#
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# Copyright (C) 2015 Google, Inc
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#
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# SPDX-License-Identifier: GPL-2.0+
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#
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=========== Table of Contents ===========
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1 U-Boot on EFI
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1.1 In God's Name, Why?
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1.2 Status
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1.3 Build Instructions
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1.4 Trying it out
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1.5 Inner workings
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1.6 EFI Application
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1.7 EFI Payload
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1.8 Tables
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1.9 Interrupts
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1.10 32/64-bit
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1.11 Future work
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1.12 Where is the code?
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2 EFI on U-Boot
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2.1 In God's Name, Why?
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2.2 How do I get it?
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2.3 Status
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2.4 Future work
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U-Boot on EFI
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=============
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This document provides information about U-Boot running on top of EFI, either
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as an application or just as a means of getting U-Boot onto a new platform.
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In God's Name, Why?
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-------------------
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This is useful in several situations:
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- You have EFI running on a board but U-Boot does not natively support it
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fully yet. You can boot into U-Boot from EFI and use that until U-Boot is
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fully ported
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- You need to use an EFI implementation (e.g. UEFI) because your vendor
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requires it in order to provide support
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- You plan to use coreboot to boot into U-Boot but coreboot support does
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not currently exist for your platform. In the meantime you can use U-Boot
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on EFI and then move to U-Boot on coreboot when ready
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- You use EFI but want to experiment with a simpler alternative like U-Boot
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Status
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------
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Only x86 is supported at present. If you are using EFI on another architecture
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you may want to reconsider. However, much of the code is generic so could be
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ported.
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U-Boot supports running as an EFI application for 32-bit EFI only. This is
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not very useful since only a serial port is provided. You can look around at
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memory and type 'help' but that is about it.
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More usefully, U-Boot supports building itself as a payload for either 32-bit
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or 64-bit EFI. U-Boot is packaged up and loaded in its entirety by EFI. Once
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started, U-Boot changes to 32-bit mode (currently) and takes over the
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machine. You can use devices, boot a kernel, etc.
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Build Instructions
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------------------
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First choose a board that has EFI support and obtain an EFI implementation
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for that board. It will be either 32-bit or 64-bit. Alternatively, you can
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opt for using QEMU [1] and the OVMF [2], as detailed below.
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To build U-Boot as an EFI application (32-bit EFI required), enable CONFIG_EFI
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and CONFIG_EFI_APP. The efi-x86 config (efi-x86_defconfig) is set up for this.
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Just build U-Boot as normal, e.g.
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make efi-x86_defconfig
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make
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To build U-Boot as an EFI payload (32-bit or 64-bit EFI can be used), adjust an
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existing config (like qemu-x86_defconfig) to enable CONFIG_EFI, CONFIG_EFI_STUB
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and either CONFIG_EFI_STUB_32BIT or CONFIG_EFI_STUB_64BIT. All of these are
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boolean Kconfig options. Then build U-Boot as normal, e.g.
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make qemu-x86_defconfig
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make
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You will end up with one of these files depending on what you build for:
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u-boot-app.efi - U-Boot EFI application
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u-boot-payload.efi - U-Boot EFI payload application
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Trying it out
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-------------
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QEMU is an emulator and it can emulate an x86 machine. Please make sure your
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QEMU version is 2.3.0 or above to test this. You can run the payload with
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something like this:
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mkdir /tmp/efi
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cp /path/to/u-boot*.efi /tmp/efi
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qemu-system-x86_64 -bios bios.bin -hda fat:/tmp/efi/
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Add -nographic if you want to use the terminal for output. Once it starts
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type 'fs0:u-boot-payload.efi' to run the payload or 'fs0:u-boot-app.efi' to
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run the application. 'bios.bin' is the EFI 'BIOS'. Check [2] to obtain a
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prebuilt EFI BIOS for QEMU or you can build one from source as well.
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To try it on real hardware, put u-boot-app.efi on a suitable boot medium,
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such as a USB stick. Then you can type something like this to start it:
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fs0:u-boot-payload.efi
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(or fs0:u-boot-app.efi for the application)
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This will start the payload, copy U-Boot into RAM and start U-Boot. Note
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that EFI does not support booting a 64-bit application from a 32-bit
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EFI (or vice versa). Also it will often fail to print an error message if
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you get this wrong.
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Inner workings
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==============
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Here follow a few implementation notes for those who want to fiddle with
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this and perhaps contribute patches.
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The application and payload approaches sound similar but are in fact
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implemented completely differently.
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EFI Application
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---------------
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For the application the whole of U-Boot is built as a shared library. The
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efi_main() function is in lib/efi/efi_app.c. It sets up some basic EFI
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functions with efi_init(), sets up U-Boot global_data, allocates memory for
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U-Boot's malloc(), etc. and enters the normal init sequence (board_init_f()
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and board_init_r()).
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Since U-Boot limits its memory access to the allocated regions very little
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special code is needed. The CONFIG_EFI_APP option controls a few things
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that need to change so 'git grep CONFIG_EFI_APP' may be instructive.
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The CONFIG_EFI option controls more general EFI adjustments.
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The only available driver is the serial driver. This calls back into EFI
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'boot services' to send and receive characters. Although it is implemented
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as a serial driver the console device is not necessarilly serial. If you
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boot EFI with video output then the 'serial' device will operate on your
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target devices's display instead and the device's USB keyboard will also
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work if connected. If you have both serial and video output, then both
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consoles will be active. Even though U-Boot does the same thing normally,
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These are features of EFI, not U-Boot.
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Very little code is involved in implementing the EFI application feature.
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U-Boot is highly portable. Most of the difficulty is in modifying the
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Makefile settings to pass the right build flags. In particular there is very
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little x86-specific code involved - you can find most of it in
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arch/x86/cpu. Porting to ARM (which can also use EFI if you are brave
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enough) should be straightforward.
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Use the 'reset' command to get back to EFI.
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EFI Payload
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-----------
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The payload approach is a different kettle of fish. It works by building
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U-Boot exactly as normal for your target board, then adding the entire
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image (including device tree) into a small EFI stub application responsible
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for booting it. The stub application is built as a normal EFI application
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except that it has a lot of data attached to it.
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The stub application is implemented in lib/efi/efi_stub.c. The efi_main()
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function is called by EFI. It is responsible for copying U-Boot from its
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original location into memory, disabling EFI boot services and starting
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U-Boot. U-Boot then starts as normal, relocates, starts all drivers, etc.
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The stub application is architecture-dependent. At present it has some
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x86-specific code and a comment at the top of efi_stub.c describes this.
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While the stub application does allocate some memory from EFI this is not
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used by U-Boot (the payload). In fact when U-Boot starts it has all of the
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memory available to it and can operate as it pleases (but see the next
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section).
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Tables
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------
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The payload can pass information to U-Boot in the form of EFI tables. At
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present this feature is used to pass the EFI memory map, an inordinately
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large list of memory regions. You can use the 'efi mem all' command to
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display this list. U-Boot uses the list to work out where to relocate
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itself.
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Although U-Boot can use any memory it likes, EFI marks some memory as used
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by 'run-time services', code that hangs around while U-Boot is running and
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is even present when Linux is running. This is common on x86 and provides
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a way for Linux to call back into the firmware to control things like CPU
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fan speed. U-Boot uses only 'conventional' memory, in EFI terminology. It
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will relocate itself to the top of the largest block of memory it can find
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below 4GB.
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Interrupts
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----------
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U-Boot drivers typically don't use interrupts. Since EFI enables interrupts
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it is possible that an interrupt will fire that U-Boot cannot handle. This
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seems to cause problems. For this reason the U-Boot payload runs with
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interrupts disabled at present.
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32/64-bit
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---------
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While the EFI application can in principle be built as either 32- or 64-bit,
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only 32-bit is currently supported. This means that the application can only
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be used with 32-bit EFI.
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The payload stub can be build as either 32- or 64-bits. Only a small amount
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of code is built this way (see the extra- line in lib/efi/Makefile).
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Everything else is built as a normal U-Boot, so is always 32-bit on x86 at
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present.
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Future work
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-----------
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This work could be extended in a number of ways:
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- Add a generic x86 EFI payload configuration. At present you need to modify
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an existing one, but mostly the low-level x86 code is disabled when booting
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on EFI anyway, so a generic 'EFI' board could be created with a suitable set
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of drivers enabled.
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- Add ARM support
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- Add 64-bit application support
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- Figure out how to solve the interrupt problem
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- Add more drivers to the application side (e.g. video, block devices, USB,
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environment access). This would mostly be an academic exercise as a strong
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use case is not readily apparent, but it might be fun.
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- Avoid turning off boot services in the stub. Instead allow U-Boot to make
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use of boot services in case it wants to. It is unclear what it might want
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though.
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Where is the code?
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------------------
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lib/efi
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payload stub, application, support code. Mostly arch-neutral
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arch/x86/lib/efi
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helper functions for the fake DRAM init, etc. These can be used by
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any board that runs as a payload.
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arch/x86/cpu/efi
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x86 support code for running as an EFI application
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board/efi/efi-x86/efi.c
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x86 board code for running as an EFI application
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common/cmd_efi.c
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the 'efi' command
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--
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Ben Stoltz, Simon Glass
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Google, Inc
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July 2015
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[1] http://www.qemu.org
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[2] http://www.tianocore.org/ovmf/
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-------------------------------------------------------------------------------
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EFI on U-Boot
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=============
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In addition to support for running U-Boot as a UEFI application, U-Boot itself
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can also expose the UEFI interfaces and thus allow UEFI payloads to run under
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it.
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In God's Name, Why?
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-------------------
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With this support in place, you can run any UEFI payload (such as the Linux
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kernel, grub2 or gummiboot) on U-Boot. This dramatically simplifies boot loader
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configuration, as U-Boot based systems now look and feel (almost) the same way
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as TianoCore based systems.
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How do I get it?
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----------------
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EFI support for 32bit ARM and AArch64 is already included in U-Boot. All you
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need to do is enable
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CONFIG_CMD_BOOTEFI=y
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CONFIG_EFI_LOADER=y
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in your .config file and you will automatically get a bootefi command to run
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an efi application as well as snippet in the default distro boot script that
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scans for removable media efi binaries as fallback.
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Status
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------
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I am successfully able to run grub2 and Linux EFI binaries with this code on
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ARMv7 as well as AArch64 systems.
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When enabled, the resulting U-Boot binary only grows by ~10KB, so it's very
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light weight.
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All storage devices are directly accessible from the uEFI payload
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Removable media booting (search for /efi/boot/boota{a64,arm}.efi) is supported.
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Simple use cases like "Plug this SD card into my ARM device and it just
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boots into grub which boots into Linux", work very well.
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Running HelloWord.efi
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---------------------
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You can run a simple 'hello world' EFI program in U-Boot.
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Enable the option CONFIG_CMD_BOOTEFI_HELLO.
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Then you can boot into U-Boot and type:
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> bootefi hello
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The 'hello world EFI' program will then run, print a message and exit.
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Future work
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-----------
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Of course, there are still a few things one could do on top:
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- Improve disk media detection (don't scan, use what information we
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have)
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- Add EFI variable support using NVRAM
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- Add GFX support
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- Make EFI Shell work
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- Network device support
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- Support for payload exit
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- Payload Watchdog support
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