Re: [PATCH v2 8/8] arch_topology: Add support to build llc_sibling on DT platforms

Sudeep Holla
2022-05-20T14:58:03Z

On Fri, May 20, 2022 at 02:33:46PM +0200, Dietmar Eggemann wrote:
> On 18/05/2022 11:33, Sudeep Holla wrote:
> > ACPI PPTT provides cache identifiers and especially the last level cache
> > identifier is used in obtaining last level cache siblings amongst CPUs.
> >
> > While we have the cpu map representing all the CPUs sharing last level
> > cache in the cacheinfo driver, it is populated quite late in the boot
> > while the information is needed to build scheduler domains quite early.
> >
> > On DT platforms we can use the pointer to the last level cache as the
> > firmware identifier for the last level cache and build the cpumap sharing
> > the last level cache based on the same.
>
> [...]
>
> > diff --git a/include/linux/arch_topology.h b/include/linux/arch_topology.h
> > index 58cbe18d825c..d8a36b0e27c9 100644
> > --- a/include/linux/arch_topology.h
> > +++ b/include/linux/arch_topology.h
> > @@ -69,6 +69,7 @@ struct cpu_topology {
> > int cluster_id;
> > int package_id;
> > int llc_id;
> > + void *llc_fw_node;
>
> Would be nicer if you could set llc_id directly to avoid all this
> addition sync `llc_id and llc_fw_node` code. ACPI PPTT has this
> ACPI_PTR_DIFF() macro which IMHO lets it create distinct ids.
>

Indeed, I initially thought so, but they are 64 bit pointers and choosing
one reference is difficult to take some sort of diff while in ACPI it is in
a static table and we could use the reference to the top of the table.

--
Regards,
Sudeep