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 payload on x86. So far only Link (Chromebook Pixel) and QEMU x86 targets have 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 is a main bootloader on Intel Edison board.

U-Boot also supports booting directly from x86 reset vector, without coreboot. In this case, known as bare mode, from the fact that it runs on the ‘bare metal’, U-Boot acts like a BIOS replacement. The following platforms are supported:

  • Bayley Bay CRB

  • Cherry Hill CRB

  • Congatec QEVAL 2.0 & conga-QA3/E3845

  • Cougar Canyon 2 CRB

  • Crown Bay CRB

  • Galileo

  • Link (Chromebook Pixel)

  • Minnowboard MAX

  • Samus (Chromebook Pixel 2015)

  • QEMU x86 (32-bit & 64-bit)

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. U-Boot supports loading an x86 VxWorks kernel. Please check README.vxworks for more details.

Build Instructions for U-Boot as BIOS replacement (bare mode)

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 may print some warnings if required binary blobs (e.g.: FSP) are not present.

CPU Microcode

Modern CPUs usually require a special bit stream called microcode 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.

SMP Support

On a multicore system, U-Boot is executed on the bootstrap processor (BSP). Additional application processors (AP) can be brought up by U-Boot. In order to have an SMP kernel to discover all of the available processors, U-Boot needs to prepare configuration tables which contain the multi-CPUs information before loading the OS kernel. Currently U-Boot supports generating two types of tables for SMP, called Simple Firmware Interface (SFI) and Multi-Processor (MP) tables. The writing of these two tables are controlled by two Kconfig options GENERATE_SFI_TABLE and GENERATE_MP_TABLE.

Driver Model

x86 has been converted to use driver model for serial, GPIO, SPI, SPI flash, keyboard, real-time clock, USB. Video is in progress.

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 useful:


Display information about Intel Firmware Support Package (FSP). This is only available on platforms which use FSP, mostly Atom.


Display I/O memory


Write I/O memory


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.

Booting Ubuntu

As an example of how to set up your boot flow with U-Boot, here are instructions for starting Ubuntu from U-Boot. These instructions have been tested on Minnowboard MAX with a SATA drive but are equally applicable on other platforms and other media. There are really only four steps and it’s a very simple script, but a more detailed explanation is provided here for completeness.

Note: It is possible to set up U-Boot to boot automatically using syslinux. It could also use the grub.cfg file (/efi/ubuntu/grub.cfg) to obtain the GUID. If you figure these out, please post patches to this README.

Firstly, you will need Ubuntu installed on an available disk. It should be possible to make U-Boot start a USB start-up disk but for now let’s assume that you used another boot loader to install Ubuntu.

Use the U-Boot command line to find the UUID of the partition you want to boot. For example our disk is SCSI device 0:

=> part list scsi 0

Partition Map for SCSI device 0  --   Partition Type: EFI

   Part      Start LBA       End LBA         Name
     Type GUID
     Partition GUID
   1 0x00000800      0x001007ff      ""
     attrs:  0x0000000000000000
     type:   c12a7328-f81f-11d2-ba4b-00a0c93ec93b
     guid:   9d02e8e4-4d59-408f-a9b0-fd497bc9291c
   2 0x00100800      0x037d8fff      ""
     attrs:  0x0000000000000000
     type:   0fc63daf-8483-4772-8e79-3d69d8477de4
     guid:   965c59ee-1822-4326-90d2-b02446050059
   3 0x037d9000      0x03ba27ff      ""
     attrs:  0x0000000000000000
     type:   0657fd6d-a4ab-43c4-84e5-0933c84b4f4f
     guid:   2c4282bd-1e82-4bcf-a5ff-51dedbf39f17

This shows that your SCSI disk has three partitions. The really long hex strings are called Globally Unique Identifiers (GUIDs). You can look up the ‘type’ ones here. On this disk the first partition is for EFI and is in VFAT format (DOS/Windows):

=> fatls scsi 0:1

0 file(s), 1 dir(s)

Partition 2 is ‘Linux filesystem data’ so that will be our root disk. It is in ext2 format:

=> ext2ls scsi 0:2
<DIR>       4096 .
<DIR>       4096 ..
<DIR>      16384 lost+found
<DIR>       4096 boot
<DIR>      12288 etc
<DIR>       4096 media
<DIR>       4096 bin
<DIR>       4096 dev
<DIR>       4096 home
<DIR>       4096 lib
<DIR>       4096 lib64
<DIR>       4096 mnt
<DIR>       4096 opt
<DIR>       4096 proc
<DIR>       4096 root
<DIR>       4096 run
<DIR>      12288 sbin
<DIR>       4096 srv
<DIR>       4096 sys
<DIR>       4096 tmp
<DIR>       4096 usr
<DIR>       4096 var
<SYM>         33 initrd.img
<SYM>         30 vmlinuz
<DIR>       4096 cdrom
<SYM>         33 initrd.img.old

and if you look in the /boot directory you will see the kernel:

=> ext2ls scsi 0:2 /boot
<DIR>       4096 .
<DIR>       4096 ..
<DIR>       4096 efi
<DIR>       4096 grub
         3381262 System.map-3.13.0-32-generic
         1162712 abi-3.13.0-32-generic
          165611 config-3.13.0-32-generic
          176500 memtest86+.bin
          178176 memtest86+.elf
          178680 memtest86+_multiboot.bin
         5798112 vmlinuz-3.13.0-32-generic
          165762 config-3.13.0-58-generic
         1165129 abi-3.13.0-58-generic
         5823136 vmlinuz-3.13.0-58-generic
        19215259 initrd.img-3.13.0-58-generic
         3391763 System.map-3.13.0-58-generic
         5825048 vmlinuz-3.13.0-58-generic.efi.signed
        28304443 initrd.img-3.13.0-32-generic

The ‘vmlinuz’ files contain a packaged Linux kernel. The format is a kind of self-extracting compressed file mixed with some ‘setup’ configuration data. Despite its size (uncompressed it is >10MB) this only includes a basic set of device drivers, enough to boot on most hardware types.

The ‘initrd’ files contain a RAM disk. This is something that can be loaded into RAM and will appear to Linux like a disk. Ubuntu uses this to hold lots of drivers for whatever hardware you might have. It is loaded before the real root disk is accessed.

The numbers after the end of each file are the version. Here it is Linux version 3.13. You can find the source code for this in the Linux tree with the tag v3.13. The ‘.0’ allows for additional Linux releases to fix problems, but normally this is not needed. The ‘-58’ is used by Ubuntu. Each time they release a new kernel they increment this number. New Ubuntu versions might include kernel patches to fix reported bugs. Stable kernels can exist for some years so this number can get quite high.

The ‘.efi.signed’ kernel is signed for EFI’s secure boot. U-Boot has its own secure boot mechanism - see this & that. It cannot read .efi files at present.

To boot Ubuntu from U-Boot the steps are as follows:

  1. Set up the boot arguments. Use the GUID for the partition you want to boot:

    => setenv bootargs root=/dev/disk/by-partuuid/965c59ee-1822-4326-90d2-b02446050059 ro

Here root= tells Linux the location of its root disk. The disk is specified by its GUID, using ‘/dev/disk/by-partuuid/’, a Linux path to a ‘directory’ containing all the GUIDs Linux has found. When it starts up, there will be a file in that directory with this name in it. It is also possible to use a device name here, see later.

  1. Load the kernel. Since it is an ext2/4 filesystem we can do:

    => ext2load scsi 0:2 03000000 /boot/vmlinuz-3.13.0-58-generic

The address 30000000 is arbitrary, but there seem to be problems with using small addresses (sometimes Linux cannot find the ramdisk). This is 48MB into the start of RAM (which is at 0 on x86).

  1. Load the ramdisk (to 64MB):

    => ext2load scsi 0:2 04000000 /boot/initrd.img-3.13.0-58-generic
  2. Start up the kernel. We need to know the size of the ramdisk, but can use a variable for that. U-Boot sets ‘filesize’ to the size of the last file it loaded:

    => zboot 03000000 0 04000000 ${filesize}

Type ‘help zboot’ if you want to see what the arguments are. U-Boot on x86 is quite verbose when it boots a kernel. You should see these messages from U-Boot:

Valid Boot Flag
Setup Size = 0x00004400
Magic signature found
Using boot protocol version 2.0c
Linux kernel version 3.13.0-58-generic (buildd@allspice) #97-Ubuntu SMP Wed Jul 8 02:56:15 UTC 2015
Building boot_params at 0x00090000
Loading bzImage at address 100000 (5805728 bytes)
Magic signature found
Initial RAM disk at linear address 0x04000000, size 19215259 bytes
Kernel command line: "root=/dev/disk/by-partuuid/965c59ee-1822-4326-90d2-b02446050059 ro"

Starting kernel ...

U-Boot prints out some bootstage timing. This is more useful if you put the above commands into a script since then it will be faster:

Timer summary in microseconds:
       Mark    Elapsed  Stage
          0          0  reset
    241,535    241,535  board_init_r
  2,421,611  2,180,076  id=64
  2,421,790        179  id=65
  2,428,215      6,425  main_loop
 48,860,584 46,432,369  start_kernel

Accumulated time:
               240,329  ahci
             1,422,704  vesa display

Now the kernel actually starts (if you want to examine kernel boot up message on the serial console, append “console=ttyS0,115200” to the kernel command line):

[    0.000000] Initializing cgroup subsys cpuset
[    0.000000] Initializing cgroup subsys cpu
[    0.000000] Initializing cgroup subsys cpuacct
[    0.000000] Linux version 3.13.0-58-generic (buildd@allspice) (gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1) ) #97-Ubuntu SMP Wed Jul 8 02:56:15 UTC 2015 (Ubuntu 3.13.0-58.97-generic 3.13.11-ckt22)
[    0.000000] Command line: root=/dev/disk/by-partuuid/965c59ee-1822-4326-90d2-b02446050059 ro console=ttyS0,115200

It continues for a long time. Along the way you will see it pick up your ramdisk:

[    0.000000] RAMDISK: [mem 0x04000000-0x05253fff]
[    0.788540] Trying to unpack rootfs image as initramfs...
[    1.540111] Freeing initrd memory: 18768K (ffff880004000000 - ffff880005254000)

Later it actually starts using it:

Begin: Running /scripts/local-premount ... done.

You should also see your boot disk turn up:

[    4.357243] scsi 1:0:0:0: Direct-Access     ATA      ADATA SP310      5.2  PQ: 0 ANSI: 5
[    4.366860] sd 1:0:0:0: [sda] 62533296 512-byte logical blocks: (32.0 GB/29.8 GiB)
[    4.375677] sd 1:0:0:0: Attached scsi generic sg0 type 0
[    4.381859] sd 1:0:0:0: [sda] Write Protect is off
[    4.387452] sd 1:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[    4.399535]  sda: sda1 sda2 sda3

Linux has found the three partitions (sda1-3). Mercifully it doesn’t print out the GUIDs. In step 1 above we could have used:

setenv bootargs root=/dev/sda2 ro

instead of the GUID. However if you add another drive to your board the numbering may change whereas the GUIDs will not. So if your boot partition becomes sdb2, it will still boot. For embedded systems where you just want to boot the first disk, you have that option.

The last thing you will see on the console is mention of plymouth (which displays the Ubuntu start-up screen) and a lot of ‘Starting’ messages:

* Starting Mount filesystems on boot                                   [ OK ]

After a pause you should see a login screen on your display and you are done.

If you want to put this in a script you can use something like this:

setenv bootargs root=UUID=b2aaf743-0418-4d90-94cc-3e6108d7d968 ro
setenv boot zboot 03000000 0 04000000 \${filesize}
setenv bootcmd "ext2load scsi 0:2 03000000 /boot/vmlinuz-3.13.0-58-generic; ext2load scsi 0:2 04000000 /boot/initrd.img-3.13.0-58-generic; run boot"

The is to tell the shell not to evaluate ${filesize} as part of the setenv command.

You can also bake this behaviour into your build by hard-coding the environment variables if you add this to minnowmax.h:

        "ext2load scsi 0:2 03000000 /boot/vmlinuz-3.13.0-58-generic; " \
        "ext2load scsi 0:2 04000000 /boot/initrd.img-3.13.0-58-generic; " \
        "run boot"

#define CFG_EXTRA_ENV_SETTINGS "boot=zboot 03000000 0 04000000 ${filesize}"

and change CONFIG_BOOTARGS value in configs/minnowmax_defconfig to:

CONFIG_BOOTARGS="root=/dev/sda2 ro"

Test with SeaBIOS

SeaBIOS is an open source implementation of a 16-bit x86 BIOS. It can run in an emulator or natively on x86 hardware with the use of U-Boot. With its help, we can boot some OSes that require 16-bit BIOS services like Windows/DOS.

As U-Boot, we have to manually create a table where SeaBIOS gets various system information (eg: E820) from. The table unfortunately has to follow the coreboot table format as SeaBIOS currently supports booting as a coreboot payload.

To support loading SeaBIOS, U-Boot should be built with CONFIG_SEABIOS on. Booting SeaBIOS is done via U-Boot’s bootelf command, like below:

=> tftp bios.bin.elf;bootelf
Using e1000#0 device
TFTP from server; our IP address is
Bytes transferred = 128748 (1f6ec hex)
## Starting application at 0x000fd269 ...
SeaBIOS (version rel-1.14.0-0-g155821a)

bios.bin.elf is the SeaBIOS image built from SeaBIOS source tree. At the time being, SeaBIOS release 1.14.0 has been tested. To build the SeaBIOS image:

$ make olddefconfig
$ make
Total size: 128512  Fixed: 69216  Free: 2560 (used 98.0% of 128KiB rom)
Creating out/bios.bin.elf

Currently this is tested on QEMU x86 target with U-Boot chain-loading SeaBIOS to install/boot a Windows XP OS (below for example command to install Windows).

# Create a 10G disk.img as the virtual hard disk
$ qemu-img create -f qcow2 disk.img 10G

# Install a Windows XP OS from an ISO image 'winxp.iso'
$ qemu-system-i386 -serial stdio -bios u-boot.rom -hda disk.img -cdrom winxp.iso -smp 2 -m 512

# Boot a Windows XP OS installed on the virutal hard disk
$ qemu-system-i386 -serial stdio -bios u-boot.rom -hda disk.img -smp 2 -m 512

This is also tested on Intel Crown Bay board with a PCIe graphics card, booting SeaBIOS then chain-loading a GRUB on a USB drive, then Linux kernel finally.

If you are using Intel Integrated Graphics Device (IGD) as the primary display device on your board, SeaBIOS needs to be patched manually to get its VGA ROM loaded and run by SeaBIOS. SeaBIOS locates VGA ROM via the PCI expansion ROM register, but IGD device does not have its VGA ROM mapped by this register. Its VGA ROM is packaged as part of u-boot.rom at a configurable flash address which is unknown to SeaBIOS. An example patch is needed for SeaBIOS below:

diff --git a/src/optionroms.c b/src/optionroms.c
index 65f7fe0..c7b6f5e 100644
--- a/src/optionroms.c
+++ b/src/optionroms.c
@@ -324,6 +324,8 @@ init_pcirom(struct pci_device *pci, int isvga, u64 *sources)
         rom = deploy_romfile(file);
     else if (RunPCIroms > 1 || (RunPCIroms == 1 && isvga))
         rom = map_pcirom(pci);
+    if (pci->bdf == pci_to_bdf(0, 2, 0))
+        rom = (struct rom_header *)0xfff90000;
     if (! rom)
         // No ROM present.

Note: the patch above expects IGD device is at PCI b.d.f 0.2.0 and its VGA ROM is at 0xfff90000 which corresponds to CONFIG_VGA_BIOS_ADDR on Minnowboard MAX. Change these two accordingly if this is not the case on your board.

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: http://review.coreboot.org/p/em100.git

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: http://www.dediprog.com/pd/programmer-accessories/EM-TC-8.

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 -m <model> create

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 -m all create

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_internal_uart() for an example.

During the U-Boot porting, one of the important steps is to write correct PIRQ routing information in the board device tree. Without it, device drivers in the Linux kernel won’t function correctly due to interrupt is not working. Please refer to U-Boot doc for the device tree bindings of Intel interrupt router. Here we have more details on the intel,pirq-routing property below.

intel,pirq-routing = <
        PCI_BDF(0, 2, 0) INTA PIRQA

As you see each entry has 3 cells. For the first one, we need describe all pci devices mounted on the board. For SoC devices, normally there is a chapter on the chipset datasheet which lists all the available PCI devices. For example on Bay Trail, this is chapter 4.3 (PCI configuration space). For the second one, we can get the interrupt pin either from datasheet or hardware via U-Boot shell. The reliable source is the hardware as sometimes chipset datasheet is not 100% up-to-date. Type ‘pci header’ plus the device’s pci bus/device/function number from U-Boot shell below:

=> pci header 0.1e.1
  vendor ID =                 0x8086
  device ID =                 0x0f08
  interrupt line =            0x09
  interrupt pin =             0x04

It shows this PCI device is using INTD pin as it reports 4 in the interrupt pin register. Repeat this until you get interrupt pins for all the devices. The last cell is the PIRQ line which a particular interrupt pin is mapped to. On Intel chipset, the power-up default mapping is INTA/B/C/D maps to PIRQA/B/C/D. This can be changed by registers in LPC bridge. So far Intel FSP does not touch those registers so we can write down the PIRQ according to the default mapping rule.

Once we get the PIRQ routing information in the device tree, the interrupt allocation and assignment will be done by U-Boot automatically. Now you can enable CONFIG_GENERATE_PIRQ_TABLE for testing Linux kernel using i8259 PIC and CONFIG_GENERATE_MP_TABLE for testing Linux kernel using local APIC and I/O APIC.

This script might be useful. If you feed it the output of ‘pci long’ from U-Boot then it will generate a device tree fragment with the interrupt configuration for each device (note it needs gawk 4.0.0):

$ cat console_output |awk '/PCI/ {device=$4} /interrupt line/ {line=$4} \
     /interrupt pin/ {pin = $4; if (pin != "0x00" && pin != "0xff") \
     {patsplit(device, bdf, "[0-9a-f]+"); \
     printf "PCI_BDF(%d, %d, %d) INT%c PIRQ%c\n", strtonum("0x" bdf[1]), \
     strtonum("0x" bdf[2]), bdf[3], strtonum(pin) + 64, 64 + strtonum(pin)}}'

Example output:


Porting Hints

Quark-specific considerations

To port U-Boot to other boards based on the Intel Quark SoC, a few things need to be taken care of. The first important part is the Memory Reference Code (MRC) parameters. Quark MRC supports memory-down configuration only. All these MRC parameters are supplied via the board device tree. To get started, first copy the MRC section of arch/x86/dts/galileo.dts to your board’s device tree, then change these values by consulting board manuals or your hardware vendor. Available MRC parameter values are listed in include/dt-bindings/mrc/quark.h. The other tricky part is with PCIe. Quark SoC integrates two PCIe root ports, but by default they are held in reset after power on. In U-Boot, PCIe initialization is properly handled as per Quark’s firmware writer guide. In your board support codes, you need provide two routines to aid PCIe initialization, which are board_assert_perst() and board_deassert_perst(). The two routines need implement a board-specific mechanism to assert/deassert PCIe PERST# pin. Care must be taken that in those routines that any APIs that may trigger PCI enumeration process are strictly forbidden, as any access to PCIe root port’s configuration registers will cause system hang while it is held in reset. For more details, check how they are implemented by the Intel Galileo board support codes in board/intel/galileo/galileo.c.


See scripts/coreboot.sed which can assist with porting coreboot code into U-Boot drivers. It will not resolve all build errors, but will perform common transformations. Remember to add attribution to coreboot for new files added to U-Boot. This should go at the top of each file and list the coreboot filename where the code originated.

Debugging ACPI issues with Windows

Windows might cache system information and only detect ACPI changes if you modify the ACPI table versions. So tweak them liberally when debugging ACPI issues with Windows.

ACPI Support Status

Advanced Configuration and Power Interface (ACPI) aims to establish industry-standard interfaces enabling OS-directed configuration, power management, and thermal management of mobile, desktop, and server platforms.

Linux can boot without ACPI with “acpi=off” command line parameter, but with ACPI the kernel gains the capabilities to handle power management. For Windows, ACPI is a must-have firmware feature since Windows Vista. CONFIG_GENERATE_ACPI_TABLE is the config option to turn on ACPI support in U-Boot. This requires Intel ACPI compiler to be installed on your host to compile ACPI DSDT table written in ASL format to AML format. You can get the compiler via “apt-get install iasl” if you are on Ubuntu or download the source from https://www.acpica.org/downloads to compile one by yourself.

Current ACPI support in U-Boot is basically complete. More optional features can be added in the future. The status as of today is:

  • Support generating RSDT, XSDT, FACS, FADT, MADT, MCFG tables.

  • Support one static DSDT table only, compiled by Intel ACPI compiler.

  • Support S0/S3/S4/S5, reboot and shutdown from OS.

  • Support booting a pre-installed Ubuntu distribution via ‘zboot’ command.

  • Support installing and booting Ubuntu 14.04 (or above) from U-Boot with the help of SeaBIOS using legacy interface (non-UEFI mode).

  • Support installing and booting Windows 8.1/10 from U-Boot with the help of SeaBIOS using legacy interface (non-UEFI mode).

  • Support ACPI interrupts with SCI only.

Features that are optional:

  • Dynamic AML bytecodes insertion at run-time. We may need this to support SSDT table generation and DSDT fix up.

  • SMI support. Since U-Boot is a modern bootloader, we don’t want to bring those legacy stuff into U-Boot. ACPI spec allows a system that does not support SMI (a legacy-free system).

ACPI was initially enabled on BayTrail based boards. Testing was done by booting a pre-installed Ubuntu 14.04 from a SATA drive. Installing Ubuntu 14.04 and Windows 8.1/10 to a SATA drive and booting from there is also tested. Most devices seem to work correctly and the board can respond a reboot/shutdown command from the OS.

For other platform boards, ACPI support status can be checked by examining their board defconfig files to see if CONFIG_GENERATE_ACPI_TABLE is set to y.

The S3 sleeping state is a low wake latency sleeping state defined by ACPI spec where all system context is lost except system memory. To test S3 resume with a Linux kernel, simply run “echo mem > /sys/power/state” and kernel will put the board to S3 state where the power is off. So when the power button is pressed again, U-Boot runs as it does in cold boot and detects the sleeping state via ACPI register to see if it is S3, if yes it means we are waking up. U-Boot is responsible for restoring the machine state as it is before sleep. When everything is done, U-Boot finds out the wakeup vector provided by OSes and jump there. To determine whether ACPI S3 resume is supported, check to see if CONFIG_HAVE_ACPI_RESUME is set for that specific board.

Note for testing S3 resume with Windows, correct graphics driver must be installed for your platform, otherwise you won’t find “Sleep” option in the “Power” submenu from the Windows start menu.

EFI Support

U-Boot supports booting as a 32-bit or 64-bit EFI payload, e.g. with UEFI. This is enabled with CONFIG_EFI_STUB to boot from both 32-bit and 64-bit UEFI BIOS. U-Boot can also run as an EFI application, with CONFIG_EFI_APP. The CONFIG_EFI_LOADER option, where U-Boot provides an EFI environment to the kernel (i.e. replaces UEFI completely but provides the same EFI run-time services) is supported too. For example, we can even use ‘bootefi’ command to load a ‘u-boot-payload.efi’, see below test logs on QEMU.

=> load ide 0 3000000 u-boot-payload.efi
489787 bytes read in 138 ms (3.4 MiB/s)
=> bootefi 3000000
Scanning disk ide.blk#0...
Found 2 disks
WARNING: booting without device tree
## Starting EFI application at 03000000 ...
U-Boot EFI Payload

U-Boot 2018.07-rc2 (Jun 23 2018 - 17:12:58 +0800)

CPU: x86_64, vendor AMD, device 663h
DRAM:  2 GiB
Video: 1024x768x32
Model: EFI x86 Payload
Net:   e1000: 52:54:00:12:34:56

Warning: e1000#0 using MAC address from ROM
eth0: e1000#0
No controllers found
Hit any key to stop autoboot:  0

See U-Boot on EFI and UEFI on U-Boot for details of EFI support in U-Boot.


U-Boot can be chain-loaded from another bootloader, such as coreboot or Slim Bootloader. Typically this is done by building for targets ‘coreboot’ or ‘slimbootloader’.

For example, at present we have a ‘coreboot’ target but this runs very different code from the bare-metal targets, such as coral. There is very little in common between them.

It is useful to be able to boot the same U-Boot on a device, with or without a first-stage bootloader. For example, with chromebook_coral, it is helpful for testing to be able to boot the same U-Boot (complete with FSP) on bare metal and from coreboot. It allows checking of things like CPU speed, comparing registers, ACPI tables and the like.

To do this you can use ll_boot_init() in appropriate places to skip init that has already been done by the previous stage. This works by setting a GD_FLG_NO_LL_INIT flag when U-Boot detects that it is running from another bootloader.

With this feature, you can build a bare-metal target and boot it from coreboot, for example.

Note that this is a development feature only. It is not intended for use in production environments. Also it is not currently part of the automated tests so may break in the future.

SMBIOS tables

To generate SMBIOS tables in U-Boot, for use by the OS, enable the CONFIG_GENERATE_SMBIOS_TABLE option. The easiest way to provide the values to use is via the device tree. For details see smbios.txt.


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