#

# Copyright (C) 2015 Google, Inc

#

# SPDX-License-Identifier: GPL-2.0+

#

=========== Table of Contents ===========

1 U-Boot on EFI

1.1 In God's Name, Why?

1.2 Status

1.3 Build Instructions

1.4 Trying it out

1.5 Inner workings

1.6 EFI Application

1.7 EFI Payload

1.8 Tables

1.9 Interrupts

1.10 32/64-bit

1.11 Future work

1.12 Where is the code?

2 EFI on U-Boot

2.1 In God's Name, Why?

2.2 How do I get it?

2.3 Status

2.4 Future work

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.

In God's Name, Why?

-------------------

This 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 config (efi-x86_defconfig) is set up for this.

Just build U-Boot as normal, e.g.

make efi-x86_defconfig

make

To build U-Boot as an EFI payload (32-bit or 64-bit EFI can be used), adjust an

existing config (like qemu-x86_defconfig) to enable CONFIG_EFI, CONFIG_EFI_STUB

and either CONFIG_EFI_STUB_32BIT or CONFIG_EFI_STUB_64BIT. All of these are

boolean Kconfig options. Then build U-Boot as normal, e.g.

make qemu-x86_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 a generic x86 EFI payload configuration. At present you need to modify

an existing one, but mostly the low-level x86 code is disabled when booting

on EFI anyway, so a generic 'EFI' board could be created with a suitable set

of drivers enabled.

- 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/lib/efi

helper functions for the fake DRAM init, etc. These can be used by

any board that runs as a payload.

arch/x86/cpu/efi

x86 support code for running as an EFI application

board/efi/efi-x86/efi.c

x86 board code for running as an EFI application

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/

-------------------------------------------------------------------------------

EFI on U-Boot

=============

In addition to support for running U-Boot as a UEFI application, U-Boot itself

can also expose the UEFI interfaces and thus allow UEFI payloads to run under

it.

In God's Name, Why?

-------------------

With this support in place, you can run any UEFI payload (such as the Linux

kernel, grub2 or gummiboot) on U-Boot. This dramatically simplifies boot loader

configuration, as U-Boot based systems now look and feel (almost) the same way

as TianoCore based systems.

How do I get it?

----------------

EFI support for 32bit ARM and AArch64 is already included in U-Boot. All you

need to do is enable

CONFIG_CMD_BOOTEFI=y

CONFIG_EFI_LOADER=y

in your .config file and you will automatically get a bootefi command to run

an efi application as well as snippet in the default distro boot script that

scans for removable media efi binaries as fallback.

Status

------

I am successfully able to run grub2 and Linux EFI binaries with this code on

ARMv7 as well as AArch64 systems.

When enabled, the resulting U-Boot binary only grows by ~10KB, so it's very

light weight.

All storage devices are directly accessible from the uEFI payload

Removable media booting (search for /efi/boot/boota{a64,arm}.efi) is supported.

Simple use cases like "Plug this SD card into my ARM device and it just

boots into grub which boots into Linux", work very well.

Running HelloWord.efi

---------------------

You can run a simple 'hello world' EFI program in U-Boot.

Enable the option CONFIG_CMD_BOOTEFI_HELLO.

Then you can boot into U-Boot and type:

> bootefi hello

The 'hello world EFI' program will then run, print a message and exit.

Future work

-----------

Of course, there are still a few things one could do on top:

- Improve disk media detection (don't scan, use what information we

have)

- Add EFI variable support using NVRAM

- Add GFX support

- Make EFI Shell work

- Network device support

- Support for payload exit

- Payload Watchdog support