Re: [PATCH v2 03/29] ACPI / PPTT: Find cache level by cache-id

Fenghua Yu
2025-10-02T04:30:56Z

On 9/10/25 13:42, 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.
>
> Signed-off-by: James Morse <james.morse@arm.com>

Other than minor comment issues as follows,

Reviewed-by: Fenghua Yu <fenghuay@nvidia.com>

> ---
> Changes since v1:
> * Droppeed the cleanup based table freeing, use acpi_get_pptt() instead.
> * Removed a confusing comment.
> * Clarified the kernel doc.
>
> 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 | 5 ++++
> 2 files changed, 67 insertions(+)
>
> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
> index 7af7d62597df..c5f2a51d280b 100644
> --- a/drivers/acpi/pptt.c
> +++ b/drivers/acpi/pptt.c
> @@ -904,3 +904,65 @@ 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 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,
s/,/./
> + *
> + * If one CPUs L2 is shared with another as L3, this function will return

This comment is not clear. Maybe it's better to say:

+ * 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)
> +{
> + u32 acpi_cpu_id;
> + 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;
> +
> + table = acpi_get_pptt();
> + if (!table)
> + return -ENOENT;
> +
> + if (table->revision < 3)
> + return -ENOENT;
> +
> + 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 -ENOENT;
> + 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)
> + return level;
> + }
> + }
> +
> + return -ENOENT;
> +}
> diff --git a/include/linux/acpi.h b/include/linux/acpi.h
> index f97a9ff678cc..5bdca5546697 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 -EINVAL;
> +}
> #endif
>
> void acpi_arch_init(void);

Thanks.

-Fenghua