[coreboot] U-Boot-x86 / coreboot Integration
graeme.russ at gmail.com
Tue May 10 16:33:56 CEST 2011
I figured it might be best to start a new, clean thread dealing with the
technical design aspects of bootstrapping U-Boot from coreboot.
I am extremely excited about this as the x86 U-Boot maintainer, but even
more so by the idea of two very mature and respected FLOSS projects
potentially becoming greater than the sum of their parts :)
OK, enough with the warm and fuzzies. Lets look first at a few facts (as I
understand them - please feel free to correct me):
- U-Boot is a bootloader for embedded systems - A market segment
dominated by ARM, PPC et al - i.e. NOT x86
- coreboot is a 'BIOS' replacement for mainstream PC's - A market segment
dominated by x86
- Both are principally designed as 'primary bootloaders' - i.e. intended
to be executed at CPU reset and responsible for low level hardware
- U-Boot has no support for modern x86 PC hardware (North and South
bridges, Dual-Core x86 CPUs, microcode, ACPI, APM etc)
- coreboot is 'dumb' (No command line, scripting etc) As I understand it
you build a 'payload' into the coreboot image which coreboot simply runs
after it has performed necessary low-level hardware initialisation
- U-Boot is 'smart' (command line, scripting, network support, environment
variables etc) but currently lacks the ability to perform low level
hardware initialisation of x86 PC hardware
- coreboot launches a 'payload' which is (typically) an ELF executable
linked to 'libpayload'. libpayload provides access to some primitive
libc functionality, I/O and coreboot generated data structures
- Our primary target OS is GNU/Linux (of course!)
- The majority of U-Boot and coreboot is licensed under the 'GPLv2 or (at
your option) any later version'
So coreboot and U-Boot are a good complement to each other so bringing
U-Boot to x86 PC mainboards via coreboot looks like a good idea - Now the
- The U-Boot source 'must' be self contained - No external libraries.
Incorporating license compatible source is OK
- coreboot payloads should be in ELF (linked to libpayload)
- There should be minimal intrusion into the core U-Boot build scripts
(Makefiles, mk.configs etc) - I would assume the same applies to
coreboot build files as well. Hacking the U-Boot x86 specific build
files should be fine
- Everything should 'just work' with a recent GNU toolchain (gcc,
- U-Boot relocates to 'Top of RAM' - This is a fundamental architectural
design and not x86 specific. This feature should be retained for
consistency with other U-Boot arches
- Do we care about legacy BIOS support (SeaBIOS) for now (I think not)?
So, a few questions
- How much of libpayload would we need to bring into U-Boot to provide
bare minimal payload delivery? U-Boot already contains it's own minimal
- How do we get VGA and USB keyboard support working? Do other U-Boot
boards implement console on anything other than serial?
- Can we add relocation support to the coreboot ELF loader?
- Does coreboot relocate into RAM? If so, what is the target address? What
guarantee is there that the target address is valid?
- Could coreboot benefit from U-Boot's 'load to top of RAM' philosophy?
- Is it worth playing around with segment registers to 'relocate' U-Boot
- What hardware should be initialised in coreboot and what should be
initialised in U-Boot? (political question ;)
- What about Chipset Microcode (CMC)
- What about System Management Mode (SMM)
I think a good start would be to create a new U-Boot target which includes
a stripped down libpayload in the U-Boot source. This target can exclude
most of the assembler startup code (resetvec.S, start16.S, start.S). I
assume the coreboot ELF loader (or libpayload) takes care of setting up the
C environment (stack etc).
We can start with U-Boot linked to a fixed location in RAM and skip
relocations then work on either extending coreboot to perform relocation
fixups or have U-Boot perform the fixups based on RAM information read from
>From there, we can start to add device support (USB, video, PCI, IDE/SATA etc)
It's getting late, so I'll stop now. Hopefully this gives a good design
platform to start from
P.S. Please keep both U-Boot and coreboot mailing lists Cc'd - Note: If you
are not on the coreboot ml, you emails will get bounced to a moderator :(
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