[RFC PATCH 07/36] ACPI / PPTT: Find cache level by cache-id

[Ben Horgan]

Hi James,

On 7/11/25 19:36, James Morse wrote:
> 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.
> 
> acpi_count_levels() expects its levels parameter to be initialised to
> zero as it passes it to acpi_find_cache_level() as starting_level.
> The existing callers do this. Document it.
This paragraph is stale. You dealt with this in the previous commit.
> 
> Signed-off-by: James Morse <james.morse at arm.com>
> Reviewed-by: Sudeep Holla <sudeep.holla at arm.com>
> ---
>   drivers/acpi/pptt.c  | 73 ++++++++++++++++++++++++++++++++++++++++++++
>   include/linux/acpi.h |  5 +++
>   2 files changed, 78 insertions(+)
> 
> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
> index 13ca2eee3b98..f53748a5df19 100644
> --- a/drivers/acpi/pptt.c
> +++ b/drivers/acpi/pptt.c
> @@ -912,3 +912,76 @@ int find_acpi_cpu_topology_hetero_id(unsigned int cpu)
>   	return find_acpi_cpu_topology_tag(cpu, PPTT_ABORT_PACKAGE,
>   					  ACPI_PPTT_ACPI_IDENTICAL);
>   }
> +
> +/**
> + * find_acpi_cache_level_from_id() - Get the level of the specified cache
> + * @cache_id: The id field of the unified cache
> + *
> + * Determine the level relative to any CPU for the unified 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 unified caches that are peers.
> + *
> + * The PPTT table must be rev 3 or later,
> + *
> + * If one CPUs L2 is shared with another as L3, this function will return
> + * an unpredictable value.
> + *
> + * Return: -ENOENT if the PPTT doesn't exist, 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)
> +{
> +	u32 acpi_cpu_id;
> +	acpi_status status;
> +	int level, cpu, num_levels;
> +	struct acpi_pptt_cache *cache;
> +	struct acpi_table_header *table;
> +	struct acpi_pptt_cache_v1 *cache_v1;
> +	struct acpi_pptt_processor *cpu_node;
> +
> +	status = acpi_get_table(ACPI_SIG_PPTT, 0, &table);
> +	if (ACPI_FAILURE(status)) {
> +		acpi_pptt_warn_missing();
> +		return -ENOENT;
> +	}
> +
> +	if (table->revision < 3) {
> +		acpi_put_table(table);
> +		return -ENOENT;
> +	}
> +
> +	/*
> +	 * If we found the cache first, we'd still need to walk from each CPU
> +	 * to find the level...
> +	 */
> +	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)
> +			break;
> +		acpi_count_levels(table, cpu_node, &num_levels, 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) {
> +				acpi_put_table(table);
> +				return level;
> +			}
> +		}
> +	}
> +
> +	acpi_put_table(table);
> +	return -ENOENT;
> +}
> diff --git a/include/linux/acpi.h b/include/linux/acpi.h
> index 8c3165c2b083..82947f6d2a43 100644
> --- a/include/linux/acpi.h
> +++ b/include/linux/acpi.h
> @@ -1542,6 +1542,7 @@ int find_acpi_cpu_topology_cluster(unsigned int cpu);
>   int find_acpi_cpu_topology_package(unsigned int cpu);
>   int find_acpi_cpu_topology_hetero_id(unsigned int cpu);
>   int acpi_pptt_get_cpus_from_container(u32 acpi_cpu_id, cpumask_t *cpus);
> +int find_acpi_cache_level_from_id(u32 cache_id);
>   #else
>   static inline int acpi_pptt_cpu_is_thread(unsigned int cpu)
>   {
> @@ -1568,6 +1569,10 @@ static inline int acpi_pptt_get_cpus_from_container(u32 acpi_cpu_id,
>   {
>   	return -EINVAL;
>   }
> +static inline int find_acpi_cache_level_from_id(u32 cache_id)
> +{
> +	return -EINVAL;
> +}
>   #endif
>   
>   void acpi_arch_init(void);

Thanks,

Ben