Re: Linux 2.6.27-rc5: System boot regression caused by commit a2bd7274b47124d2fc4dfdb8c0591f545ba749dd

Linus Torvalds
2008-08-31T03:03:00Z

On Sat, 30 Aug 2008, Yinghai Lu wrote:
> >
> > /* Don't touch classless devices or host bridges or ioapics. */
> > if (class == PCI_CLASS_NOT_DEFINED ||
> > class == PCI_CLASS_BRIDGE_HOST)
> > continue;
> >
> >
> > it skips the host bridge...
>
> what's story for not touching host bridges?

Ahh. Exactly because of things like this. The hist bridge BAR's are often
special.

That code comes from almost four years ago, the commit message was:

Author: Maciej W. Rozycki <macro@mips.com>
Date: Thu Dec 16 21:44:31 2004 -0800

[PATCH] PCI: Don't touch BARs of host bridges

BARs of host bridges often have special meaning and AFAIK are best left
to be setup by the firmware or system-specific startup code and kept
intact by the generic resource handler. For example a couple of host
bridges used for MIPS processors interpret BARs as target-mode decoders
for accessing host memory by PCI masters (which is quite reasonable).
For them it's desirable to keep their decoded address range overlapping
with the host RAM for simplicity if nothing else (I can imagine running
out of address space with lots of memory and 32-bit PCI with no DAC
support in the participating devices).

This is already the case with the i386 and ppc platform-specific PCI
resource allocators. Please consider the following change for the generic
allocator. Currently we have a pile of hacks implemented for host bridges
to be left untouched and I'd be pleased to remove them.

From: "Maciej W. Rozycki" <macro@mips.com>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

and we've had other things where host bridges are special (ie iirc, if you
turn off PCI_COMMAND_MEM from a host bridge, it stops access to real RAM
from the CPU for some bridges - so you must never turn those things off or
you get a dead system).

(But at least Intel host bridges will just ignore writes to the CMD
register, I think - you cannot turn MEM off).

Linus