RE: [RFT] major libata update

["Brown, Len" <len.brown@intel.com>]

>Many BIOS ACPI tables from years ago simply _assumed_ that you have 
>hardcoded irq 14/15, even...  Their irq descriptors for 14/15 would be 
>absent or completely non-functional.
>
>Or maybe its the $pirq table I'm recalling.  One of the two, anyway.

For x86, the ACPI interrupt configuration process is to identity-map
the IOAPIC entries below 16 1:1 PIC:IOAPIC,
unless there are interrupt source overrides
(such as commonly done to swizzle IRQ0 from a different pin)

This makes legacy-mode ATA happy.  Hard code ATA to 14/15 and
off you go.

But there is a gray area where the ATA controller registers
as a PCI device, but Linux goes off and looks in the ACPI PRT
for that PCI-dev and finds no entry.  So if you didn't
have the hard-coded 14/15, you'd be dead.

Then there are cases where the PRT specifies something
_other_ than 14/15 for ATA, and in that cases the hard-coded
default is the wrong thing to do; and the workaround is
to use BIOS SETUP options to be sure that ATA is set up
in legacy mode.

I suspect Linux could be smarter here.  The 14/15 should be
the backup default for when the tables
don't give us anything else; not the only option.

-Len
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