u-boot/doc/arch/x86.rst

.. SPDX-License-Identifier: GPL-2.0+
.. Copyright (C) 2014, Simon Glass <sjg@chromium.org>
.. Copyright (C) 2014, Bin Meng <bmeng.cn@gmail.com>

x86
===

This document describes the information about U-Boot running on x86 targets,
including supported boards, build instructions, todo list, etc.

Status
------
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:

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

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
        Attributes
        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
               efi/

   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.

2. 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).

3. Load the ramdisk (to 64MB)::

   => ext2load scsi 0:2 04000000 /boot/initrd.img-3.13.0-58-generic

4. 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"
   saveenv

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:

.. code-block:: c

	#undef CONFIG_BOOTCOMMAND
	#define CONFIG_BOOTCOMMAND	\
		"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"

	#undef CONFIG_EXTRA_ENV_SETTINGS
	#define CONFIG_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 10.10.0.100; our IP address is 10.10.0.108
   ...
   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::

   $ echo -e 'CONFIG_COREBOOT=y\nCONFIG_COREBOOT_FLASH=n\nCONFIG_DEBUG_SERIAL=y\nCONFIG_DEBUG_COREBOOT=n' > .config
   $ 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).

.. code-block:: none

   # 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:

.. code-block:: none

   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.
            return;

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 <doc/device-tree-bindings/misc/intel,irq-router.txt>`_ for
the device tree bindings of Intel interrupt router. Here we have more details
on the intel,pirq-routing property below.

.. code-block:: none

	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::

   PCI_BDF(0, 2, 0) INTA PIRQA
   PCI_BDF(0, 3, 0) INTA PIRQA
   ...

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.

coreboot
^^^^^^^^

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.

.. code-block:: none

  => 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
  MMC:
  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 :doc:`../develop/uefi/u-boot_on_efi` and :doc:`../develop/uefi/uefi` for
details of EFI support in U-Boot.

Chain-loading
-------------
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
:download:`smbios.txt <../device-tree-bindings/sysinfo/smbios.txt>`.

TODO List
---------
- Audio
- Chrome OS verified boot

.. _coreboot: http://www.coreboot.org
.. _QEMU: http://www.qemu.org
.. _microcode: http://en.wikipedia.org/wiki/Microcode
.. _SFI: http://simplefirmware.org
.. _MP: http://www.intel.com/design/archives/processors/pro/docs/242016.htm
.. _here: https://en.wikipedia.org/wiki/GUID_Partition_Table
.. _this: http://events.linuxfoundation.org/sites/events/files/slides/chromeos_and_diy_vboot_0.pdf
.. _that: http://events.linuxfoundation.org/sites/events/files/slides/elce-2014.pdf
.. _SeaBIOS: http://www.seabios.org/SeaBIOS
.. _ACPI: http://www.acpi.info