Subject: Re: [PATCH 04/33] ACPI / PPTT: Find cache level by cache-id

From: Jonathan Cameron <[email-masked]>
Date: Mon, 10 Nov 2025 16:02:18 +0000

On Fri, 7 Nov 2025 12:34:21 +0000
Ben Horgan <[email-masked]> wrote:

> From: James Morse <[email-masked]>
> 
> The MPAM table identifies caches by id. The MPAM driver also wants to know
> the cache level to determine if the platform is of the shape that can be
> managed via resctrl. Cacheinfo has this information, but only for CPUs that
> are online.
> 
> Waiting for all CPUs to come online is a problem for platforms where
> CPUs are brought online late by user-space.
> 
> Add a helper that walks every possible cache, until it finds the one
> identified by cache-id, then return the level.
> 
> Signed-off-by: James Morse <[email-masked]>
> Signed-off-by: Ben Horgan <[email-masked]>

A few things inline.

> ---
> Changes since v3:
> Tags dropped due to rework
> Fallout/simplification from adding acpi_pptt_cache_v1_full
> Look for each cache type before incrementing level
> ---
>  drivers/acpi/pptt.c  | 63 ++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/acpi.h |  5 ++++
>  2 files changed, 68 insertions(+)
> 
> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
> index 1ed2099c0d1a..71841c106020 100644
> --- a/drivers/acpi/pptt.c
> +++ b/drivers/acpi/pptt.c
> @@ -918,3 +918,66 @@ void acpi_pptt_get_cpus_from_container(u32 acpi_cpu_id, cpumask_t *cpus)
>  				     entry->length);
>  	}
>  }
> +
> +/**
> + * find_acpi_cache_level_from_id() - Get the level of the specified cache
> + * @cache_id: The id field of the cache
> + *
> + * Determine the level relative to any CPU for the cache identified by
> + * cache_id. This allows the property to be found even if the CPUs are offline.
> + *
> + * The returned level can be used to group caches that are peers.
> + *
> + * The PPTT table must be rev 3 or later.
> + *
> + * If one CPU's L2 is shared with another CPU as L3, this function will return
> + * an unpredictable value.
> + *
> + * Return: -ENOENT if the PPTT doesn't exist, the revision isn't supported or
> + * the cache cannot be found.
> + * Otherwise returns a value which represents the level of the specified cache.
> + */
> +int find_acpi_cache_level_from_id(u32 cache_id)
> +{
> +	int cpu;
> +	struct acpi_table_header *table;
> +
> +	table = acpi_get_pptt();
> +	if (!table)
> +		return -ENOENT;
> +
> +	if (table->revision < 3)
> +		return -ENOENT;
> +
> +	for_each_possible_cpu(cpu) {
> +		bool not_empty = true;
> +		u32 acpi_cpu_id;
> +		struct acpi_pptt_cache_v1_full *cache;
> +		struct acpi_pptt_processor *cpu_node;
> +
> +		acpi_cpu_id = get_acpi_id_for_cpu(cpu);

Might as well combine this one with declaration.

> +		cpu_node = acpi_find_processor_node(table, acpi_cpu_id);
> +		if (!cpu_node)
> +			continue;
> +
> +		for (int level = 1; not_empty; level++) {

This smells very much like a while loop rather than a for loop. Make
it a do/while and you can avoid the somewhat nasty setting not_empty = true
just to get in for first iteration.

		int level = 1;
		do {
			int cache_type[] = { CACHE_TYPE_INST, CACHE_TYPE_DATA, CACHE_TYPE_UNIFIED };

			not_empty = false;
			for (int i = 0; i < ARRAY_SIZE(cache_type); i++) {
				cache = acpi_find_cache_node(table, acpi_cpu_id, cache_type[i],
							     level, &cpu_node);
				if (!cache)
					continue;

				not_empty = true;

				if (acpi_pptt_cache_id_is_valid(cache) &&
				    cache->extra.cache_id == cache_id)
					return level;
			}
		} while (not_empty);

Maybe flip sense of that bool to be empty and !empty for the test.

> +			int cache_type[] = {CACHE_TYPE_INST, CACHE_TYPE_DATA, CACHE_TYPE_UNIFIED};
> +
> +			not_empty = false;
> +			for (int i = 0; i < ARRAY_SIZE(cache_type); i++) {
> +				cache = acpi_find_cache_node(table, acpi_cpu_id, cache_type[i],
> +							     level, &cpu_node);
> +				if (!cache)
> +					continue;
> +
> +				not_empty = true;
> +
> +				if (acpi_pptt_cache_id_is_valid(cache) &&
> +				    cache->extra.cache_id == cache_id)
> +					return level;
> +			}
> +		}
> +	}
> +
> +	return -ENOENT;
> +}