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

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Date:   Fri, 20 May 2022 14:33:46 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Sudeep Holla <sudeep.holla@....com>,
        Atish Patra <atishp@...shpatra.org>,
        linux-kernel@...r.kernel.org
Cc:     Atish Patra <atishp@...osinc.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 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.

>  	cpumask_t thread_sibling;
>  	cpumask_t core_sibling;
>  	cpumask_t cluster_sibling;