AHCI: using bios after linux

[msundius@u1.netgate.com]

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My employer is using Linux as a boot-loader of sorts which eventually boots
another OS. We have some proprietary code which runs in real mode between the
time Linux runs and the next operating system is chain-booted (we use kexec
return to real mode after Linux). This proprietary code makes use of the BIOS
disk calls via int13. In short we need to the BIOS Int13 after Linux.

Our system seems to work well on most machines, however it does not work on any
machine that make use of AHCI. As soon as we make the first Int13 call after the 
Linux AHCI driver has been initialized, the machine goes off into the weeds. 

I been looking though your archives and reading the AHCI spec. It seems clear
that the hardware has at least some state contained within it; I'm wondering if
there is any hope of putting the chip back to a known state that the BIOS will
be able to handle. 

1)In reading the AHCI spec, I notice in chapter 10.6 they talk about a bit that
when set by the OS driver, control of the AHCI hardware is irrevocably passed to
the OS, your driver does not seem to touch that bit so I'm hoping that it is
reasonable to pass back control to the BIOS by either resetting the chip or
simply placing it into a quiescent state.
2)I also have noticed that you do not do anything special when the AHCI module
is removed. That is, you simply use the ata_pci_remove_one() function instead of
one particular to the AHCI chip. 

Could anyone lend some guidance as to what has to be done to put the hardware
back into a sane state for BIOS, or if there is a good reason that this is not
achievable. 

Any prior work or manuals that I should “rtfm” or other wisdom would be greatly
appreciated.

Thank you.

Mike