2 SPDX-License-Identifier: GPL-2.0+
4 Copyright (c) 2018 Heinrich Schuchardt
9 The Unified Extensible Firmware Interface Specification (UEFI) [1] has become
10 the default for booting on AArch64 and x86 systems. It provides a stable API for
11 the interaction of drivers and applications with the firmware. The API comprises
12 access to block storage, network, and console to name a few. The Linux kernel
13 and boot loaders like GRUB or the FreeBSD loader can be executed.
17 The implementation of UEFI in U-Boot strives to reach the minimum requirements
18 described in "Server Base Boot Requirements System Software on ARM Platforms -
21 A full blown UEFI implementation would contradict the U-Boot design principle
26 The UEFI standard supports only little-endian systems. The UEFI support can be
27 activated for ARM and x86 by specifying
34 Support for attaching virtual block devices, e.g. iSCSI drives connected by the
35 loaded UEFI application [3], requires
40 ### Executing a UEFI binary
42 The bootefi command is used to start UEFI applications or to install UEFI
43 drivers. It takes two parameters
45 bootefi <image address> [fdt address]
47 * image address - the memory address of the UEFI binary
48 * fdt address - the memory address of the flattened device tree
50 Below you find the output of an example session starting GRUB.
52 => load mmc 0:2 ${fdt_addr_r} boot/dtb
53 29830 bytes read in 14 ms (2 MiB/s)
54 => load mmc 0:1 ${kernel_addr_r} efi/debian/grubaa64.efi
55 reading efi/debian/grubaa64.efi
56 120832 bytes read in 7 ms (16.5 MiB/s)
57 => bootefi ${kernel_addr_r} ${fdt_addr_r}
59 The environment variable 'bootargs' is passed as load options in the UEFI system
60 table. The Linux kernel EFI stub uses the load options as command line
63 ### Executing the boot manager
65 The UEFI specification foresees to define boot entries and boot sequence via UEFI
66 variables. Booting according to these variables is possible via
68 bootefi bootmgr [fdt address]
70 As of U-Boot v2018.03 UEFI variables are not persisted and cannot be set at
73 ### Executing the built in hello world application
75 A hello world UEFI application can be built with
77 CONFIG_CMD_BOOTEFI_HELLO_COMPILE=y
79 It can be embedded into the U-Boot binary with
81 CONFIG_CMD_BOOTEFI_HELLO=y
83 The bootefi command is used to start the embedded hello world application.
85 bootefi hello [fdt address]
87 Below you find the output of an example session.
89 => bootefi hello ${fdtcontroladdr}
90 ## Starting EFI application at 01000000 ...
91 WARNING: using memory device/image path, this may confuse some payloads!
96 Load options: root=/dev/sdb3 init=/sbin/init rootwait ro
97 ## Application terminated, r = 0
99 The environment variable fdtcontroladdr points to U-Boot's internal device tree
102 ### Executing the built-in self-test
104 An UEFI self-test suite can be embedded in U-Boot by building with
106 CONFIG_CMD_BOOTEFI_SELFTEST=y
108 For testing the UEFI implementation the bootefi command can be used to start the
111 bootefi selftest [fdt address]
113 The environment variable 'efi_selftest' can be used to select a single test. If
114 it is not provided all tests are executed except those marked as 'on request'.
115 If the environment variable is set to 'list' a list of all tests is shown.
117 Below you can find the output of an example session.
119 => setenv efi_selftest simple network protocol
121 Testing EFI API implementation
122 Selected test: 'simple network protocol'
123 Setting up 'simple network protocol'
124 Setting up 'simple network protocol' succeeded
125 Executing 'simple network protocol'
127 DHCP reply received from 192.168.76.2 (52:55:c0:a8:4c:02)
128 as broadcast message.
129 Executing 'simple network protocol' succeeded
130 Tearing down 'simple network protocol'
131 Tearing down 'simple network protocol' succeeded
132 Boot services terminated
134 Preparing for reset. Press any key.
136 ## The UEFI life cycle
138 After the U-Boot platform has been initialized the UEFI API provides two kinds
144 The API can be extended by loading UEFI drivers which come in two variants
149 UEFI drivers are installed with U-Boot's bootefi command. With the same command
150 UEFI applications can be executed.
152 Loaded images of UEFI drivers stay in memory after returning to U-Boot while
153 loaded images of applications are removed from memory.
155 An UEFI application (e.g. an operating system) that wants to take full control
156 of the system calls ExitBootServices. After a UEFI application calls
159 * boot services are not available anymore
160 * timer events are stopped
161 * the memory used by U-Boot except for runtime services is released
162 * the memory used by boot time drivers is released
164 So this is a point of no return. Afterwards the UEFI application can only return
165 to U-Boot by rebooting.
167 ## The UEFI object model
169 UEFI offers a flexible and expandable object model. The objects in the UEFI API
170 are devices, drivers, and loaded images. These objects are referenced by
173 The interfaces implemented by the objects are referred to as protocols. These
174 are identified by GUIDs. They can be installed and uninstalled by calling the
175 appropriate boot services.
177 Handles are created by the InstallProtocolInterface or the
178 InstallMultipleProtocolinterfaces service if NULL is passed as handle.
180 Handles are deleted when the last protocol has been removed with the
181 UninstallProtocolInterface or the UninstallMultipleProtocolInterfaces service.
183 Devices offer the EFI_DEVICE_PATH_PROTOCOL. A device path is the concatenation
184 of device nodes. By their device paths all devices of a system are arranged in a
187 Drivers offer the EFI_DRIVER_BINDING_PROTOCOL. This protocol is used to connect
188 a driver to devices (which are referenced as controllers in this context).
190 Loaded images offer the EFI_LOADED_IMAGE_PROTOCOL. This protocol provides meta
191 information about the image and a pointer to the unload callback function.
195 In the UEFI terminology an event is a data object referencing a notification
196 function which is queued for calling when the event is signaled. The following
197 types of events exist:
199 * periodic and single shot timer events
200 * exit boot services events, triggered by calling the ExitBootServices() service
201 * virtual address change events
202 * memory map change events
203 * read to boot events
204 * reset system events
205 * system table events
206 * events that are only triggered programmatically
208 Events can be created with the CreateEvent service and deleted with CloseEvent
211 Events can be assigned to an event group. If any of the events in a group is
212 signaled, all other events in the group are also set to the signaled state.
214 ## The UEFI driver model
216 A driver is specific for a single protocol installed on a device. To install a
217 driver on a device the ConnectController service is called. In this context
218 controller refers to the device for which the driver is installed.
220 The relevant drivers are identified using the EFI_DRIVER_BINDING_PROTOCOL. This
221 protocol has has three functions:
223 * supported - determines if the driver is compatible with the device
224 * start - installs the driver by opening the relevant protocol with
225 attribute EFI_OPEN_PROTOCOL_BY_DRIVER
226 * stop - uninstalls the driver
228 The driver may create child controllers (child devices). E.g. a driver for block
229 IO devices will create the device handles for the partitions. The child
230 controllers will open the supported protocol with the attribute
231 EFI_OPEN_PROTOCOL_BY_CHILD_CONTROLLER.
233 A driver can be detached from a device using the DisconnectController service.
235 ## U-Boot devices mapped as UEFI devices
237 Some of the U-Boot devices are mapped as UEFI devices
244 As of U-Boot 2018.03 the logic for doing this is hard coded.
246 The development target is to integrate the setup of these UEFI devices with the
247 U-Boot driver model. So when a U-Boot device is discovered a handle should be
248 created and the device path protocol and the relevant IO protocol should be
249 installed. The UEFI driver then would be attached by calling ConnectController.
250 When a U-Boot device is removed DisconnectController should be called.
252 ## UEFI devices mapped as U-Boot devices
254 UEFI drivers binaries and applications may create new (virtual) devices, install
255 a protocol and call the ConnectController service. Now the matching UEFI driver
256 is determined by iterating over the implementations of the
257 EFI_DRIVER_BINDING_PROTOCOL.
259 It is the task of the UEFI driver to create a corresponding U-Boot device and to
260 proxy calls for this U-Boot device to the controller.
262 In U-Boot 2018.03 this has only been implemented for block IO devices.
266 An UEFI uclass driver (lib/efi_driver/efi_uclass.c) has been created that
267 takes care of initializing the UEFI drivers and providing the
268 EFI_DRIVER_BINDING_PROTOCOL implementation for the UEFI drivers.
270 A linker created list is used to keep track of the UEFI drivers. To create an
271 entry in the list the UEFI driver uses the U_BOOT_DRIVER macro specifying
272 UCLASS_EFI as the ID of its uclass, e.g.
274 /* Identify as UEFI driver */
275 U_BOOT_DRIVER(efi_block) = {
276 .name = "EFI block driver",
281 The available operations are defined via the structure struct efi_driver_ops.
283 struct efi_driver_ops {
284 const efi_guid_t *protocol;
285 const efi_guid_t *child_protocol;
286 int (*bind)(efi_handle_t handle, void *interface);
289 When the supported() function of the EFI_DRIVER_BINDING_PROTOCOL is called the
290 uclass checks if the protocol GUID matches the protocol GUID of the UEFI driver.
291 In the start() function the bind() function of the UEFI driver is called after
293 The stop() function of the EFI_DRIVER_BINDING_PROTOCOL disconnects the child
294 controllers created by the UEFI driver and the UEFI driver. (In U-Boot v2013.03
295 this is not yet completely implemented.)
297 ### UEFI block IO driver
299 The UEFI block IO driver supports devices exposing the EFI_BLOCK_IO_PROTOCOL.
301 When connected it creates a new U-Boot block IO device with interface type
302 IF_TYPE_EFI, adds child controllers mapping the partitions, and installs the
303 EFI_SIMPLE_FILE_SYSTEM_PROTOCOL on these. This can be used together with the
304 software iPXE to boot from iSCSI network drives [3].
306 This driver is only available if U-Boot is configured with
311 ## TODOs as of U-Boot 2019.04
313 * unimplemented or incompletely implemented boot services
314 * Exit - call unload function, unload applications only
315 * ProtocolRegisterNotify
318 * unimplemented or incompletely implemented runtime services
319 * SetVariable() ignores attribute EFI_VARIABLE_APPEND_WRITE
320 * QueryVariableInfo is not implemented
322 * unimplemented events
324 * EVT_SIGNAL_VIRTUAL_ADDRESS_CHANGE
327 * manage configuration tables in a linked list
330 * support DisconnectController for UEFI block devices.
332 * support for CONFIG_EFI_LOADER in the sandbox (CONFIG_SANDBOX=y)
338 * incompletely implemented protocols
339 * support version 0x00020000 of the EFI file protocol
343 * [1](http://uefi.org/specifications)
344 http://uefi.org/specifications - UEFI specifications
345 * [2](./driver-model/README.txt) doc/driver-model/README.txt - Driver model
346 * [3](./README.iscsi) doc/README.iscsi - iSCSI booting with U-Boot and iPXE
347 * [4](https://developer.arm.com/docs/den0044/latest/server-base-boot-requirements-system-software-on-arm-platforms-version-11)
348 Server Base Boot Requirements System Software on ARM Platforms - Version 1.1