Interrupt routing in prep_pci.c

Gabriel Paubert paubert at
Thu Feb 11 20:07:26 EST 1999

On Wed, 10 Feb 1999, Jeremy Fitzhardinge wrote:

> Hi Cort/all,
> I'm trying to work out what kernel/prep_pci.c is doing with interrupt routing. 
> I'm looking at the stuff in the standard 2.2 kernel, so perhaps there's a better
> place for me to look.
> Firstly, in route_pci_interrupts it does an inb(0x800) to get something to
> determine the interrupt architecture of the board.  What is this value, and
> where does it come from?

It's a value which give sthe board type on rather old IBM/Motorola boards.

> Rather than having hard-coded tables, can't the interrupt routing be determined
> from either firmware or the interrupt controller itself?  The comment at the
> top of the file is that this will go away soon.  What are you planning to
> replace it?

Indeed, that's what I do from residual data on PreP machines and it seems
to work on several kinds of boards. You'll find it embedded somewhere
in the patch file at:

> Background: I'm porting LinuxPPC to a Yellowknife-like board running Open
> Firmware.  It works well except that the drivers can't find their interrupts.

Note that this patch includes a new bootloader for PreP machines, I'd like
to extend it to support OF too but don't have te time right now.


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