Re: [RFC PATCH v3 3/4] arm64:thunder: Add initial dts for Cavium's Thunder SoC in 2 Node topology.

[Lorenzo Pieralisi <lorenzo.pieralisi-5wv7dgnIgG8@public.gmane.org>]

On Wed, Jan 07, 2015 at 08:18:50AM +0000, Arnd Bergmann wrote:
> On Wednesday 07 January 2015 12:37:51 Ganapatrao Kulkarni wrote:
> > Hi Arnd,
> > 
> > On Wed, Jan 7, 2015 at 1:32 AM, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:
> > > On Tuesday 06 January 2015 15:04:26 Ganapatrao Kulkarni wrote:
> > >> On Sat, Jan 3, 2015 at 2:47 AM, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:
> > >> > On Wednesday 31 December 2014 13:03:27 Ganapatrao Kulkarni wrote:
> > >> >> +             cpu@00f {
> > >> >> +                     device_type = "cpu";
> > >> >> +                     compatible = "cavium,thunder", "arm,armv8";
> > >> >> +                     reg = <0x0 0x00f>;
> > >> >> +                     enable-method = "psci";
> > >> >> +                     arm,associativity = <0 0 0x00f>;
> > >> >> +             };
> > >> >> +             cpu@100 {
> > >> >> +                     device_type = "cpu";
> > >> >> +                     compatible = "cavium,thunder", "arm,armv8";
> > >> >> +                     reg = <0x0 0x100>;
> > >> >> +                     enable-method = "psci";
> > >> >> +                     arm,associativity = <0 0 0x100>;
> > >> >> +             };
> > >> >
> > >> > What is the 0x100 offset in the last-level topology field? Does this have
> > >> > no significance to topology at all? I would expect that to be something
> > >> > like cluster number that is relevant to caching and should be represented
> > >> > as a separate level.
> > >>
> > >> i did not understand, can you please explain little more about "
> > >> should be represented as a separate level."
> > >> at present, i have put the hwid of a cpu.
> > >
> > > From what I undertand, the hwid of the CPU contains the "cluster" number in
> > > this bit position, so you typically have a shared L2 or L3 cache between
> > > all cores within a cluster, but separate caches in other clusters.
> > >
> > > If this is the case, there will be a measurable difference in performance
> > > between two processes sharing memory when running on the same cluster,
> > > or when running on different clusters on the same socket. If the
> > > performance difference is relevant, it should be described as a separate
> > > level in the associativity property.
> > you mean, the associativity as array of  <board> <socket> <cluster>
> 
> No, that would leave out the core number, which is required to identify
> the individual thread. I meant adding an extra level such as
> 
> <board> <socket> <cluster> <core>
> 
> A lot of machines will leave out the <board> number because they are
> built with SoCs that don't have a long-distance coherency protocol.

Can't we use phandles to cpu-map nodes instead of a list of numbers (and
yet another topology binding description) ?

Is arm,associativity used solely to map "devices" (inclusive of caches)
to a set of cpus ?

cpu-map misses a notion of distance between hierarchy layers, but we can
add to that.

Lorenzo
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