linux-kernel - Re: [PATCH 06/33] ACPI / PPTT: Add a helper to fill a cpumask from a cache

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Message-ID: <aK7ju2caTjqf1+VN@e133380.arm.com>
Date: Wed, 27 Aug 2025 11:53:47 +0100
From: Dave Martin <Dave.Martin@....com>
To: James Morse <james.morse@....com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-acpi@...r.kernel.org, devicetree@...r.kernel.org,
	shameerali.kolothum.thodi@...wei.com,
	D Scott Phillips OS <scott@...amperecomputing.com>,
	carl@...amperecomputing.com, lcherian@...vell.com,
	bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
	baolin.wang@...ux.alibaba.com, Jamie Iles <quic_jiles@...cinc.com>,
	Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com,
	dfustini@...libre.com, amitsinght@...vell.com,
	David Hildenbrand <david@...hat.com>,
	Rex Nie <rex.nie@...uarmicro.com>, Koba Ko <kobak@...dia.com>,
	Shanker Donthineni <sdonthineni@...dia.com>, fenghuay@...dia.com,
	baisheng.gao@...soc.com,
	Jonathan Cameron <jonathan.cameron@...wei.com>,
	Rob Herring <robh@...nel.org>, Rohit Mathew <rohit.mathew@....com>,
	Rafael Wysocki <rafael@...nel.org>, Len Brown <lenb@...nel.org>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Hanjun Guo <guohanjun@...wei.com>,
	Sudeep Holla <sudeep.holla@....com>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Danilo Krummrich <dakr@...nel.org>
Subject: Re: [PATCH 06/33] ACPI / PPTT: Add a helper to fill a cpumask from a
 cache_id

Hi James,

On Fri, Aug 22, 2025 at 03:29:47PM +0000, James Morse wrote:
> MPAM identifies CPUs by the cache_id in the PPTT cache structure.
> 
> The driver needs to know which CPUs are associated with the cache,
> the CPUs may not all be online, so cacheinfo does not have the
> information.

Nit: cacheinfo lacking the information is not a consequence of the
driver needing it.

Maybe split the sentence:

-> "[...] associated with the cache. The CPUs may not [...]"

> 
> Add a helper to pull this information out of the PPTT.
> 
> CC: Rohit Mathew <Rohit.Mathew@....com>
> Signed-off-by: James Morse <james.morse@....com>
> Reviewed-by: Sudeep Holla <sudeep.holla@....com>
> ---
> Changes since RFC:
>  * acpi_count_levels() now returns a value.
>  * Converted the table-get stuff to use Jonathan's cleanup helper.
>  * Dropped Sudeep's Review tag due to the cleanup change.
> ---
>  drivers/acpi/pptt.c  | 62 ++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/acpi.h |  6 +++++
>  2 files changed, 68 insertions(+)
> 
> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
> index 660457644a5b..cb93a9a7f9b6 100644
> --- a/drivers/acpi/pptt.c
> +++ b/drivers/acpi/pptt.c
> @@ -971,3 +971,65 @@ int find_acpi_cache_level_from_id(u32 cache_id)
>  
>  	return -ENOENT;
>  }
> +
> +/**
> + * acpi_pptt_get_cpumask_from_cache_id() - Get the cpus associated with the
> + *					   specified cache
> + * @cache_id: The id field of the unified cache
> + * @cpus: Where to build the cpumask
> + *
> + * Determine which CPUs are below this cache in the PPTT. This allows the property
> + * to be found even if the CPUs are offline.
> + *
> + * The PPTT table must be rev 3 or later,
> + *
> + * Return: -ENOENT if the PPTT doesn't exist, or the cache cannot be found.
> + * Otherwise returns 0 and sets the cpus in the provided cpumask.
> + */
> +int acpi_pptt_get_cpumask_from_cache_id(u32 cache_id, cpumask_t *cpus)
> +{
> +	u32 acpi_cpu_id;
> +	int level, cpu, num_levels;
> +	struct acpi_pptt_cache *cache;
> +	struct acpi_pptt_cache_v1 *cache_v1;
> +	struct acpi_pptt_processor *cpu_node;
> +	struct acpi_table_header *table __free(acpi_table) = acpi_get_table_ret(ACPI_SIG_PPTT, 0);
> +
> +	cpumask_clear(cpus);
> +
> +	if (IS_ERR(table))
> +		return -ENOENT;
> +
> +	if (table->revision < 3)
> +		return -ENOENT;
> +
> +	/*
> +	 * If we found the cache first, we'd still need to walk from each cpu.
> +	 */
> +	for_each_possible_cpu(cpu) {
> +		acpi_cpu_id = get_acpi_id_for_cpu(cpu);
> +		cpu_node = acpi_find_processor_node(table, acpi_cpu_id);
> +		if (!cpu_node)
> +			return 0;
> +		num_levels = acpi_count_levels(table, cpu_node, NULL);
> +
> +		/* Start at 1 for L1 */
> +		for (level = 1; level <= num_levels; level++) {
> +			cache = acpi_find_cache_node(table, acpi_cpu_id,
> +						     ACPI_PPTT_CACHE_TYPE_UNIFIED,
> +						     level, &cpu_node);
> +			if (!cache)
> +				continue;
> +
> +			cache_v1 = ACPI_ADD_PTR(struct acpi_pptt_cache_v1,
> +						cache,
> +						sizeof(struct acpi_pptt_cache));
> +
> +			if (cache->flags & ACPI_PPTT_CACHE_ID_VALID &&
> +			    cache_v1->cache_id == cache_id)
> +				cpumask_set_cpu(cpu, cpus);

Again, it feels like we are repeating the same walk multiple times to
determine how deep the table is (on which point the table is self-
describing anyway), and then again to derive some static property, and
then we are then doing all of that work multiple times to derive
different static properties, etc.

Can we not just walk over the tables once and stash the derived
properties somewhere?

I'm still getting my head around this parsing code, so I'm not saying
that the approach is incorrect here -- just wondering whether there is
a way to make it simpler.

[...]

Cheers
---Dave