
[LibraryClasses]
  UefiDriverEntryPoint