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

["Yinghai Lu" <yhlu.kernel@gmail.com>]

On Sat, Aug 30, 2008 at 8:00 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
>
> 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).

then
1. we should not probe them in probe.c
2. at least we should not try to request_resource for them in
pcibios_resource_survey...

just pretend that they are not existing.

YH