On 5/9/11 5:53 PM, Graeme Russ wrote:
Yes. The information is published by coreboot in the "coreboot table"
AKA cbtable. libpayload already has code to find it.
Hmm - I don't think linking U-Boot against libpayload is the right
solution. U-Boot is designed to be the primary bootloader
much like coreboot is - It is not a 'payload'. There is a rather special
case with NAND booting where a bootstrap loader loads U-Boot into RAM,
but even in this case U-Boot is treated like a binary blob exectuted
in RAM with no real concept of how it got there (i.e. no link-backs to
the 'loader')
That's not the case for all platforms. On Nvidia Tegra2 there is a boot
rom loading uboot. I remember on the Dbox uboot was piggybacked on the
original firmware, too. At the least, uboot should play nice in such
scenarios (which I think it does)
U-Boot is build with the target address in Flash. It initialises memory
and copies itself into RAM at an address calculated by the RAM init code.
It then processes the ELF symbol table (embedded in the raw binary image)
to adjust memory addresses in the in-RAM copy before jumping out of flash
and into RAM
We should add a configuration option to avoid this copying with respect
to boot time.
So what would be really neat is if coreboot calculated where the final
in-RAM location of U-Boot needs to be, copy U-Boot there, do the
relocation fixups and jumps into the in-RAM copy of U-Boot.
Another option would be to have uboot do a virtual/physical mapping that
allows us to skip the run time linking. As the copying, it's a (small
but measurable) waste of time, and it makes the code unnecessarily
complex. I'm not suggesting to remove this feature, but I think it
should be configurable (and disabled for coreboot and anyone else who
aims at booting quickly)
U-Boot would
then skip all it's own relocation code. The other option is to not use
coreboot's ELF loader and simply have coreboot jump to the U-Boot binary
image in Flash. This is a far simpler method, and all the code already
exists in U-Boot anyway - all we need to do is tell U-Boot the highest
available memory address.
coreboot does its ELF parsing at build time, not at runtime. This way we
save yet another copy while preserving the option of compressing the
payload (e.g. uboot) in flash.
The sequence would then be coreboot->SeaBIOS->U-Boot. Note that
coreboot is not a BIOS and does not want to be a BIOS.
Neither is (or does) U-Boot. But in order to load many OSs, there needs
(unfortunately) to be some BIOS functionality.
Piggybacking coreboot->seabios->uboot should not be a big deal, but the
most common use case for uboot is booting Linux, not Windows or other
bios prone OSes. Let's do one step after another.
- Can we move more hardware init and drivers from coreboot into
u-boot and provide more commands in u-boot for coreboot
supported boards
Code duplication? *shudder*
The point being that U-Boot provides a shell and command line with
diagnostic features. These require hooks into the hardware drivers.
coreboot does not generally have drivers. coreboot knows how to do
the init, but the payload is responsible for driving any hardware
that needs to be driven.
One could argue that coreboot is only a collection of CPU and chipset
drivers, which so far have very few if any knobs.
U-Boot is similar, but having a command line and shell which can run
scripts means that it can do a bit more with the hardware (set Ethernet
MAC addresses, intialise devices on I2C busses, display a bitmap splash
screen, toggle LEDs etc)
Those things are often not available or required on the systems coreboot
runs on.
- VGA& Keyboard support
- U-Boot splash screen support
libpayload
- Flash updates from u-boot
libflashrom (work in progress)
I don't think we want to plug flashrom into uboot (not in the beginning
anyways) nor libpayload. Let's keep this minimal and then see what's
actually missing. A clean design is more important right now than
overplugging uboot with foreign libraries.
As I said earlier, I don't want to tie U-Boot to libpayload. With regard
to VGA and keyboard support, how can we make them usable in U-Boot (i.e.
so the command prompt is displayed on a monitor and the user can use the
keyboard to enter commands?)
Does uboot provide a USB stack on PCs (e.g. UHCI/OHCI/EHCI/XHCI)...
those might be interesting parts of libpayload in the longer run.
For now, just adding a simple keyboard and vga driver should be fairly
simple. It would be nice to also have an option for running on a linear
framebuffer (with the framebuffer specs read from the coreboot table)
Stefan
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