3.9.1 How drivers use device paths

UEFI drivers that manage physical devices must be aware of device paths. When possible, UEFI drivers treat device paths as data structures. In general, UEFI Drivers are not required to parse or understand the beginning of the device path. They usually only need to understand the device path node associated with the specific controller the UEFI Driver is managing and, potentially, the device path node associated with child controllers the UEFI Driver may generate by appending a new device path node to the end of the device path from the parent controller.

  • Root bridge drivers are required only to produce the device paths for the root bridges, which typically contain only a single ACPI device path node.

  • For a child device, bus drivers usually just append a single device path node to that of the parent device. The bus drivers should not parse the contents of the parent device path. Instead, a bus driver appends the one device path node that it is required to understand to the device path of the parent device.

For example, consider a SCSI Bus Driver that produces child handles for the mass storage devices on a SCSI channel. This UEFI Driver builds a device path for each mass storage device. The device path is constructed by appending a SCSI device path node to the device path of the SCSI channel. The SCSI device path node simply contains the Physical Unit Number (PUN) and Logical Unit Number (LUN) of the SCSI mass storage device.

The mechanism described above allows the construction of device paths to be a distributed process. The bus drivers at each level of the system hierarchy are required only to understand the device path nodes for their child devices. Bus drivers understand their local view of the device path, and a group of bus drivers from each level of the system bus hierarchy work together to produce complete device paths for the console and boot devices that are used to install and boot operating systems.

There are a number of functions in the EFI Device Path Utilities Protocol defined by the UEFI Specification to help manage device paths. The MdePkg in the EDK II also provides a Device Path Library with many useful functions and macros to manage device paths.