-# SPDX-License-Identifier: GPL-2.0+
-#
-# Copyright (C) 2015 Google, Inc
+.. SPDX-License-Identifier: GPL-2.0+
+.. Copyright (C) 2015 Google, Inc
U-Boot on EFI
=============
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
+ 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
+ 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
+ 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
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.
+for this. Just build U-Boot as normal, e.g.::
make efi-x86_app_defconfig
make
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.
+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
+* 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:
+something like this::
mkdir /tmp/efi
cp /path/to/u-boot*.efi /tmp/efi
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:
+such as a USB stick. Then you can type something like this to start it::
fs0:u-boot-payload.efi
Inner workings
-==============
+--------------
Here follow a few implementation notes for those who want to fiddle with
this and perhaps contribute patches.
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
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
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
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.
- 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.
+ 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.
+ use of boot services in case it wants to. It is unclear what it might want
+ though.
Where is the code?
------------------
Google, Inc
July 2015
-[1] http://www.qemu.org
-[2] http://www.tianocore.org/ovmf/
+* [1] http://www.qemu.org
+* [2] http://www.tianocore.org/ovmf/
projects / platform / kernel / u-boot.git / blobdiff