[PATCH] pci: add quirk for non-symmetric-mode irq routing to versions 0 and 4 of the MCP55 northbridge

[Simon Horman]

On Tue, Sep 21, 2010 at 01:54:39PM -0400, Neil Horman wrote:
>     	A long time ago I worked on a RHEL5 bug in which kdump hung during boot
>     on a set of systems.  The systems hung because they never received timer
>     interrupts during calibrate_delay.  These systems also all had Opteron
>     processors on a hypertransport bus, bridged to a pci bus via an Nvidia MCP55
>     northbridge chip.  AFter much wrangling I managed to learn from Nvidia that they
>     have an undocumented register in some versions of that chip which control how
>     legacy interrupts are send to the cpu complex when the ioapic isn't active.
>     Nvidia defaults this register to only send legacy interrupts to the BSP, so if
>     kdump happens to boot on an AP, we never get timer interrupts and boom.  I had
>     initially used this quirk as a workaround, with my intent being to move apic
>     initalization to an earlier point in the boot process, so the setting of the
>     register would be irrelevant.  Given the work involved in doing that however,
>     the fragile nature of the apic initalization code, and the fact that, over the 2
>     years since we found this bug, the MCP55 is the only chip which seems to have
>     this issue, I've figure at this point its likely safer to just carry the quirk
>     around.  By setting the referenced bits in this hidden register, interrupts will
>     be broadcast to all cpus when the ioapic isn't active on the above described
>     systems.
>     
>     Signed-off-by: Neil Horman <nhorman at tuxdriver.com>

FWIW, Acked-by: Simon Horman <horms at verge.net.au>