Message-ID: <baa93014-e71d-4098-a6bc-1d75d4d819ee@arm.com>
Date: Fri, 19 Sep 2025 17:10:46 +0100
From: James Morse <james.morse@....com>
To: Jonathan Cameron <jonathan.cameron@...wei.com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
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Subject: Re: [PATCH v2 03/29] ACPI / PPTT: Find cache level by cache-id

Hi Jonathan,

On 11/09/2025 11:59, Jonathan Cameron wrote:
> On Wed, 10 Sep 2025 20:42:43 +0000
> James Morse <james.morse@....com> 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);
>>  	}
>>  }
>> +
>> +/*
> /**
> 
> It's an exposed interface so nice to have formal kernel-doc and automatic
> checks that brings.
> 
>> + * 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.

> Silly question but why do we care if this a unified cache?

/me returns from the time-machine trip....

This is legacy, but results in parity with the DT approach.
Really early versions of this generated an ID based on the associated CPUs - like DT
does/would-do today. This value isn't unique for non-unified caches as they have the same
set of CPUs below them, so every use of cache-id used to have to check it was a unified cache.
This isn't a problem for MPAM as there is never likely to be an L1 MSC, but you're right-
it hinders re-use of this.

Since ACPI then went and added the ID to the PPTT, we don't need to check this here.


> It's a bit odd to have a general sounding function fail for split caches.
> The handling would have to be more complex but if we really don't want
> to do it maybe rename the function to find_acpi_unifiedcache_level_from_id()
> and if the general version gets added later we can switch to that.

I'll add the extra work - this avoids the call to acpi_count_levels() which was annoying
Dave as its another walk of the whole table. (I didn't twig in that conversation that the
unified check may no longer be necessary)


>> + *
>> + * The PPTT table must be rev 3 or later,
>> + *
>> + * 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;
>> +		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));
> 
> sizeof(*cache) to me makes this more obvious.

Would be the only instance of this in the file - but I agree its more readable, and
results in fewer line breaks, which also helps.


>> +
>> +			if (cache->flags & ACPI_PPTT_CACHE_ID_VALID &&
>> +			    cache_v1->cache_id == cache_id)
>> +				return level;
>> +		}
>> +	}
>> +
>> +	return -ENOENT;
>> +}


Thanks,

James

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