Re: Exposing ROM's though sysfs

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

> So you can do port I/O on low addresses but not memory accesses?  For some 
> cards simply throwing away reads and writes to the low memory area is 
> probably ok since it'll just be doing things like printing a BIOS banner or a 
> pretty logo.  But then again, I've never disassembled one so I can't be sure.  
> Alan would probably know though :)

Yes, typically, on Apple north bridges, you have the PIO space memory
mapped somewhere in the CPU address space, generating IO cycles from 0
to N (usually 16Mb), but the MMIO space is directly mapped 1:1 from
0x8000000.

> If you can't do legacy port I/O on a given bus though, I think you'd be out of 
> luck wrt POSTing a card with an x86 BIOS.

Port IO should be fine in most cases (there might be one or 2 weird cases
with earlier models, but overall, it's fine). The typical case is a machine
using Apple "UniNorth" chipset (the one that supports AGP, been out for a while
now), since the AGP slot and the PCI bus are on 2 separate domains. Both can
do Port IO at any low address, but they are completely separate domains.

All this could be very nicely dealt with by the kernel driver.

Ben.