can someone explain...[ PCI interrupts]

Julian Elischer julian at
Tue Dec 6 14:57:35 PST 2005

In short words for the likes of me,
Can someone give a quicj roundup on PCI routing in 4.x and -current.


How much is set up by the BIOS and how much is set up by the OS (4.x and 

How much of my fondly held knowledge of the old hardware PICS (where
an interrupt corresponded to an interrupt line) is now out of date?

In this world of multiple PCI to PCI bridges, how much latitude doe the 
OS have in
deciding where an interrupt turns up?
(in 4.x and -current)

An example is my Dell PE2850 (on 4.x), in which my 4 port ethernet card 
seems to
be assigned irqs 10,14,14,2 in that order sometimes and 16,17,17,2 at 

Who is making those decisions? Is it the BIOS and 4.x is just playing along?

Assuming the much maligned "boot interrupt" comes in on irq2, does the OS
have the oportunity to put my 4th port somewhere else? On 4.x it collides
with one of my ether ports but in -current it doesn't

Linux and -current on teh same box a;;ocate way different irqs, and
they agree about it.. i.e. Linux and -current assign my 4 port card IRQs 

Do they agree becasue something else has decided it (the bios again?) or 
becasue they
use the same algorythm to work it out..

Also, if the "boot interrupt" was previously set to 2, is that likely to 
have changed in -current?
Am I now going to get clobbered on IRQ16?  If yes, is this something 
that teh BIOS writers
decided, or something that the Motherboard designers decided?

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