1# 2# Copyright (C) 2015 Google, Inc 3# 4# SPDX-License-Identifier: GPL-2.0+ 5# 6 7U-Boot on EFI 8============= 9This document provides information about U-Boot running on top of EFI, either 10as an application or just as a means of getting U-Boot onto a new platform. 11 12 13=========== Table of Contents =========== 14 15Motivation 16Status 17Build Instructions 18Trying it out 19Inner workings 20EFI Application 21EFI Payload 22Tables 23Interrupts 2432/64-bit 25Future work 26Where is the code? 27 28 29Motivation 30---------- 31Running U-Boot on EFI is useful in several situations: 32 33- You have EFI running on a board but U-Boot does not natively support it 34fully yet. You can boot into U-Boot from EFI and use that until U-Boot is 35fully ported 36 37- You need to use an EFI implementation (e.g. UEFI) because your vendor 38requires it in order to provide support 39 40- You plan to use coreboot to boot into U-Boot but coreboot support does 41not currently exist for your platform. In the meantime you can use U-Boot 42on EFI and then move to U-Boot on coreboot when ready 43 44- You use EFI but want to experiment with a simpler alternative like U-Boot 45 46 47Status 48------ 49Only x86 is supported at present. If you are using EFI on another architecture 50you may want to reconsider. However, much of the code is generic so could be 51ported. 52 53U-Boot supports running as an EFI application for 32-bit EFI only. This is 54not very useful since only a serial port is provided. You can look around at 55memory and type 'help' but that is about it. 56 57More usefully, U-Boot supports building itself as a payload for either 32-bit 58or 64-bit EFI. U-Boot is packaged up and loaded in its entirety by EFI. Once 59started, U-Boot changes to 32-bit mode (currently) and takes over the 60machine. You can use devices, boot a kernel, etc. 61 62 63Build Instructions 64------------------ 65First choose a board that has EFI support and obtain an EFI implementation 66for that board. It will be either 32-bit or 64-bit. Alternatively, you can 67opt for using QEMU [1] and the OVMF [2], as detailed below. 68 69To build U-Boot as an EFI application (32-bit EFI required), enable CONFIG_EFI 70and CONFIG_EFI_APP. The efi-x86 config (efi-x86_defconfig) is set up for this. 71Just build U-Boot as normal, e.g. 72 73 make efi-x86_defconfig 74 make 75 76To build U-Boot as an EFI payload (32-bit or 64-bit EFI can be used), adjust an 77existing config (like qemu-x86_defconfig) to enable CONFIG_EFI, CONFIG_EFI_STUB 78and either CONFIG_EFI_STUB_32BIT or CONFIG_EFI_STUB_64BIT. All of these are 79boolean Kconfig options. Then build U-Boot as normal, e.g. 80 81 make qemu-x86_defconfig 82 make 83 84You will end up with one of these files depending on what you build for: 85 86 u-boot-app.efi - U-Boot EFI application 87 u-boot-payload.efi - U-Boot EFI payload application 88 89 90Trying it out 91------------- 92QEMU is an emulator and it can emulate an x86 machine. Please make sure your 93QEMU version is 2.3.0 or above to test this. You can run the payload with 94something like this: 95 96 mkdir /tmp/efi 97 cp /path/to/u-boot*.efi /tmp/efi 98 qemu-system-x86_64 -bios bios.bin -hda fat:/tmp/efi/ 99 100Add -nographic if you want to use the terminal for output. Once it starts 101type 'fs0:u-boot-payload.efi' to run the payload or 'fs0:u-boot-app.efi' to 102run the application. 'bios.bin' is the EFI 'BIOS'. Check [2] to obtain a 103prebuilt EFI BIOS for QEMU or you can build one from source as well. 104 105To try it on real hardware, put u-boot-app.efi on a suitable boot medium, 106such as a USB stick. Then you can type something like this to start it: 107 108 fs0:u-boot-payload.efi 109 110(or fs0:u-boot-app.efi for the application) 111 112This will start the payload, copy U-Boot into RAM and start U-Boot. Note 113that EFI does not support booting a 64-bit application from a 32-bit 114EFI (or vice versa). Also it will often fail to print an error message if 115you get this wrong. 116 117 118Inner workings 119============== 120Here follow a few implementation notes for those who want to fiddle with 121this and perhaps contribute patches. 122 123The application and payload approaches sound similar but are in fact 124implemented completely differently. 125 126EFI Application 127--------------- 128For the application the whole of U-Boot is built as a shared library. The 129efi_main() function is in lib/efi/efi_app.c. It sets up some basic EFI 130functions with efi_init(), sets up U-Boot global_data, allocates memory for 131U-Boot's malloc(), etc. and enters the normal init sequence (board_init_f() 132and board_init_r()). 133 134Since U-Boot limits its memory access to the allocated regions very little 135special code is needed. The CONFIG_EFI_APP option controls a few things 136that need to change so 'git grep CONFIG_EFI_APP' may be instructive. 137The CONFIG_EFI option controls more general EFI adjustments. 138 139The only available driver is the serial driver. This calls back into EFI 140'boot services' to send and receive characters. Although it is implemented 141as a serial driver the console device is not necessarilly serial. If you 142boot EFI with video output then the 'serial' device will operate on your 143target devices's display instead and the device's USB keyboard will also 144work if connected. If you have both serial and video output, then both 145consoles will be active. Even though U-Boot does the same thing normally, 146These are features of EFI, not U-Boot. 147 148Very little code is involved in implementing the EFI application feature. 149U-Boot is highly portable. Most of the difficulty is in modifying the 150Makefile settings to pass the right build flags. In particular there is very 151little x86-specific code involved - you can find most of it in 152arch/x86/cpu. Porting to ARM (which can also use EFI if you are brave 153enough) should be straightforward. 154 155Use the 'reset' command to get back to EFI. 156 157EFI Payload 158----------- 159The payload approach is a different kettle of fish. It works by building 160U-Boot exactly as normal for your target board, then adding the entire 161image (including device tree) into a small EFI stub application responsible 162for booting it. The stub application is built as a normal EFI application 163except that it has a lot of data attached to it. 164 165The stub application is implemented in lib/efi/efi_stub.c. The efi_main() 166function is called by EFI. It is responsible for copying U-Boot from its 167original location into memory, disabling EFI boot services and starting 168U-Boot. U-Boot then starts as normal, relocates, starts all drivers, etc. 169 170The stub application is architecture-dependent. At present it has some 171x86-specific code and a comment at the top of efi_stub.c describes this. 172 173While the stub application does allocate some memory from EFI this is not 174used by U-Boot (the payload). In fact when U-Boot starts it has all of the 175memory available to it and can operate as it pleases (but see the next 176section). 177 178Tables 179------ 180The payload can pass information to U-Boot in the form of EFI tables. At 181present this feature is used to pass the EFI memory map, an inordinately 182large list of memory regions. You can use the 'efi mem all' command to 183display this list. U-Boot uses the list to work out where to relocate 184itself. 185 186Although U-Boot can use any memory it likes, EFI marks some memory as used 187by 'run-time services', code that hangs around while U-Boot is running and 188is even present when Linux is running. This is common on x86 and provides 189a way for Linux to call back into the firmware to control things like CPU 190fan speed. U-Boot uses only 'conventional' memory, in EFI terminology. It 191will relocate itself to the top of the largest block of memory it can find 192below 4GB. 193 194Interrupts 195---------- 196U-Boot drivers typically don't use interrupts. Since EFI enables interrupts 197it is possible that an interrupt will fire that U-Boot cannot handle. This 198seems to cause problems. For this reason the U-Boot payload runs with 199interrupts disabled at present. 200 20132/64-bit 202--------- 203While the EFI application can in principle be built as either 32- or 64-bit, 204only 32-bit is currently supported. This means that the application can only 205be used with 32-bit EFI. 206 207The payload stub can be build as either 32- or 64-bits. Only a small amount 208of code is built this way (see the extra- line in lib/efi/Makefile). 209Everything else is built as a normal U-Boot, so is always 32-bit on x86 at 210present. 211 212Future work 213----------- 214This work could be extended in a number of ways: 215 216- Add a generic x86 EFI payload configuration. At present you need to modify 217an existing one, but mostly the low-level x86 code is disabled when booting 218on EFI anyway, so a generic 'EFI' board could be created with a suitable set 219of drivers enabled. 220 221- Add ARM support 222 223- Add 64-bit application support 224 225- Figure out how to solve the interrupt problem 226 227- Add more drivers to the application side (e.g. video, block devices, USB, 228environment access). This would mostly be an academic exercise as a strong 229use case is not readily apparent, but it might be fun. 230 231- Avoid turning off boot services in the stub. Instead allow U-Boot to make 232use of boot services in case it wants to. It is unclear what it might want 233though. 234 235Where is the code? 236------------------ 237lib/efi 238 payload stub, application, support code. Mostly arch-neutral 239 240arch/x86/lib/efi 241 helper functions for the fake DRAM init, etc. These can be used by 242 any board that runs as a payload. 243 244arch/x86/cpu/efi 245 x86 support code for running as an EFI application 246 247board/efi/efi-x86/efi.c 248 x86 board code for running as an EFI application 249 250common/cmd_efi.c 251 the 'efi' command 252 253-- 254Ben Stoltz, Simon Glass 255Google, Inc 256July 2015 257 258[1] http://www.qemu.org 259[2] http://www.tianocore.org/ovmf/ 260