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

James Morse
2025-09-19T16:11:00Z

Hi Lorenzo,

On 11/09/2025 16:27, Lorenzo Pieralisi wrote:
> On Wed, Sep 10, 2025 at 08:42:43PM +0000, 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.

>> 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,
>
> * The PPTT table must be rev 3 or later.

Fixed,

>> + *
>> + * 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;
>
> Same comment as in another patch - don't think you want to stop parsing
> here.

Yes - this is me throwing my hands up in the air at a nonsense firmware table.
'continue' would be a try harder approach.

>> + 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
>> @@ -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;

> return -ENOENT;

Yes, same bug as before. Fixed.

> Reviewed-by: Lorenzo Pieralisi <lpieralisi@kernel.org>

Thanks - I think the change to ignoring unified caches and searching all types is
substantive enough that I won't pick this up because of the changes for v3...

Thanks,

James