[SeaBIOS] Starting SEABIOS as Grub payload and NOT loading roms/configuration from CBFS

[Dennis Guse]

Thanks - works like charm!
For reproducability: http://g00se.org/2016/11/seabios-on-libreboot.html

On Fri, Nov 18, 2016 at 4:45 PM, Kevin O'Connor <kevin at koconnor.net> wrote:
> On Fri, Nov 18, 2016 at 10:48:02AM +0100, Dennis Guse wrote:
>> I know usually Seabios is loaded by Coreboot and then Seabios loads
>> its modules from CBFS.
>> I want to do it slightly different:
>> * Hardware starts coreboot (actually libreboot flashed to a Lenovo X60)
>> * Coreboot starts Grub2
>> * Grub2 chainloads Seabios from external storage, e.g., USB0:
>> `chainloader (usb0,msdos1)/bios.bin.elf`
>> * Seabios starts and loads its roms and configuration NOT from CBFS
>>
>> My actual reason for not accessing CBFS is that I want to avoid
>> flashing the CBFS to hardware for every change of Seabios or its
>> configuration.
>> However, I want to have VGA output...
>
> The multiboot support should allow one to pass modules from grub2 to
> SeaBIOS.  I've not used multiboot, but this was an example posted in
> the past:
>
> menuentry "SeaBIOS (mb)" --unrestricted {
>    root=ahci0,2
>    multiboot /bios.bin.elf
>    module /vgabios_x230.rom name=pci8086,0166.rom
> }
>

Just for documentation, I used for SeaVGABIOS:

  multiboot (usb0,msdos1)/bios.elf.bin
  module (usb0,msdos1)/vgabios.bin name=vgaroms/seavgabios.bin
  boot

>> So far, I can chainload Seabios from Grub2 (mostly via USB0).
>> Then Seabios is accessing the CBFS and automatically loads Grub2 from CBFS.
>
> I don't see why SeaBIOS would automatically chainload grub2 unless you
> asked it to.  SeaBIOS only brings in payloads that are stored in the
> "img/" directory of CBFS - as long as one doesn't put images in that
> directory, SeaBIOS wont attempt to boot them.

Grub2 is loaded by SeaBIOS if CBFS support is enabled.
As Grub2 resides in img/ to be loaded by coreboot, SeaBIOS finds and loads it.
I simply disabled CBFS support in SeaBIOS to avoid this.

>> For my case, it would be perfect if the roms (mainly VGA.rom) and
>> configuration would be compiled into the `bios.bin.elf`.
>>
>> I looked into the source and did not find a built in option for this
>> (I think nobody ever came with such a use case).
>> What would be a good starting point for me?
>
> There's no support for compiling in binaries to SeaBIOS.
>
> I'm CC'ing Vladimir as he added the multiboot support.

Thanks for implementing it!

> -Kevin