path: root/doc/README.x86

#
# Copyright (C) 2014, Simon Glass <sjg@chromium.org>
# Copyright (C) 2014, Bin Meng <bmeng.cn@gmail.com>
#
# 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.

Status
------
U-Boot supports running as a coreboot [1] payload on x86. So far only Link
(Chromebook Pixel) and QEMU [2] 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 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. Currently Link, QEMU x86
targets and all Intel boards 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 for U-Boot as coreboot payload
-------------------------------------------------
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 QEMU board.
To build a coreboot payload against another board, you can change the build
configuration during the 'make menuconfig' process.

x86 architecture  --->
	...
	(qemu-x86) Board configuration file
	(qemu-x86_i440fx) Board Device Tree Source (dts) file
	(0x01920000) 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.

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 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.

---

Chromebook Link specific instructions for bare mode:

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 http://review.coreboot.org/p/blobs.git
$ cd blobs

Find the following files:

* ./mainboard/google/link/descriptor.bin
* ./mainboard/google/link/me.bin
* ./northbridge/intel/sandybridge/systemagent-r6.bin

The 3rd one should be renamed to mrc.bin.
As for the video ROM, you can get it here [3] and rename it to vga.bin.
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 for bare mode:

U-Boot support of Intel Crown Bay board [4] relies on a binary blob called
Firmware Support Package [5] 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.

* ./FSP/QUEENSBAY_FSP_GOLD_001_20-DECEMBER-2013.fd
* ./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.

As for the video ROM, you need manually extract it from the Intel provided
BIOS for Crown Bay here [6], using the AMI MMTool [7]. Check PCI option ROM
ID 8086:4108, extract and save it as vga.bin in the board directory.

Now you can build U-Boot and obtain u-boot.rom

$ make crownbay_defconfig
$ make all

---

Intel Minnowboard Max instructions for bare mode:

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:
board/intel/minnowmax/fsp.bin

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. The first comes from the original
firmware image available from:

http://firmware.intel.com/sites/default/files/2014-WW42.4-MinnowBoardMax.73-64-bit.bin_Release.zip

Unzip it:

   $ unzip 2014-WW42.4-MinnowBoardMax.73-64-bit.bin_Release.zip

Use ifdtool in the U-Boot tools directory to extract the images from that
file, for example:

   $ ./tools/ifdtool -x MNW2MAX1.X64.0073.R02.1409160934.bin

This will provide the descriptor file - copy this into the correct place:

   $ cp flashregion_0_flashdescriptor.bin board/intel/minnowmax/descriptor.bin

Then do the same with the sample SPI image provided in the FSP (SPI.bin at
the time of writing) to obtain the last image. Note that this will also
produce a flash descriptor file, but it does not seem to work, probably
because it is not designed for the Minnowmax. That is why you need to get
the flash descriptor from the original firmware as above.

   $ ./tools/ifdtool -x BayleyBay/SPI.bin
   $ cp flashregion_2_intel_me.bin board/intel/minnowmax/me.bin

Now you can build U-Boot and obtain u-boot.rom

$ make minnowmax_defconfig
$ make all

Checksums are as follows (but note that newer versions will invalidate this):

$ md5sum -b board/intel/minnowmax/*.bin
ffda9a3b94df5b74323afb328d51e6b4  board/intel/minnowmax/descriptor.bin
69f65b9a580246291d20d08cbef9d7c5  board/intel/minnowmax/fsp.bin
894a97d371544ec21de9c3e8e1716c4b  board/intel/minnowmax/me.bin
a2588537da387da592a27219d56e9962  board/intel/minnowmax/vga.bin

The ROM image is broken up into these parts:

Offset   Description         Controlling config
------------------------------------------------------------
000000   descriptor.bin      Hard-coded to 0 in ifdtool
001000   me.bin              Set by the descriptor
500000   <spare>
700000   u-boot-dtb.bin      CONFIG_SYS_TEXT_BASE
790000   vga.bin             CONFIG_VGA_BIOS_ADDR
7c0000   fsp.bin             CONFIG_FSP_ADDR
7f8000   <spare>             (depends on size of fsp.bin)
7fe000   Environment         CONFIG_ENV_OFFSET
7ff800   U-Boot 16-bit boot  CONFIG_SYS_X86_START16

Overall ROM image size is controlled by CONFIG_ROM_SIZE.

---

Intel Galileo instructions for bare mode:

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

QEMU x86 target instructions:

To build u-boot.rom for QEMU x86 targets, just simply run

$ make qemu-x86_defconfig
$ make all

Note this default configuration will build a U-Boot for the QEMU x86 i440FX
board. To build a U-Boot against QEMU x86 Q35 board, you can change the build
configuration during the 'make menuconfig' process like below:

Device Tree Control  --->
	...
	(qemu-x86_q35) Default Device Tree for DT control

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 0x1110000

Make sure 0x1110000 matches CONFIG_SYS_TEXT_BASE, which is the symbol address
of _x86boot_start (in arch/x86/cpu/start.S).

If you want to use ELF as the coreboot payload, change U-Boot configuration to
use CONFIG_OF_EMBED instead of CONFIG_OF_SEPARATE.

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.

Test with QEMU for bare mode
----------------------------
QEMU is a fancy emulator that can enable us to test U-Boot without access to
a real x86 board. Please make sure your QEMU version is 2.3.0 or above test
U-Boot. To launch QEMU with u-boot.rom, call QEMU as follows:

$ qemu-system-i386 -nographic -bios path/to/u-boot.rom

This will instantiate an emulated x86 board with i440FX and PIIX chipset. QEMU
also supports emulating an x86 board with Q35 and ICH9 based chipset, which is
also supported by U-Boot. To instantiate such a machine, call QEMU with:

$ qemu-system-i386 -nographic -bios path/to/u-boot.rom -M q35

Note by default QEMU instantiated boards only have 128 MiB system memory. But
it is enough to have U-Boot boot and function correctly. You can increase the
system memory by pass '-m' parameter to QEMU if you want more memory:

$ qemu-system-i386 -nographic -bios path/to/u-boot.rom -m 1024

This creates a board with 1 GiB system memory. Currently U-Boot for QEMU only
supports 3 GiB maximum system memory and reserves the last 1 GiB address space
for PCI device memory-mapped I/O and other stuff, so the maximum value of '-m'
would be 3072.

QEMU emulates a graphic card which U-Boot supports. Removing '-nographic' will
show QEMU's VGA console window. Note this will disable QEMU's serial output.
If you want to check both consoles, use '-serial stdio'.

Multicore is also supported by QEMU via '-smp n' where n is the number of cores
to instantiate. Currently the default U-Boot built for QEMU supports 2 cores.
In order to support more cores, you need add additional cpu nodes in the device
tree and change CONFIG_MAX_CPUS accordingly.

CPU Microcode
-------------
Modern CPUs usually require a special bit stream called microcode [8] 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) [9] and Multi-Processor (MP)
[10] 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 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
useful:

hob  - Display information about Firmware Support Package (FSP) Hand-off
	 Block. 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 driver but are equally applicable on
other platforms and other media. There are really only four steps and its 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 Ubunutu 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 [11]. 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 [12] [13] and 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: "console=ttyS0,115200 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:

   [    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: console=ttyS0,115200 root=/dev/disk/by-partuuid/965c59ee-1822-4326-90d2-b02446050059 ro

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 will also need to add this to your board configuration file, e.g.
include/configs/minnowmax.h:

   #define CONFIG_BOOTDELAY	2

Now when you reset your board it wait a few seconds (in case you want to
interrupt) and then should boot straight into Ubuntu.

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

#undef CONFIG_BOOTARGS
#undef CONFIG_BOOTCOMMAND

#define CONFIG_BOOTARGS		\
	"root=/dev/sda2 ro"
#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}"


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_early_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 [14] 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:
   PCI_BDF(0, 2, 0) INTA PIRQA
   PCI_BDF(0, 3, 0) INTA PIRQA
...

TODO List
---------
- Audio
- Chrome OS verified boot
- SMI and ACPI support, to provide platform info and facilities to Linux

References
----------
[1] http://www.coreboot.org
[2] http://www.qemu.org
[3] http://www.coreboot.org/~stepan/pci8086,0166.rom
[4] http://www.intel.com/content/www/us/en/embedded/design-tools/evaluation-platforms/atom-e660-eg20t-development-kit.html
[5] http://www.intel.com/fsp
[6] http://www.intel.com/content/www/us/en/secure/intelligent-systems/privileged/e6xx-35-b1-cmc22211.html
[7] http://www.ami.com/products/bios-uefi-tools-and-utilities/bios-uefi-utilities/
[8] http://en.wikipedia.org/wiki/Microcode
[9] http://simplefirmware.org
[10] http://www.intel.com/design/archives/processors/pro/docs/242016.htm
[11] https://en.wikipedia.org/wiki/GUID_Partition_Table
[12] http://events.linuxfoundation.org/sites/events/files/slides/chromeos_and_diy_vboot_0.pdf
[13] http://events.linuxfoundation.org/sites/events/files/slides/elce-2014.pdf
[14] doc/device-tree-bindings/misc/intel,irq-router.txt