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

Jonathan Cameron
2025-11-10T16:02:24Z

On Fri, 7 Nov 2025 12:34:21 +0000
Ben Horgan <ben.horgan@arm.com> wrote:

> From: James Morse <james.morse@arm.com>
>
> 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 <james.morse@arm.com>
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>

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;
> +}