Re: [PATCH v3 03/29] ACPI / PPTT: Find cache level by cache-id

[Ben Horgan <ben.horgan@arm.com>]

Hi James,

On 10/17/25 19:56, 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>
> Reviewed-by: Fenghua Yu <fenghuay@nvidia.com>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Tested-by: Fenghua Yu <fenghuay@nvidia.com>
> ---
> Changes sinec v2:
>  * Search all caches, not just unified caches. This removes the need to count
>    the caches first, but means a failure to find the table walks the table
>    three times for different cache types.
>  * Fixed return value of the no-acpi stub.
>  * Punctuation typo in a comment,
>  * Keep trying to parse the table even if a bogus CPU is encountered.
>  * Specified CPUs share caches with other CPUs.
> 
> 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  | 82 ++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/acpi.h |  5 +++
>  2 files changed, 87 insertions(+)
> 
> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
> index 63c3a344c075..50c8f2a3c927 100644
> --- a/drivers/acpi/pptt.c
> +++ b/drivers/acpi/pptt.c
> @@ -350,6 +350,27 @@ static struct acpi_pptt_cache *acpi_find_cache_node(struct acpi_table_header *ta
>  	return found;
>  }
>  
> +static struct acpi_pptt_cache *
> +acpi_find_any_type_cache_node(struct acpi_table_header *table_hdr,
> +			      u32 acpi_cpu_id, unsigned int level,
> +			      struct acpi_pptt_processor **node)
> +{
> +	struct acpi_pptt_cache *cache;
> +
> +	cache = acpi_find_cache_node(table_hdr, acpi_cpu_id, CACHE_TYPE_UNIFIED,
> +				     level, node);
> +	if (cache)
> +		return cache;
> +
> +	cache = acpi_find_cache_node(table_hdr, acpi_cpu_id, CACHE_TYPE_DATA,
> +				     level, node);
> +	if (cache)
> +		return cache;
> +
> +	return acpi_find_cache_node(table_hdr, acpi_cpu_id, CACHE_TYPE_INST,
> +				    level, node);
> +}
> +
>  /**
>   * update_cache_properties() - Update cacheinfo for the given processor
>   * @this_leaf: Kernel cache info structure being updated
> @@ -903,3 +924,64 @@ 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 level, cpu;
> +	u32 acpi_cpu_id;
> +	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)
> +			continue;
> +
> +		/* Start at 1 for L1 */
> +		level = 1;
> +		cache = acpi_find_any_type_cache_node(table, acpi_cpu_id, level,
> +						      &cpu_node);
> +		while (cache) {
> +			cache_v1 = ACPI_ADD_PTR(struct acpi_pptt_cache_v1,
> +						cache, sizeof(*cache));
> +
> +			if (cache->flags & ACPI_PPTT_CACHE_ID_VALID &&
> +			    cache_v1->cache_id == cache_id)
> +				return level;
> +
> +			level++;

If there is more than one type of cache at a given level only one is
checked. For example, if there is an L1 data cache and an L1 instruction
cache then the L1 instruction cache will never be considered.

> +			cache = acpi_find_any_type_cache_node(table, acpi_cpu_id,
> +							      level, &cpu_node);
> +		}
> +	}
> +
> +	return -ENOENT;
> +}
Thanks,

Ben