Re: [PATCH 06/33] ACPI / PPTT: Add a helper to fill a cpumask from a cache_id

[Lorenzo Pieralisi <lpieralisi@kernel.org>]

On Fri, Aug 22, 2025 at 03:30:21PM +0000, James Morse wrote:
> MPAM identifies CPUs by the cache_id in the PPTT cache structure.
> 
> The driver needs to know which CPUs are associated with the cache,
> the CPUs may not all be online, so cacheinfo does not have the
> information.
> 
> Add a helper to pull this information out of the PPTT.
> 
> CC: Rohit Mathew <Rohit.Mathew@arm.com>
> Signed-off-by: James Morse <james.morse@arm.com>
> Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
> ---
> 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 |  6 +++++
>  2 files changed, 68 insertions(+)
> 
> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
> index 660457644a5b..cb93a9a7f9b6 100644
> --- a/drivers/acpi/pptt.c
> +++ b/drivers/acpi/pptt.c
> @@ -971,3 +971,65 @@ int find_acpi_cache_level_from_id(u32 cache_id)
>  
>  	return -ENOENT;
>  }
> +
> +/**
> + * acpi_pptt_get_cpumask_from_cache_id() - Get the cpus associated with the
> + *					   specified cache
> + * @cache_id: The id field of the unified cache
> + * @cpus: Where to build the cpumask
> + *
> + * Determine which CPUs are below this cache in the PPTT. This allows the property
> + * to be found even if the CPUs are offline.
> + *
> + * The PPTT table must be rev 3 or later,
> + *
> + * Return: -ENOENT if the PPTT doesn't exist, or the cache cannot be found.
> + * Otherwise returns 0 and sets the cpus in the provided cpumask.
> + */
> +int acpi_pptt_get_cpumask_from_cache_id(u32 cache_id, cpumask_t *cpus)
> +{
> +	u32 acpi_cpu_id;
> +	int level, cpu, num_levels;
> +	struct acpi_pptt_cache *cache;
> +	struct acpi_pptt_cache_v1 *cache_v1;
> +	struct acpi_pptt_processor *cpu_node;
> +	struct acpi_table_header *table __free(acpi_table) = acpi_get_table_ret(ACPI_SIG_PPTT, 0);
> +
> +	cpumask_clear(cpus);
> +
> +	if (IS_ERR(table))
> +		return -ENOENT;
> +
> +	if (table->revision < 3)
> +		return -ENOENT;
> +
> +	/*
> +	 * If we found the cache first, we'd still need to walk from each cpu.
> +	 */
> +	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 0;

If for a possible cpu you don't get an acpi_pptt_processor node we return 0,
is that correct ? Should not the loop continue ? Forgive me if that's a
dumb question.

> +		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)
> +				cpumask_set_cpu(cpu, cpus);
> +		}
> +	}
> +
> +	return 0;
> +}
> diff --git a/include/linux/acpi.h b/include/linux/acpi.h
> index 30c10b1dcdb2..4ad08f5f1d83 100644
> --- a/include/linux/acpi.h
> +++ b/include/linux/acpi.h
> @@ -1555,6 +1555,7 @@ 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);
> +int acpi_pptt_get_cpumask_from_cache_id(u32 cache_id, cpumask_t *cpus);
>  #else
>  static inline int acpi_pptt_cpu_is_thread(unsigned int cpu)
>  {
> @@ -1582,6 +1583,11 @@ static inline int find_acpi_cache_level_from_id(u32 cache_id)
>  {
>  	return -EINVAL;
>  }
> +static inline int acpi_pptt_get_cpumask_from_cache_id(u32 cache_id,
> +						      cpumask_t *cpus)
> +{
> +	return -EINVAL;

Nit: You might want the return value here to be coherent with what the function
documentation states (ie return -ENOENT;)

Other than that:

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

> +}
>  #endif
>  
>  void acpi_arch_init(void);
> -- 
> 2.20.1
>