[PATCH v6 04/34] ACPI / PPTT: Find cache level by cache-id

[Jeremy Linton]

On 11/19/25 6:22 AM, Ben Horgan wrote:
> From: James Morse <james.morse at 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.

Per V5:

Reviewed-by: Jeremy Linton <jeremy.linton at arm.com>>
> Signed-off-by: James Morse <james.morse at arm.com>
> Signed-off-by: Ben Horgan <ben.horgan at arm.com>
> Reviewed-by: Gavin Shan <gshan at redhat.com>
> Reviewed-by: Fenghua Yu <fenghuay at nvidia.com>
> Reviewed-by: Shaopeng Tan <tan.shaopeng at jp.fujitsu.com>
> Reviewed-by: Jonathan Cameron <jonathan.cameron at huawei.com>
> Tested-by: Fenghua Yu <fenghuay at nvidia.com>
> Tested-by: Shaopeng Tan <tan.shaopeng at jp.fujitsu.com>
> Tested-by: Carl Worth <carl at os.amperecomputing.com>
> Tested-by: Gavin Shan <gshan at redhat.com>
> Tested-by: Zeng Heng <zengheng4 at huawei.com>
> Tested-by: Hanjun Guo <guohanjun at huawei.com>
> ---
> Changes since v4:
> Initialise acpi_cpu_id at declaration
> Convert a for loop to do/while (Jonathan)
> Use new version of acpi_pptt_cache_v1_full
> 
> 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  | 66 ++++++++++++++++++++++++++++++++++++++++++++
>   include/linux/acpi.h |  5 ++++
>   2 files changed, 71 insertions(+)
> 
> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
> index ef39b176dc00..da49b56a1ef2 100644
> --- a/drivers/acpi/pptt.c
> +++ b/drivers/acpi/pptt.c
> @@ -932,3 +932,69 @@ 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 empty;
> +		int level = 1;
> +		u32 acpi_cpu_id = get_acpi_id_for_cpu(cpu);
> +		struct acpi_pptt_cache *cache;
> +		struct acpi_pptt_processor *cpu_node;
> +
> +		cpu_node = acpi_find_processor_node(table, acpi_cpu_id);
> +		if (!cpu_node)
> +			continue;
> +
> +		do {
> +			int cache_type[] = {CACHE_TYPE_INST, CACHE_TYPE_DATA, CACHE_TYPE_UNIFIED};
> +
> +			empty = true;
> +			for (int i = 0; i < ARRAY_SIZE(cache_type); i++) {
> +				struct acpi_pptt_cache_v1_full *cache_v1;
> +
> +				cache = acpi_find_cache_node(table, acpi_cpu_id, cache_type[i],
> +							     level, &cpu_node);
> +				if (!cache)
> +					continue;
> +
> +				empty = false;
> +
> +				cache_v1 = upgrade_pptt_cache(cache);
> +				if (cache_v1 && cache_v1->cache_id == cache_id)
> +					return level;
> +			}
> +			level++;
> +		} while (!empty);
> +	}
> +
> +	return -ENOENT;
> +}
> diff --git a/include/linux/acpi.h b/include/linux/acpi.h
> index 4752ebd48132..be074bdfd4d1 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);
>   void 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)
>   {
> @@ -1565,6 +1566,10 @@ static inline int find_acpi_cpu_topology_hetero_id(unsigned int cpu)
>   }
>   static inline void acpi_pptt_get_cpus_from_container(u32 acpi_cpu_id,
>   						     cpumask_t *cpus) { }
> +static inline int find_acpi_cache_level_from_id(u32 cache_id)
> +{
> +	return -ENOENT;
> +}
>   #endif
>   
>   void acpi_arch_init(void);