linux-kernel - Re: [PATCH 06/33] ACPI / PPTT: Add a helper to fill a cpumask from a cache

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Message-ID: <cc3b3335-8474-404a-a424-adc070760a9f@arm.com>
Date: Wed, 10 Sep 2025 20:18:04 +0100
From: James Morse <james.morse@....com>
To: Lorenzo Pieralisi <lpieralisi@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-acpi@...r.kernel.org, devicetree@...r.kernel.org,
 shameerali.kolothum.thodi@...wei.com,
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 baolin.wang@...ux.alibaba.com, Jamie Iles <quic_jiles@...cinc.com>,
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 David Hildenbrand <david@...hat.com>, Rex Nie <rex.nie@...uarmicro.com>,
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 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Catalin Marinas <catalin.marinas@....com>,
 Will Deacon <will@...nel.org>,
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 Danilo Krummrich <dakr@...nel.org>
Subject: Re: [PATCH 06/33] ACPI / PPTT: Add a helper to fill a cpumask from a
 cache_id

Hi Lorenzo,

On 10/09/2025 17:06, Lorenzo Pieralisi wrote:
> 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.

>> 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.

That looks like me throwing my hands up in the air and bailing out!
Yes, the loop continue-ing would be better as only possible CPUs that are missing a
PPTT description (...and cache hierarchy...) would be missing form the bitmap.

It's probably worth a WARN_ON_ONCE() too.

Thanks for spotting that!


>> +		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);

>> @@ -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;)

Makes sense,


> Other than that:
> 
> Reviewed-by: Lorenzo Pieralisi <lpieralisi@...nel.org>


Thanks!

James