xref: /openbmc/u-boot/doc/README.u-boot_on_efi (revision a674313c)
1# SPDX-License-Identifier: GPL-2.0+
2#
3# Copyright (C) 2015 Google, Inc
4
5U-Boot on EFI
6=============
7This document provides information about U-Boot running on top of EFI, either
8as an application or just as a means of getting U-Boot onto a new platform.
9
10
11=========== Table of Contents ===========
12
13Motivation
14Status
15Build Instructions
16Trying it out
17Inner workings
18EFI Application
19EFI Payload
20Tables
21Interrupts
2232/64-bit
23Future work
24Where is the code?
25
26
27Motivation
28----------
29Running U-Boot on EFI is useful in several situations:
30
31- You have EFI running on a board but U-Boot does not natively support it
32fully yet. You can boot into U-Boot from EFI and use that until U-Boot is
33fully ported
34
35- You need to use an EFI implementation (e.g. UEFI) because your vendor
36requires it in order to provide support
37
38- You plan to use coreboot to boot into U-Boot but coreboot support does
39not currently exist for your platform. In the meantime you can use U-Boot
40on EFI and then move to U-Boot on coreboot when ready
41
42- You use EFI but want to experiment with a simpler alternative like U-Boot
43
44
45Status
46------
47Only x86 is supported at present. If you are using EFI on another architecture
48you may want to reconsider. However, much of the code is generic so could be
49ported.
50
51U-Boot supports running as an EFI application for 32-bit EFI only. This is
52not very useful since only a serial port is provided. You can look around at
53memory and type 'help' but that is about it.
54
55More usefully, U-Boot supports building itself as a payload for either 32-bit
56or 64-bit EFI. U-Boot is packaged up and loaded in its entirety by EFI. Once
57started, U-Boot changes to 32-bit mode (currently) and takes over the
58machine. You can use devices, boot a kernel, etc.
59
60
61Build Instructions
62------------------
63First choose a board that has EFI support and obtain an EFI implementation
64for that board. It will be either 32-bit or 64-bit. Alternatively, you can
65opt for using QEMU [1] and the OVMF [2], as detailed below.
66
67To build U-Boot as an EFI application (32-bit EFI required), enable CONFIG_EFI
68and CONFIG_EFI_APP. The efi-x86_app config (efi-x86_app_defconfig) is set up
69for this. Just build U-Boot as normal, e.g.
70
71   make efi-x86_app_defconfig
72   make
73
74To build U-Boot as an EFI payload (32-bit or 64-bit EFI can be used), enable
75CONFIG_EFI, CONFIG_EFI_STUB, and select either CONFIG_EFI_STUB_32BIT or
76CONFIG_EFI_STUB_64BIT. The efi-x86_payload configs (efi-x86_payload32_defconfig
77and efi-x86_payload32_defconfig) are set up for this. Then build U-Boot as
78normal, e.g.
79
80   make efi-x86_payload32_defconfig (or efi-x86_payload64_defconfig)
81   make
82
83You will end up with one of these files depending on what you build for:
84
85   u-boot-app.efi      - U-Boot EFI application
86   u-boot-payload.efi  - U-Boot EFI payload application
87
88
89Trying it out
90-------------
91QEMU is an emulator and it can emulate an x86 machine. Please make sure your
92QEMU version is 2.3.0 or above to test this. You can run the payload with
93something like this:
94
95   mkdir /tmp/efi
96   cp /path/to/u-boot*.efi /tmp/efi
97   qemu-system-x86_64 -bios bios.bin -hda fat:/tmp/efi/
98
99Add -nographic if you want to use the terminal for output. Once it starts
100type 'fs0:u-boot-payload.efi' to run the payload or 'fs0:u-boot-app.efi' to
101run the application. 'bios.bin' is the EFI 'BIOS'. Check [2] to obtain a
102prebuilt EFI BIOS for QEMU or you can build one from source as well.
103
104To try it on real hardware, put u-boot-app.efi on a suitable boot medium,
105such as a USB stick. Then you can type something like this to start it:
106
107   fs0:u-boot-payload.efi
108
109(or fs0:u-boot-app.efi for the application)
110
111This will start the payload, copy U-Boot into RAM and start U-Boot. Note
112that EFI does not support booting a 64-bit application from a 32-bit
113EFI (or vice versa). Also it will often fail to print an error message if
114you get this wrong.
115
116
117Inner workings
118==============
119Here follow a few implementation notes for those who want to fiddle with
120this and perhaps contribute patches.
121
122The application and payload approaches sound similar but are in fact
123implemented completely differently.
124
125EFI Application
126---------------
127For the application the whole of U-Boot is built as a shared library. The
128efi_main() function is in lib/efi/efi_app.c. It sets up some basic EFI
129functions with efi_init(), sets up U-Boot global_data, allocates memory for
130U-Boot's malloc(), etc. and enters the normal init sequence (board_init_f()
131and board_init_r()).
132
133Since U-Boot limits its memory access to the allocated regions very little
134special code is needed. The CONFIG_EFI_APP option controls a few things
135that need to change so 'git grep CONFIG_EFI_APP' may be instructive.
136The CONFIG_EFI option controls more general EFI adjustments.
137
138The only available driver is the serial driver. This calls back into EFI
139'boot services' to send and receive characters. Although it is implemented
140as a serial driver the console device is not necessarilly serial. If you
141boot EFI with video output then the 'serial' device will operate on your
142target devices's display instead and the device's USB keyboard will also
143work if connected. If you have both serial and video output, then both
144consoles will be active. Even though U-Boot does the same thing normally,
145These are features of EFI, not U-Boot.
146
147Very little code is involved in implementing the EFI application feature.
148U-Boot is highly portable. Most of the difficulty is in modifying the
149Makefile settings to pass the right build flags. In particular there is very
150little x86-specific code involved - you can find most of it in
151arch/x86/cpu. Porting to ARM (which can also use EFI if you are brave
152enough) should be straightforward.
153
154Use the 'reset' command to get back to EFI.
155
156EFI Payload
157-----------
158The payload approach is a different kettle of fish. It works by building
159U-Boot exactly as normal for your target board, then adding the entire
160image (including device tree) into a small EFI stub application responsible
161for booting it. The stub application is built as a normal EFI application
162except that it has a lot of data attached to it.
163
164The stub application is implemented in lib/efi/efi_stub.c. The efi_main()
165function is called by EFI. It is responsible for copying U-Boot from its
166original location into memory, disabling EFI boot services and starting
167U-Boot. U-Boot then starts as normal, relocates, starts all drivers, etc.
168
169The stub application is architecture-dependent. At present it has some
170x86-specific code and a comment at the top of efi_stub.c describes this.
171
172While the stub application does allocate some memory from EFI this is not
173used by U-Boot (the payload). In fact when U-Boot starts it has all of the
174memory available to it and can operate as it pleases (but see the next
175section).
176
177Tables
178------
179The payload can pass information to U-Boot in the form of EFI tables. At
180present this feature is used to pass the EFI memory map, an inordinately
181large list of memory regions. You can use the 'efi mem all' command to
182display this list. U-Boot uses the list to work out where to relocate
183itself.
184
185Although U-Boot can use any memory it likes, EFI marks some memory as used
186by 'run-time services', code that hangs around while U-Boot is running and
187is even present when Linux is running. This is common on x86 and provides
188a way for Linux to call back into the firmware to control things like CPU
189fan speed. U-Boot uses only 'conventional' memory, in EFI terminology. It
190will relocate itself to the top of the largest block of memory it can find
191below 4GB.
192
193Interrupts
194----------
195U-Boot drivers typically don't use interrupts. Since EFI enables interrupts
196it is possible that an interrupt will fire that U-Boot cannot handle. This
197seems to cause problems. For this reason the U-Boot payload runs with
198interrupts disabled at present.
199
20032/64-bit
201---------
202While the EFI application can in principle be built as either 32- or 64-bit,
203only 32-bit is currently supported. This means that the application can only
204be used with 32-bit EFI.
205
206The payload stub can be build as either 32- or 64-bits. Only a small amount
207of code is built this way (see the extra- line in lib/efi/Makefile).
208Everything else is built as a normal U-Boot, so is always 32-bit on x86 at
209present.
210
211Future work
212-----------
213This work could be extended in a number of ways:
214
215- Add ARM support
216
217- Add 64-bit application support
218
219- Figure out how to solve the interrupt problem
220
221- Add more drivers to the application side (e.g. video, block devices, USB,
222environment access). This would mostly be an academic exercise as a strong
223use case is not readily apparent, but it might be fun.
224
225- Avoid turning off boot services in the stub. Instead allow U-Boot to make
226use of boot services in case it wants to. It is unclear what it might want
227though.
228
229Where is the code?
230------------------
231lib/efi
232	payload stub, application, support code. Mostly arch-neutral
233
234arch/x86/cpu/efi
235	x86 support code for running as an EFI application and payload
236
237board/efi/efi-x86_app/efi.c
238	x86 board code for running as an EFI application
239
240board/efi/efi-x86_payload
241	generic x86 EFI payload board support code
242
243common/cmd_efi.c
244	the 'efi' command
245
246--
247Ben Stoltz, Simon Glass
248Google, Inc
249July 2015
250
251[1] http://www.qemu.org
252[2] http://www.tianocore.org/ovmf/
253