linux-kernel - Re: [PATCH v2 8/8] arch_topology: Add support to build llc

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Date:   Fri, 20 May 2022 15:56:39 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Atish Patra <atishp@...shpatra.org>, linux-kernel@...r.kernel.org,
        Atish Patra <atishp@...osinc.com>,
        Sudeep Holla <sudeep.holla@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qing Wang <wangqing@...o.com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH v2 8/8] arch_topology: Add support to build llc_sibling
 on DT platforms

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