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

James Morse
2025-09-10T20:43:44Z

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>
---
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,
+ *
+ * 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, 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);
--
2.39.5