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Copyright (c) 2018 Heinrich Schuchardt
SPDX-License-Identifier: GPL-2.0+
# UEFI on U-Boot
The Unified Extensible Firmware Interface Specification (UEFI) [1] has become
the default for booting on AArch64 and x86 systems. It provides a stable API for
the interaction of drivers and applications with the firmware. The API comprises
access to block storage, network, and console to name a few. The Linux kernel
and boot loaders like GRUB or the FreeBSD loader can be executed.
## Building for UEFI
The UEFI standard supports only little endian systems. The UEFI support can be
activated for ARM and x86 by specifying
in the .config file.
Support for attaching virtual block devices, e.g. iSCSI drives connected by the
loaded UEFI application [3], requires
### Executing a UEFI binary
The bootefi command is used to start UEFI applications or to install UEFI
drivers. It takes two parameters
bootefi <image address> [fdt address]
* image address - the memory address of the UEFI binary
* fdt address - the memory address of the flattened device tree
Below you find the output of an example session starting GRUB.
=> load mmc 0:2 ${fdt_addr_r} boot/dtb
29830 bytes read in 14 ms (2 MiB/s)
=> load mmc 0:1 ${kernel_addr_r} efi/debian/grubaa64.efi
reading efi/debian/grubaa64.efi
120832 bytes read in 7 ms (16.5 MiB/s)
=> bootefi ${kernel_addr_r} ${fdt_addr_r}
The environment variable 'bootargs' is passed as load options in the UEFI system
table. The Linux kernel EFI stub uses the load options as command line
### Executing the boot manager
The UEFI specfication foresees to define boot entries and boot sequence via UEFI
variables. Booting according to these variables is possible via
bootefi bootmgr [fdt address]
As of U-Boot v2018.03 UEFI variables are not persisted and cannot be set at
### Executing the built in hello world application
A hello world UEFI application can be built with
It can be embedded into the U-Boot binary with
The bootefi command is used to start the embedded hello world application.
bootefi hello [fdt address]
Below you find the output of an example session.
=> bootefi hello ${fdtcontroladdr}
## Starting EFI application at 01000000 ...
WARNING: using memory device/image path, this may confuse some payloads!
Hello, world!
Running on UEFI 2.7
Have SMBIOS table
Have device tree
Load options: root=/dev/sdb3 init=/sbin/init rootwait ro
## Application terminated, r = 0
The environment variable fdtcontroladdr points to U-Boot's internal device tree
(if available).
### Executing the built-in selftest
An UEFI selftest suite can be embedded in U-Boot by building with
For testing the UEFI implementation the bootefi command can be used to start the
bootefi selftest [fdt address]
The environment variable 'efi_selftest' can be used to select a single test. If
it is not provided all tests are executed except those marked as 'on request'.
If the environment variable is set to 'list' a list of all tests is shown.
Below you can find the output of an example session.
=> setenv efi_selftest simple network protocol
=> bootefi selftest
Testing EFI API implementation
Selected test: 'simple network protocol'
Setting up 'simple network protocol'
Setting up 'simple network protocol' succeeded
Executing 'simple network protocol'
DHCP Discover
DHCP reply received from (52:55:c0:a8:4c:02)
as broadcast message.
Executing 'simple network protocol' succeeded
Tearing down 'simple network protocol'
Tearing down 'simple network protocol' succeeded
Boot services terminated
Summary: 0 failures
Preparing for reset. Press any key.
## The UEFI life cycle
After the U-Boot platform has been initialized the UEFI API provides two kinds
of services
* boot services and
* runtime services.
The API can be extended by loading UEFI drivers which come in two variants
* boot drivers and
* runtime drivers.
UEFI drivers are installed with U-Boot's bootefi command. With the same command
UEFI applications can be executed.
Loaded images of UEFI drivers stay in memory after returning to U-Boot while
loaded images of applications are removed from memory.
An UEFI application (e.g. an operating system) that wants to take full control
of the system calls ExitBootServices. After a UEFI application calls
* boot services are not available anymore
* timer events are stopped
* the memory used by U-Boot except for runtime services is released
* the memory used by boot time drivers is released
So this is a point of no return. Afterwards the UEFI application can only return
to U-Boot by rebooting.
## The UEFI object model
UEFI offers a flexible and expandable object model. The objects in the UEFI API
are devices, drivers, and loaded images. These objects are referenced by
The interfaces implemented by the objects are referred to as protocols. These
are identified by GUIDs. They can be installed and uninstalled by calling the
appropriate boot services.
Handles are created by the InstallProtocolInterface or the
InstallMultipleProtocolinterfaces service if NULL is passed as handle.
Handles are deleted when the last protocol has been removed with the
UninstallProtocolInterface or the UninstallMultipleProtocolInterfaces service.
Devices offer the EFI_DEVICE_PATH_PROTOCOL. A device path is the concatenation
of device nodes. By their device paths all devices of a system are arranged in a
Drivers offer the EFI_DRIVER_BINDING_PROTOCOL. This protocol is used to connect
a driver to devices (which are referenced as controllers in this context).
Loaded images offer the EFI_LOADED_IMAGE_PROTOCOL. This protocol provides meta
information about the image and a pointer to the unload callback function.
## The UEFI events
In the UEFI terminology an event is a data object referencing a notification
function which is queued for calling when the event is signaled. The following
types of events exist:
* periodic and single shot timer events
* exit boot services events, triggered by calling the ExitBootServices() service
* virtual address change events
* memory map change events
* read to boot events
* reset system events
* system table events
* events that are only triggered programmatically
Events can be created with the CreateEvent service and deleted with CloseEvent
Events can be assigned to an event group. If any of the events in a group is
signaled, all other events in the group are also set to the signaled state.
## The UEFI driver model
A driver is specific for a single protocol installed on a device. To install a
driver on a device the ConnectController service is called. In this context
controller refers to the device for which the driver is installed.
The relevant drivers are identified using the EFI_DRIVER_BINDING_PROTOCOL. This
protocol has has three functions:
* supported - determines if the driver is compatible with the device
* start - installs the driver by opening the relevant protocol with
* stop - uninstalls the driver
The driver may create child controllers (child devices). E.g. a driver for block
IO devices will create the device handles for the partitions. The child
controllers will open the supported protocol with the attribute
A driver can be detached from a device using the DisconnectController service.
## U-Boot devices mapped as UEFI devices
Some of the U-Boot devices are mapped as UEFI devices
* block IO devices
* console
* graphical output
* network adapter
As of U-Boot 2018.03 the logic for doing this is hard coded.
The development target is to integrate the setup of these UEFI devices with the
U-Boot driver model. So when a U-Boot device is discovered a handle should be
created and the device path protocol and the relevant IO protocol should be
installed. The UEFI driver then would be attached by calling ConnectController.
When a U-Boot device is removed DisconnectController should be called.
## UEFI devices mapped as U-Boot devices
UEFI drivers binaries and applications may create new (virtual) devices, install
a protocol and call the ConnectController service. Now the matching UEFI driver
is determined by iterating over the implementations of the
It is the task of the UEFI driver to create a corresponding U-Boot device and to
proxy calls for this U-Boot device to the controller.
In U-Boot 2018.03 this has only been implemented for block IO devices.
### UEFI uclass
An UEFI uclass driver (lib/efi_driver/efi_uclass.c) has been created that
takes care of initializing the UEFI drivers and providing the
EFI_DRIVER_BINDING_PROTOCOL implementation for the UEFI drivers.
A linker created list is used to keep track of the UEFI drivers. To create an
entry in the list the UEFI driver uses the U_BOOT_DRIVER macro specifying
UCLASS_EFI as the ID of its uclass, e.g.
/* Identify as UEFI driver */
U_BOOT_DRIVER(efi_block) = {
.name = "EFI block driver",
.ops = &driver_ops,
The available operations are defined via the structure struct efi_driver_ops.
struct efi_driver_ops {
const efi_guid_t *protocol;
const efi_guid_t *child_protocol;
int (*bind)(efi_handle_t handle, void *interface);
When the supported() function of the EFI_DRIVER_BINDING_PROTOCOL is called the
uclass checks if the protocol GUID matches the protocol GUID of the UEFI driver.
In the start() function the bind() function of the UEFI driver is called after
checking the GUID.
The stop() function of the EFI_DRIVER_BINDING_PROTOCOL disconnects the child
controllers created by the UEFI driver and the UEFI driver. (In U-Boot v2013.03
this is not yet completely implemented.)
### UEFI block IO driver
The UEFI block IO driver supports devices exposing the EFI_BLOCK_IO_PROTOCOL.
When connected it creates a new U-Boot block IO device with interface type
IF_TYPE_EFI, adds child controllers mapping the partitions, and installs the
EFI_SIMPLE_FILE_SYSTEM_PROTOCOL on these. This can be used together with the
software iPXE to boot from iSCSI network drives [3].
This driver is only available if U-Boot is configured with
## TODOs as of U-Boot 2018.03
* unimplemented or incompletely implemented boot services
* Exit - call unload function, unload applications only
* ReinstallProtocolInterface
* UnloadImage
* unimplemented events
* event groups
* data model
* manage events in a linked list
* manage configuration tables in a linked list
* UEFI drivers
* support DisconnectController for UEFI block devices.
* support for CONFIG_EFI_LOADER in the sandbox (CONFIG_SANDBOX=y)
* UEFI variables
* persistence
* runtime support
* support bootefi booting ARMv7 in non-secure mode (CONFIG_ARMV7_NONSEC=y)
## Links
* [1]( - UEFI specifications
* [2](./driver-model/README.txt) doc/driver-model/README.txt - Driver model
* [3](./README.iscsi) doc/README.iscsi - iSCSI booting with U-Boot and iPXE