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

Ben Horgan
2025-11-07T12:35:49Z

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>
---
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);
+ cpu_node = acpi_find_processor_node(table, acpi_cpu_id);
+ if (!cpu_node)
+ continue;
+
+ for (int level = 1; not_empty; level++) {
+ 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;
+}
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);
--
2.43.0