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diff --git a/doc/README.u-boot_on_efi b/doc/README.u-boot_on_efi deleted file mode 100644 index e12dd4e3e65..00000000000 --- a/doc/README.u-boot_on_efi +++ /dev/null @@ -1,252 +0,0 @@ -# SPDX-License-Identifier: GPL-2.0+ -# -# Copyright (C) 2015 Google, Inc - -U-Boot on EFI -============= -This document provides information about U-Boot running on top of EFI, either -as an application or just as a means of getting U-Boot onto a new platform. - - -=========== Table of Contents =========== - -Motivation -Status -Build Instructions -Trying it out -Inner workings -EFI Application -EFI Payload -Tables -Interrupts -32/64-bit -Future work -Where is the code? - - -Motivation ----------- -Running U-Boot on EFI is useful in several situations: - -- You have EFI running on a board but U-Boot does not natively support it -fully yet. You can boot into U-Boot from EFI and use that until U-Boot is -fully ported - -- You need to use an EFI implementation (e.g. UEFI) because your vendor -requires it in order to provide support - -- You plan to use coreboot to boot into U-Boot but coreboot support does -not currently exist for your platform. In the meantime you can use U-Boot -on EFI and then move to U-Boot on coreboot when ready - -- You use EFI but want to experiment with a simpler alternative like U-Boot - - -Status ------- -Only x86 is supported at present. If you are using EFI on another architecture -you may want to reconsider. However, much of the code is generic so could be -ported. - -U-Boot supports running as an EFI application for 32-bit EFI only. This is -not very useful since only a serial port is provided. You can look around at -memory and type 'help' but that is about it. - -More usefully, U-Boot supports building itself as a payload for either 32-bit -or 64-bit EFI. U-Boot is packaged up and loaded in its entirety by EFI. Once -started, U-Boot changes to 32-bit mode (currently) and takes over the -machine. You can use devices, boot a kernel, etc. - - -Build Instructions ------------------- -First choose a board that has EFI support and obtain an EFI implementation -for that board. It will be either 32-bit or 64-bit. Alternatively, you can -opt for using QEMU [1] and the OVMF [2], as detailed below. - -To build U-Boot as an EFI application (32-bit EFI required), enable CONFIG_EFI -and CONFIG_EFI_APP. The efi-x86_app config (efi-x86_app_defconfig) is set up -for this. Just build U-Boot as normal, e.g. - - make efi-x86_app_defconfig - make - -To build U-Boot as an EFI payload (32-bit or 64-bit EFI can be used), enable -CONFIG_EFI, CONFIG_EFI_STUB, and select either CONFIG_EFI_STUB_32BIT or -CONFIG_EFI_STUB_64BIT. The efi-x86_payload configs (efi-x86_payload32_defconfig -and efi-x86_payload32_defconfig) are set up for this. Then build U-Boot as -normal, e.g. - - make efi-x86_payload32_defconfig (or efi-x86_payload64_defconfig) - make - -You will end up with one of these files depending on what you build for: - - u-boot-app.efi - U-Boot EFI application - u-boot-payload.efi - U-Boot EFI payload application - - -Trying it out -------------- -QEMU is an emulator and it can emulate an x86 machine. Please make sure your -QEMU version is 2.3.0 or above to test this. You can run the payload with -something like this: - - mkdir /tmp/efi - cp /path/to/u-boot*.efi /tmp/efi - qemu-system-x86_64 -bios bios.bin -hda fat:/tmp/efi/ - -Add -nographic if you want to use the terminal for output. Once it starts -type 'fs0:u-boot-payload.efi' to run the payload or 'fs0:u-boot-app.efi' to -run the application. 'bios.bin' is the EFI 'BIOS'. Check [2] to obtain a -prebuilt EFI BIOS for QEMU or you can build one from source as well. - -To try it on real hardware, put u-boot-app.efi on a suitable boot medium, -such as a USB stick. Then you can type something like this to start it: - - fs0:u-boot-payload.efi - -(or fs0:u-boot-app.efi for the application) - -This will start the payload, copy U-Boot into RAM and start U-Boot. Note -that EFI does not support booting a 64-bit application from a 32-bit -EFI (or vice versa). Also it will often fail to print an error message if -you get this wrong. - - -Inner workings -============== -Here follow a few implementation notes for those who want to fiddle with -this and perhaps contribute patches. - -The application and payload approaches sound similar but are in fact -implemented completely differently. - -EFI Application ---------------- -For the application the whole of U-Boot is built as a shared library. The -efi_main() function is in lib/efi/efi_app.c. It sets up some basic EFI -functions with efi_init(), sets up U-Boot global_data, allocates memory for -U-Boot's malloc(), etc. and enters the normal init sequence (board_init_f() -and board_init_r()). - -Since U-Boot limits its memory access to the allocated regions very little -special code is needed. The CONFIG_EFI_APP option controls a few things -that need to change so 'git grep CONFIG_EFI_APP' may be instructive. -The CONFIG_EFI option controls more general EFI adjustments. - -The only available driver is the serial driver. This calls back into EFI -'boot services' to send and receive characters. Although it is implemented -as a serial driver the console device is not necessarilly serial. If you -boot EFI with video output then the 'serial' device will operate on your -target devices's display instead and the device's USB keyboard will also -work if connected. If you have both serial and video output, then both -consoles will be active. Even though U-Boot does the same thing normally, -These are features of EFI, not U-Boot. - -Very little code is involved in implementing the EFI application feature. -U-Boot is highly portable. Most of the difficulty is in modifying the -Makefile settings to pass the right build flags. In particular there is very -little x86-specific code involved - you can find most of it in -arch/x86/cpu. Porting to ARM (which can also use EFI if you are brave -enough) should be straightforward. - -Use the 'reset' command to get back to EFI. - -EFI Payload ------------ -The payload approach is a different kettle of fish. It works by building -U-Boot exactly as normal for your target board, then adding the entire -image (including device tree) into a small EFI stub application responsible -for booting it. The stub application is built as a normal EFI application -except that it has a lot of data attached to it. - -The stub application is implemented in lib/efi/efi_stub.c. The efi_main() -function is called by EFI. It is responsible for copying U-Boot from its -original location into memory, disabling EFI boot services and starting -U-Boot. U-Boot then starts as normal, relocates, starts all drivers, etc. - -The stub application is architecture-dependent. At present it has some -x86-specific code and a comment at the top of efi_stub.c describes this. - -While the stub application does allocate some memory from EFI this is not -used by U-Boot (the payload). In fact when U-Boot starts it has all of the -memory available to it and can operate as it pleases (but see the next -section). - -Tables ------- -The payload can pass information to U-Boot in the form of EFI tables. At -present this feature is used to pass the EFI memory map, an inordinately -large list of memory regions. You can use the 'efi mem all' command to -display this list. U-Boot uses the list to work out where to relocate -itself. - -Although U-Boot can use any memory it likes, EFI marks some memory as used -by 'run-time services', code that hangs around while U-Boot is running and -is even present when Linux is running. This is common on x86 and provides -a way for Linux to call back into the firmware to control things like CPU -fan speed. U-Boot uses only 'conventional' memory, in EFI terminology. It -will relocate itself to the top of the largest block of memory it can find -below 4GB. - -Interrupts ----------- -U-Boot drivers typically don't use interrupts. Since EFI enables interrupts -it is possible that an interrupt will fire that U-Boot cannot handle. This -seems to cause problems. For this reason the U-Boot payload runs with -interrupts disabled at present. - -32/64-bit ---------- -While the EFI application can in principle be built as either 32- or 64-bit, -only 32-bit is currently supported. This means that the application can only -be used with 32-bit EFI. - -The payload stub can be build as either 32- or 64-bits. Only a small amount -of code is built this way (see the extra- line in lib/efi/Makefile). -Everything else is built as a normal U-Boot, so is always 32-bit on x86 at -present. - -Future work ------------ -This work could be extended in a number of ways: - -- Add ARM support - -- Add 64-bit application support - -- Figure out how to solve the interrupt problem - -- Add more drivers to the application side (e.g. video, block devices, USB, -environment access). This would mostly be an academic exercise as a strong -use case is not readily apparent, but it might be fun. - -- Avoid turning off boot services in the stub. Instead allow U-Boot to make -use of boot services in case it wants to. It is unclear what it might want -though. - -Where is the code? ------------------- -lib/efi - payload stub, application, support code. Mostly arch-neutral - -arch/x86/cpu/efi - x86 support code for running as an EFI application and payload - -board/efi/efi-x86_app/efi.c - x86 board code for running as an EFI application - -board/efi/efi-x86_payload - generic x86 EFI payload board support code - -common/cmd_efi.c - the 'efi' command - --- -Ben Stoltz, Simon Glass -Google, Inc -July 2015 - -[1] http://www.qemu.org -[2] http://www.tianocore.org/ovmf/ |