Invalid INTLN assignment (0x28) in bridged config

[Steven Kan <steven@kan.org>]

We develop CompactPCI-based instrumentation cards, and we deploy them in
a bridged-chassis configuration with a single-board-computer and a
CPCI-CPCI bridge from SBS Technologies (210-2 and 212-2) cards. Our
instrument cards use the PLX9052 PCI bridge chip

We have been having an intermittent problem with INTLN assignments. Most
of the cards receive a valid INTLN assignment at boot (e.g. 05 hex or 09
hex). But some of our cards are being assigned an invalid value (28
hex). 

It seems to occur only in the slave chassis, and it is repeatable for a
given card order. But it's not very logical. I can swap two otherwise
identical cards and make the problem disappear, then swap them back in
make it reappear. Or I can move a card from slot 3 to slot 4 and make
the problem move from slot 5 to slot 6. Weird.

The invalid interrupt number doesn't appear to follow any particular
card or slot, but it _is_ repeatable.

We're doing some investigation of timing, etc., but I was wondering
whether anyone had seen this before and whether the 28 hex value had any
special meaning.

Thanks!