[SeaBIOS] Bootorder Failover (Patch)

[Kevin O'Connor]

On Fri, Jul 13, 2012 at 09:32:21AM -0600, Steve Goodrich wrote:
> Legacy MBRs on HDDs and USB MSCs allow booting only from device id 0x80.
> This is OK for platforms that have only one HDD or USB device to boot from,
> but causes problems if the end user wants to have more than one HDD or USB
> MSC device in the boot order (and if those devices are present, but not
> bootable).
[...]
> With the code in this patch, the system would behave like this:
> * Assign HDD1 as device 0x80 and attempt to boot
> * boot fails because the device is not bootable
> * Rotate the boot order to be HDD2, Floppy, USB1, USB2, HDD1, making HDD2
>   device 0x80

The problem with this approach is that there is no way to change the
drive id of hard drives controlled via option roms (eg, scsi drives).
So, if a scsi drive is made 0x80 this scheme does not work at all, and
if a scsi drive is not 0x80 there is no way to boot from it.

> Thoughts?

I don't think I understand the use case you are trying to address.  If
a user has a stable set of drives attached they can configure the boot
order via the bootorder file.  If the user is attaching drives for
one-off boots, they can hit F12 and choose which drive to boot from.
What is the use case for a user that does not have a stable attachment
of drives and is unable to hit F12?

On a technical note, the patch has some implementation issues: the use
of "#if"s is undesirable - the compiler can weed out unused code and
variables; SeaBIOS no longer uses the EBDA for internal storage - use
global variables with the "VARLOW" decoration instead; it's not safe
to alter IDMap in the boot phase as global variables are read-only
after POST phase ends.

-Kevin