[PATCH 2/4] arm64: pmu: add support for interrupt-affinity property

[mark.rutland@arm.com (Mark Rutland)]

On Thu, Feb 05, 2015 at 12:12:25PM +0000, Will Deacon wrote:
> On Thu, Feb 05, 2015 at 11:56:01AM +0000, Mark Rutland wrote:
> > On Mon, Jan 26, 2015 at 05:54:16PM +0000, Will Deacon wrote:
> > > Historically, the PMU devicetree bindings have expected SPIs to be
> > > listed in order of *logical* CPU number. This is problematic for
> > > bootloaders, especially when the boot CPU (logical ID 0) isn't listed
> > > first in the devicetree.
> > > 
> > > This patch adds a new optional property, interrupt-affinity, to the
> > > PMU node which allows the interrupt affinity to be described using
> > > a list of phandled to CPU nodes, with each entry in the list
> > > corresponding to the SPI at the same index in the interrupts property.
> > > 
> > > Cc: Mark Rutland <mark.rutland@arm.com>
> > > Signed-off-by: Will Deacon <will.deacon@arm.com>
> > > ---
> > >  Documentation/devicetree/bindings/arm/pmu.txt |  6 +++
> > >  arch/arm64/include/asm/pmu.h                  |  1 +
> > >  arch/arm64/kernel/perf_event.c                | 57 +++++++++++++++++++++++++--
> > >  3 files changed, 60 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/Documentation/devicetree/bindings/arm/pmu.txt b/Documentation/devicetree/bindings/arm/pmu.txt
> > > index 75ef91d08f3b..a9281fc48743 100644
> > > --- a/Documentation/devicetree/bindings/arm/pmu.txt
> > > +++ b/Documentation/devicetree/bindings/arm/pmu.txt
> > > @@ -24,6 +24,12 @@ Required properties:
> > >  
> > >  Optional properties:
> > >  
> > > +- interrupt-affinity : Valid only when using SPIs, specifies a list of phandles
> > > +                       to CPU nodes corresponding directly to the affinity of
> > > +		       the SPIs listed in the interrupts property. If absent,
> > > +		       the interrupts are assumed to be listed in logical CPU
> > > +		       order.
> > 
> > This covers the case we care about today, but it's problematic in cases
> > where the number of interrupts is not equal to the number of CPUs affine
> > to that interrupt. For example:
> > 
> > * PPIs in big.LITTLE systems, where we may need a node per cluster, and
> >   will need a way of associating a PMU node with a subset of all CPUs,
> >   despite having only one interrupt.
> > 
> > * Muxed SPIs per-cluster (is this likely to happen?)
> > 
> > The former can be covered by allowing multiple entries in
> > interrupt-affintiy for PPIs.
> 
> Yes, that sounds like a sensible extension in the future if we have to
> support such a platform.
> 
> > I'm not sure if the latter is something we need to cater for. If we do,
> > then perhaps we need an interruptN-affinity property per interrupt (though
> > that's ugly and painful to deal with).
> 
> I'm not keen to handle this, so I'd rather defer it to whoever ends up
> building it. Trying to design for every possibility is usually impossible
> in my experience and you just end up carrying something that isn't useful.

I suspected that would be the case. Just thought I should raise it as a
potential problem.

Thanks,
Mark.