Bug#731709: grub-efi UEFI support based on debian-cd work complete (repos)

["Thomas Schmitt" <scdbackup@gmx.net>]

Hi,

after checking El Torito specs, i have to correct my statements
about ranking of El Torito boot images.

The first entry in the boot catalog not only has the title
"Initial" but "Initial/Default".

"4.4 Boot Entry Selection
If the CD has several boot entries, a default entry which boots a 
election program may be provided as the Default/Initial catalog entry.
This image will usually be a floppy image which loads a program that
selects the proper boot image by examining the system configuration or
questioning the user."

Insofar, the first boot image in the catalog is something special.
It is to be preferred over the other boot images with the same
Platform Id. (Nevertheless, it is not obliged to really offer
booting of the others of the same Platform Id.)

I would plan for only one El Torito boot image per platform:
0 = 80x86 BIOS , 0xef = EFI.
But if there is a use case for offering BIOS boot images from GRUB2
and ISOLINUX in the same El Torito catalog, then the sequence matters
if one of them occupies the first entry in the catalog.
If you put the EFI boot image first, then both BIOS boot images get
equal rank.


In the light of this, it seems wise to stay with the sequence that is
tested since a few years:
First the BIOS boot image, then the EFI boot image, no further BIOS
boot images.

This makes it clear to the firmware:
BIOS will hop on the first entry, because it has the right Platform Id
and is Default. BIOS will most probably not enforce that the first
boot image is a floppy image, because there is no other BIOS boot image.
(You always have to expect that the BIOS programmers read the specs
with tired eyes.)
EFI will not hop on the Default entry, because the Platform Id
does not match.

The MBR x86 BIOS code, too, would have to decide on which BIOS boot
image binary to hop. If there is only one, the decision is easy.


Have a nice day :)

Thomas