[parisc-linux] EISA support

[Grant Grundler]

> Instead of generating a PCI/EISA interrupt, generate a write of an
> allocated value to the CPU's EIRR (External Interrupt register).

Correct. That's what txn_alloc_irq() is for.
That interface needs to change slightly if someone really wants to start
using it on a broad range of platforms and busses. FWIW, this could
work on any platform which a "local SAPIC" (ie IA64).

> This way, the CPU jumps straight to the scsi chip's interrupt handler
> rather than Asp's, which then has to decode which Asp IRQ it was;
> then jump to the EISA interrupt handler, decode which EISA IRQ it was,
> finally calling the scsi interrupt handler.

excalty - this also cuts out a bunch of IO space reads (and possibly writes).

> We might need to work out some other infrastructure to handle this,
> but it's something we should look into at some point.  Clearly I need
> to get EISA interrupts working anyway :-)

I think all the needed functions are in place already.

grant