4.2.15 Do not use hard-coded device path nodes

The ACPI() node in the EFI Device Path Protocol identifies the PCI root bridge in the ACPI namespace. The ACPI Specification allows _HID to describe vendor-specific capability and _CID to describe compatibility. Therefore, there is no requirement for all platforms to use the PNP0A03 identifier in the _HID to identify the PCI root bridge. The following are the only requirements for the PCI root bridge:
  • The PNP0A03 identifier must appear in _HID if a vendor-specific capability description isn't needed.
  • The PNP0A03 identifier must appear in _CID if _HID contains a vendor-specific identifier.
To avoid problems with platform differences, UEFI drivers should not create UEFI device paths from hard-coded information. Instead, UEFI bus drivers should append new device path nodes to the device path from the parent device handle.

4.2.15.1 PNPID byte order for UEFI

The ACPI PNPID format (byte order) follows the original EISA ID format. UEFI also uses PNPID in the device path ACPI nodes. However, for a given string, ACPI and UEFI do not generate the same numbers. For example:
HID = "PNP0501"
ACPI = 0x0105D041
EFI = 0x050141D0
The significance is that operating systems that try to match the UEFI ACPI device path node to the ACPI name space must perform a translation.
Refer to Chapter 4 of this guide for information about lengths of words on 32-bit versus 64-bit architectures.

4.2.15.2 Working with UEFI Device Path Nodes

UEFI Device Paths Nodes are not required to be aligned. If the proper coding style is used when working with device paths, a UEFI Driver can be implemented to guarantee all that fields of UEFI Device Path Nodes are accessed with natural alignment. This improves platform compatibility, especially for IPF platforms.
TIP: Do not assume that, when given a device path, that the path is aligned. Copy pieces of the device path to a known-aligned device path before accessing it. The device path may then be accessed safely. Alternatively, use EDK II BaseLib functions to perform unaligned reads and writes.