The Intel 82426EX ISA Bridge (IB), a part of the Intel 82420EX PCIset, 
implements PCI interrupt steering with a PIRQ router in the form of two 
PIRQ Route Control registers, available in the PCI configuration space 
at locations 0x66 and 0x67 for the PIRQ0# and PIRQ1# lines respectively.

The semantics is the same as with the PIIX router, however it is not
clear if BIOSes use register indices or line numbers as the cookie to
identify PCI interrupts in their routing tables and therefore support
either scheme.

The IB is directly attached to the Intel 82425EX PCI System Controller 
(PSC) component of the chipset via a dedicated PSC/IB Link interface 
rather than the host bus or PCI.  Therefore it does not itself appear in 
the PCI configuration space even though it responds to configuration 
cycles addressing registers it implements.  Use 82425EX's identification 
then for determining the presence of the IB.

References:

[1] "82420EX PCIset Data Sheet, 82425EX PCI System Controller (PSC) and 
    82426EX ISA Bridge (IB)", Intel Corporation, Order Number: 
    290488-004, December 1995, Section 3.3.18 "PIRQ1RC/PIRQ0RC--PIRQ 
    Route Control Registers", p. 61
Signed-off-by: NMaciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2107200213490.9461@angie.orcam.me.uk