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

James Morse
2025-10-10T16:55:42Z

Hi Fenghua,

On 02/10/2025 05:30, Fenghua Yu wrote:
> On 9/10/25 13:42, 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>
>
> Other than minor comment issues as follows,
>
> Reviewed-by: Fenghua Yu <fenghuay@nvidia.com>

Thanks,

>> 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,

> s/,/./

Yup, already fixed. (I need to clean my glasses more often!)

>> + *
>> + * If one CPUs L2 is shared with another as L3, this function will return
>
> This comment is not clear. Maybe it's better to say:
>
> + * If one CPU's L2 is shared with another CPU as L3, this function will return

Sure,

>> + * 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.
>> + */

Thanks,

James