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