Re: default IRQ affinity change in v2.6.27 (breaking several SMP PPC based systems)

On Oct 24, 2008, at 10:17 AM, Chris Snook wrote:

Kumar Gala wrote:

quoted

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