Still IRQ routing problems with bridged devices.

Bernd Walter ticso at
Fri Jan 2 14:40:47 PST 2004

On Fri, Jan 02, 2004 at 04:30:44PM -0500, John Baldwin wrote:
> On 02-Jan-2004 Bernd Walter wrote:
> > On Fri, Jan 02, 2004 at 02:19:53PM -0500, John Baldwin wrote:
> >> On 01-Jan-2004 Bernd Walter wrote:
> >> > On Thu, Jan 01, 2004 at 10:12:23AM -0700, M. Warner Losh wrote:
> >> >> In message: <20040101155100.GF11668 at>
> > I have no clue about $PIR and therefor have no idea where irq4 comes
> > from - any pointer to $PIR documents are welcome.
> Erm, according to the PCI spec, the devices behind the bridges on the
> add-in cards will swizzle their interrupts.  Thus, device 8.0 will
> use INTA on the add-in card's connect, 9.0 will use INTB for its
> INTA, etc.  We use the $PIR to then figure out what IRQ to use for
> INT[ABCD] on that slot as appropriate.  Thus, it would work something
> like this:
>  1) device 1.8.0 wants to route INTA so it asks the bridge for the IRQ
>  2) the bridges translates that into INTA on itself, and asks its parent
>     for a route to INTA at its slot (say 0.2.0).

Yes - I know how PCI bridge routing works.
What I don't know is the x86 sepcific evilness.

>  3) the host bridge lookes up 0.2.0 INTA in the $PIR, chooses an IRQ from
>     the possible list (defaults to just using first IRQ) and returns it.
>     This step should be skipping IRQ 4 adn using IRQ 10 or 11 instead

That's the interesting part.
What exactly is in the $PIR table?
Fact is that the 4 PCI connectors share the same intlines wired in
different INTA-D order which is board specific.
The intlines are setup by the BIOS to 5, 10, 11 and 12.
Now FreeBSD has found out that it needs 0.2.0 INTA for the bridged device
1.8.0 INTA.
It now incorrectly selects IRQ4 for 0.2.0 INTA, which is already
in use for a ISA device by an PnP On-Board component.
It also has to connect the intline with IRQ4 anywhere in the chipset,
which doesn't seem to happen, because the IRQ doesn't even work.
And I don't see the point why this is not a problem for non bridged
devices, which would also require an IRQ for 0.2.0 INTA.

B.Walter                   BWCT      
ticso at                                  info at

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