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 UEFI standard supports only little endian systems. The UEFI support can be
18 activated for ARM and x86 by specifying
25 Support for attaching virtual block devices, e.g. iSCSI drives connected by the
26 loaded UEFI application [3], requires
31 ### Executing a UEFI binary
33 The bootefi command is used to start UEFI applications or to install UEFI
34 drivers. It takes two parameters
36 bootefi <image address> [fdt address]
38 * image address - the memory address of the UEFI binary
39 * fdt address - the memory address of the flattened device tree
41 Below you find the output of an example session starting GRUB.
43 => load mmc 0:2 ${fdt_addr_r} boot/dtb
44 29830 bytes read in 14 ms (2 MiB/s)
45 => load mmc 0:1 ${kernel_addr_r} efi/debian/grubaa64.efi
46 reading efi/debian/grubaa64.efi
47 120832 bytes read in 7 ms (16.5 MiB/s)
48 => bootefi ${kernel_addr_r} ${fdt_addr_r}
50 The environment variable 'bootargs' is passed as load options in the UEFI system
51 table. The Linux kernel EFI stub uses the load options as command line
54 ### Executing the boot manager
56 The UEFI specfication foresees to define boot entries and boot sequence via UEFI
57 variables. Booting according to these variables is possible via
59 bootefi bootmgr [fdt address]
61 As of U-Boot v2018.03 UEFI variables are not persisted and cannot be set at
64 ### Executing the built in hello world application
66 A hello world UEFI application can be built with
68 CONFIG_CMD_BOOTEFI_HELLO_COMPILE=y
70 It can be embedded into the U-Boot binary with
72 CONFIG_CMD_BOOTEFI_HELLO=y
74 The bootefi command is used to start the embedded hello world application.
76 bootefi hello [fdt address]
78 Below you find the output of an example session.
80 => bootefi hello ${fdtcontroladdr}
81 ## Starting EFI application at 01000000 ...
82 WARNING: using memory device/image path, this may confuse some payloads!
87 Load options: root=/dev/sdb3 init=/sbin/init rootwait ro
88 ## Application terminated, r = 0
90 The environment variable fdtcontroladdr points to U-Boot's internal device tree
93 ### Executing the built-in selftest
95 An UEFI selftest suite can be embedded in U-Boot by building with
97 CONFIG_CMD_BOOTEFI_SELFTEST=y
99 For testing the UEFI implementation the bootefi command can be used to start the
102 bootefi selftest [fdt address]
104 The environment variable 'efi_selftest' can be used to select a single test. If
105 it is not provided all tests are executed except those marked as 'on request'.
106 If the environment variable is set to 'list' a list of all tests is shown.
108 Below you can find the output of an example session.
110 => setenv efi_selftest simple network protocol
112 Testing EFI API implementation
113 Selected test: 'simple network protocol'
114 Setting up 'simple network protocol'
115 Setting up 'simple network protocol' succeeded
116 Executing 'simple network protocol'
118 DHCP reply received from 192.168.76.2 (52:55:c0:a8:4c:02)
119 as broadcast message.
120 Executing 'simple network protocol' succeeded
121 Tearing down 'simple network protocol'
122 Tearing down 'simple network protocol' succeeded
123 Boot services terminated
125 Preparing for reset. Press any key.
127 ## The UEFI life cycle
129 After the U-Boot platform has been initialized the UEFI API provides two kinds
135 The API can be extended by loading UEFI drivers which come in two variants
140 UEFI drivers are installed with U-Boot's bootefi command. With the same command
141 UEFI applications can be executed.
143 Loaded images of UEFI drivers stay in memory after returning to U-Boot while
144 loaded images of applications are removed from memory.
146 An UEFI application (e.g. an operating system) that wants to take full control
147 of the system calls ExitBootServices. After a UEFI application calls
150 * boot services are not available anymore
151 * timer events are stopped
152 * the memory used by U-Boot except for runtime services is released
153 * the memory used by boot time drivers is released
155 So this is a point of no return. Afterwards the UEFI application can only return
156 to U-Boot by rebooting.
158 ## The UEFI object model
160 UEFI offers a flexible and expandable object model. The objects in the UEFI API
161 are devices, drivers, and loaded images. These objects are referenced by
164 The interfaces implemented by the objects are referred to as protocols. These
165 are identified by GUIDs. They can be installed and uninstalled by calling the
166 appropriate boot services.
168 Handles are created by the InstallProtocolInterface or the
169 InstallMultipleProtocolinterfaces service if NULL is passed as handle.
171 Handles are deleted when the last protocol has been removed with the
172 UninstallProtocolInterface or the UninstallMultipleProtocolInterfaces service.
174 Devices offer the EFI_DEVICE_PATH_PROTOCOL. A device path is the concatenation
175 of device nodes. By their device paths all devices of a system are arranged in a
178 Drivers offer the EFI_DRIVER_BINDING_PROTOCOL. This protocol is used to connect
179 a driver to devices (which are referenced as controllers in this context).
181 Loaded images offer the EFI_LOADED_IMAGE_PROTOCOL. This protocol provides meta
182 information about the image and a pointer to the unload callback function.
186 In the UEFI terminology an event is a data object referencing a notification
187 function which is queued for calling when the event is signaled. The following
188 types of events exist:
190 * periodic and single shot timer events
191 * exit boot services events, triggered by calling the ExitBootServices() service
192 * virtual address change events
193 * memory map change events
194 * read to boot events
195 * reset system events
196 * system table events
197 * events that are only triggered programmatically
199 Events can be created with the CreateEvent service and deleted with CloseEvent
202 Events can be assigned to an event group. If any of the events in a group is
203 signaled, all other events in the group are also set to the signaled state.
205 ## The UEFI driver model
207 A driver is specific for a single protocol installed on a device. To install a
208 driver on a device the ConnectController service is called. In this context
209 controller refers to the device for which the driver is installed.
211 The relevant drivers are identified using the EFI_DRIVER_BINDING_PROTOCOL. This
212 protocol has has three functions:
214 * supported - determines if the driver is compatible with the device
215 * start - installs the driver by opening the relevant protocol with
216 attribute EFI_OPEN_PROTOCOL_BY_DRIVER
217 * stop - uninstalls the driver
219 The driver may create child controllers (child devices). E.g. a driver for block
220 IO devices will create the device handles for the partitions. The child
221 controllers will open the supported protocol with the attribute
222 EFI_OPEN_PROTOCOL_BY_CHILD_CONTROLLER.
224 A driver can be detached from a device using the DisconnectController service.
226 ## U-Boot devices mapped as UEFI devices
228 Some of the U-Boot devices are mapped as UEFI devices
235 As of U-Boot 2018.03 the logic for doing this is hard coded.
237 The development target is to integrate the setup of these UEFI devices with the
238 U-Boot driver model. So when a U-Boot device is discovered a handle should be
239 created and the device path protocol and the relevant IO protocol should be
240 installed. The UEFI driver then would be attached by calling ConnectController.
241 When a U-Boot device is removed DisconnectController should be called.
243 ## UEFI devices mapped as U-Boot devices
245 UEFI drivers binaries and applications may create new (virtual) devices, install
246 a protocol and call the ConnectController service. Now the matching UEFI driver
247 is determined by iterating over the implementations of the
248 EFI_DRIVER_BINDING_PROTOCOL.
250 It is the task of the UEFI driver to create a corresponding U-Boot device and to
251 proxy calls for this U-Boot device to the controller.
253 In U-Boot 2018.03 this has only been implemented for block IO devices.
257 An UEFI uclass driver (lib/efi_driver/efi_uclass.c) has been created that
258 takes care of initializing the UEFI drivers and providing the
259 EFI_DRIVER_BINDING_PROTOCOL implementation for the UEFI drivers.
261 A linker created list is used to keep track of the UEFI drivers. To create an
262 entry in the list the UEFI driver uses the U_BOOT_DRIVER macro specifying
263 UCLASS_EFI as the ID of its uclass, e.g.
265 /* Identify as UEFI driver */
266 U_BOOT_DRIVER(efi_block) = {
267 .name = "EFI block driver",
272 The available operations are defined via the structure struct efi_driver_ops.
274 struct efi_driver_ops {
275 const efi_guid_t *protocol;
276 const efi_guid_t *child_protocol;
277 int (*bind)(efi_handle_t handle, void *interface);
280 When the supported() function of the EFI_DRIVER_BINDING_PROTOCOL is called the
281 uclass checks if the protocol GUID matches the protocol GUID of the UEFI driver.
282 In the start() function the bind() function of the UEFI driver is called after
284 The stop() function of the EFI_DRIVER_BINDING_PROTOCOL disconnects the child
285 controllers created by the UEFI driver and the UEFI driver. (In U-Boot v2013.03
286 this is not yet completely implemented.)
288 ### UEFI block IO driver
290 The UEFI block IO driver supports devices exposing the EFI_BLOCK_IO_PROTOCOL.
292 When connected it creates a new U-Boot block IO device with interface type
293 IF_TYPE_EFI, adds child controllers mapping the partitions, and installs the
294 EFI_SIMPLE_FILE_SYSTEM_PROTOCOL on these. This can be used together with the
295 software iPXE to boot from iSCSI network drives [3].
297 This driver is only available if U-Boot is configured with
302 ## TODOs as of U-Boot 2018.07
304 * unimplemented or incompletely implemented boot services
305 * Exit - call unload function, unload applications only
306 * ProtocolRegisterNotify
309 * unimplemented or incompletely implemented runtime services
310 * SetVariable() ignores attribute EFI_VARIABLE_APPEND_WRITE
311 * GetNextVariableName is not implemented
312 * QueryVariableInfo is not implemented
314 * unimplemented events
316 * EVT_SIGNAL_VIRTUAL_ADDRESS_CHANGE
320 * manage events in a linked list
321 * manage configuration tables in a linked list
324 * support DisconnectController for UEFI block devices.
326 * support for CONFIG_EFI_LOADER in the sandbox (CONFIG_SANDBOX=y)
332 * support bootefi booting ARMv7 in non-secure mode (CONFIG_ARMV7_NONSEC=y)
336 * [1](http://uefi.org/specifications)
337 http://uefi.org/specifications - UEFI specifications
338 * [2](./driver-model/README.txt) doc/driver-model/README.txt - Driver model
339 * [3](./README.iscsi) doc/README.iscsi - iSCSI booting with U-Boot and iPXE