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SubjectRe: default IRQ affinity change in v2.6.27 (breaking several SMP PPC based systems)
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On Oct 24, 2008, at 10:17 AM, Chris Snook wrote:

> Kumar Gala wrote:
>> It appears the default IRQ affinity changes from being just cpu 0
>> to all cpu's. This breaks several PPC SMP systems in which only a
>> single processor is allowed to be selected as the destination of
>> the IRQ.
>> What is the right answer in fixing this? Should we:
>> cpumask_t irq_default_affinity = 1;
>> instead of
>> cpumask_t irq_default_affinity = CPU_MASK_ALL?
>
> On those systems, perhaps, but not universally. There's plenty of
> hardware where the physical topology of the machine is abstracted
> away from the OS, and you need to leave the mask wide open and let
> the APIC figure out where to map the IRQs. Ideally, we should
> probably make this decision based on the APIC, but if there's no PPC
> hardware that uses this technique, then it would suffice to make
> this arch-specific.


What did those systems do before this patch? Its one thing to expose
a mask in the ability to change the default mask in /proc/irq/
default_smp_affinity. Its another (and a regression in my opinion) to
change the mask value itself.

As for making it ARCH specific, that doesn't really help since not all
PPC hw has the limitation I spoke of. Not even all MPIC (in our
cases) have the limitation.

- k


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