CONFIG_X86_GOOD_APIC is enabled, but there is no IO-APIC in this
VAIO and /proc/interrupts shows all XT-PIC.

It matters if you have APIC/IO-APIC support enabled. In marcelo's tree
disable_irq is broken in APIC mode for non SMP.

WRT. the VAIO, I originally suspected some of the DMI table changes
that went into 2.4.21, but I reverted that (and nearly all of the
other i386 specific 2.4.21 changes) to no avail.

They really wouldn't relate to the IDE code at all. IDE or block layer
stuff is by far the most likely.

Interesting.  I've never seen any evidence from Ben or anyone else who
saw the problem on Sparc say much other than "2.6.0-test1 doesn't have
the problem, 2.4.x does"

Can you show the patch in question?  I guess it's in 2.6.0-test1
already?

I got mail from two people confirming some "patch" fixed it, but nobody
ever sent me the patch in question. I'd guess its making sure the PIO side
is always set up at boot