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# Copyright (C) 2014, Simon Glass <>
# Copyright (C) 2014, Bin Meng <>
# SPDX-License-Identifier: GPL-2.0+
U-Boot on x86
This document describes the information about U-Boot running on x86 targets,
including supported boards, build instructions, todo list, etc.
U-Boot supports running as a coreboot [1] payload on x86. So far only Link
(Chromebook Pixel) has been tested, but it should work with minimal adjustments
on other x86 boards since coreboot deals with most of the low-level details.
U-Boot also supports booting directly from x86 reset vector without coreboot,
aka raw support or bare support. Currently Link, Intel Crown Bay, Intel
Minnowboard Max and Intel Galileo support running U-Boot 'bare metal'.
As for loading an OS, U-Boot supports directly booting a 32-bit or 64-bit
Linux kernel as part of a FIT image. It also supports a compressed zImage.
Build Instructions
Building U-Boot as a coreboot payload is just like building U-Boot for targets
on other architectures, like below:
$ make coreboot-x86_defconfig
$ make all
Note this default configuration will build a U-Boot payload for the Link board.
To build a coreboot payload against another board, you can change the build
configuration during the 'make menuconfig' process.
x86 architecture --->
(chromebook_link) Board configuration file
(chromebook_link) Board Device Tree Source (dts) file
(0x19200000) Board specific Cache-As-RAM (CAR) address
(0x4000) Board specific Cache-As-RAM (CAR) size
Change the 'Board configuration file' and 'Board Device Tree Source (dts) file'
to point to a new board. You can also change the Cache-As-RAM (CAR) related
settings here if the default values do not fit your new board.
Building a ROM version of U-Boot (hereafter referred to as u-boot.rom) is a
little bit tricky, as generally it requires several binary blobs which are not
shipped in the U-Boot source tree. Due to this reason, the u-boot.rom build is
not turned on by default in the U-Boot source tree. Firstly, you need turn it
on by enabling the ROM build:
$ export BUILD_ROM=y
This tells the Makefile to build u-boot.rom as a target.
Link-specific instructions:
First, you need the following binary blobs:
* descriptor.bin - Intel flash descriptor
* me.bin - Intel Management Engine
* mrc.bin - Memory Reference Code, which sets up SDRAM
* video ROM - sets up the display
You can get these binary blobs by:
$ git clone
$ cd blobs
Find the following files:
* ./mainboard/google/link/descriptor.bin
* ./mainboard/google/link/me.bin
* ./northbridge/intel/sandybridge/systemagent-ivybridge.bin
The 3rd one should be renamed to mrc.bin.
As for the video ROM, you can get it here [2].
Make sure all these binary blobs are put in the board directory.
Now you can build U-Boot and obtain u-boot.rom:
$ make chromebook_link_defconfig
$ make all
Intel Crown Bay specific instructions:
U-Boot support of Intel Crown Bay board [3] relies on a binary blob called
Firmware Support Package [4] to perform all the necessary initialization steps
as documented in the BIOS Writer Guide, including initialization of the CPU,
memory controller, chipset and certain bus interfaces.
Download the Intel FSP for Atom E6xx series and Platform Controller Hub EG20T,
install it on your host and locate the FSP binary blob. Note this platform
also requires a Chipset Micro Code (CMC) state machine binary to be present in
the SPI flash where u-boot.rom resides, and this CMC binary blob can be found
in this FSP package too.
* ./Microcode/C0_22211.BIN
Rename the first one to fsp.bin and second one to cmc.bin and put them in the
board directory.
Note the FSP release version 001 has a bug which could cause random endless
loop during the FspInit call. This bug was published by Intel although Intel
did not describe any details. We need manually apply the patch to the FSP
binary using any hex editor (eg: bvi). Go to the offset 0x1fcd8 of the FSP
binary, change the following five bytes values from orginally E8 42 FF FF FF
to B8 00 80 0B 00.
Now you can build U-Boot and obtain u-boot.rom
$ make crownbay_defconfig
$ make all
Intel Minnowboard Max instructions:
This uses as FSP as with Crown Bay, except it is for the Atom E3800 series.
Download this and get the .fd file (BAYTRAIL_FSP_GOLD_003_16-SEP-2014.fd at
the time of writing). Put it in the board directory:
Obtain the VGA RAM (Vga.dat at the time of writing) and put it into the same
directory: board/intel/minnowmax/vga.bin
You still need two more binary blobs. These come from the sample SPI image
provided in the FSP (SPI.bin at the time of writing).
Use ifdtool in the U-Boot tools directory to extract the images from that
file, for example:
$ ./tools/ifdtool -x BayleyBay/SPI.bin
$ cp flashregion_2_intel_me.bin board/intel/minnowmax/me.bin
$ cp flashregion_0_flashdescriptor.bin board/intel/minnowmax/descriptor.bin
Now you can build U-Boot and obtain u-boot.rom
$ make minnowmax_defconfig
$ make all
Intel Galileo instructions:
Only one binary blob is needed for Remote Management Unit (RMU) within Intel
Quark SoC. Not like FSP, U-Boot does not call into the binary. The binary is
needed by the Quark SoC itself.
You can get the binary blob from Quark Board Support Package from Intel website:
* ./QuarkSocPkg/QuarkNorthCluster/Binary/QuarkMicrocode/RMU.bin
Rename the file and put it to the board directory by:
$ cp RMU.bin board/intel/galileo/rmu.bin
Now you can build U-Boot and obtain u-boot.rom
$ make galileo_defconfig
$ make all
Test with coreboot
For testing U-Boot as the coreboot payload, there are things that need be paid
attention to. coreboot supports loading an ELF executable and a 32-bit plain
binary, as well as other supported payloads. With the default configuration,
U-Boot is set up to use a separate Device Tree Blob (dtb). As of today, the
generated u-boot-dtb.bin needs to be packaged by the cbfstool utility (a tool
provided by coreboot) manually as coreboot's 'make menuconfig' does not provide
this capability yet. The command is as follows:
# in the coreboot root directory
$ ./build/util/cbfstool/cbfstool build/coreboot.rom add-flat-binary \
-f u-boot-dtb.bin -n fallback/payload -c lzma -l 0x1110000 -e 0x1110015
Make sure 0x1110000 matches CONFIG_SYS_TEXT_BASE and 0x1110015 matches the
symbol address of _start (in arch/x86/cpu/start.S).
If you want to use ELF as the coreboot payload, change U-Boot configuration to
To enable video you must enable these options in coreboot:
- Set framebuffer graphics resolution (1280x1024 32k-color (1:5:5))
- Keep VESA framebuffer
At present it seems that for Minnowboard Max, coreboot does not pass through
the video information correctly (it always says the resolution is 0x0). This
works correctly for link though.
CPU Microcode
Modern CPUs usually require a special bit stream called microcode [5] to be
loaded on the processor after power up in order to function properly. U-Boot
has already integrated these as hex dumps in the source tree.
Driver Model
x86 has been converted to use driver model for serial and GPIO.
Device Tree
x86 uses device tree to configure the board thus requires CONFIG_OF_CONTROL to
be turned on. Not every device on the board is configured via device tree, but
more and more devices will be added as time goes by. Check out the directory
arch/x86/dts/ for these device tree source files.
Useful Commands
In keeping with the U-Boot philosophy of providing functions to check and
adjust internal settings, there are several x86-specific commands that may be
hob - Display information about Firmware Support Package (FSP) Hand-off
Block. This is only available on platforms which use FSP, mostly
iod - Display I/O memory
iow - Write I/O memory
mtrr - List and set the Memory Type Range Registers (MTRR). These are used to
tell the CPU whether memory is cacheable and if so the cache write
mode to use. U-Boot sets up some reasonable values but you can
adjust then with this command.
Development Flow
These notes are for those who want to port U-Boot to a new x86 platform.
Since x86 CPUs boot from SPI flash, a SPI flash emulator is a good investment.
The Dediprog em100 can be used on Linux. The em100 tool is available here:
On Minnowboard Max the following command line can be used:
sudo em100 -s -p LOW -d u-boot.rom -c W25Q64DW -r
A suitable clip for connecting over the SPI flash chip is here:
This allows you to override the SPI flash contents for development purposes.
Typically you can write to the em100 in around 1200ms, considerably faster
than programming the real flash device each time. The only important
limitation of the em100 is that it only supports SPI bus speeds up to 20MHz.
This means that images must be set to boot with that speed. This is an
Intel-specific feature - e.g. tools/ifttool has an option to set the SPI
speed in the SPI descriptor region.
If your chip/board uses an Intel Firmware Support Package (FSP) it is fairly
easy to fit it in. You can follow the Minnowboard Max implementation, for
example. Hopefully you will just need to create new files similar to those
in arch/x86/cpu/baytrail which provide Bay Trail support.
If you are not using an FSP you have more freedom and more responsibility.
The ivybridge support works this way, although it still uses a ROM for
graphics and still has binary blobs containing Intel code. You should aim to
support all important peripherals on your platform including video and storage.
Use the device tree for configuration where possible.
For the microcode you can create a suitable device tree file using the
microcode tool:
./tools/microcode-tool -d microcode.dat create <model>
or if you only have header files and not the full Intel microcode.dat database:
./tools/microcode-tool -H BAY_TRAIL_FSP_KIT/Microcode/M0130673322.h \
-H BAY_TRAIL_FSP_KIT/Microcode/M0130679901.h \
create all
These are written to arch/x86/dts/microcode/ by default.
Note that it is possible to just add the micrcode for your CPU if you know its
model. U-Boot prints this information when it starts
CPU: x86_64, vendor Intel, device 30673h
so here we can use the M0130673322 file.
If you platform can display POST codes on two little 7-segment displays on
the board, then you can use post_code() calls from C or assembler to monitor
boot progress. This can be good for debugging.
If not, you can try to get serial working as early as possible. The early
debug serial port may be useful here. See setup_early_uart() for an example.
- Audio
- Chrome OS verified boot
- SMI and ACPI support, to provide platform info and facilities to Linux