linux-kernel - Re: [PATCH v2 04/29] ACPI / PPTT: Add a helper to fill a cpumask from a cache

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Message-ID: <7fe33c0b-affb-4e30-a3fd-24e4c2013654@arm.com>
Date: Fri, 19 Sep 2025 17:10:57 +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,
 linux-acpi@...r.kernel.org,
 D Scott Phillips OS <scott@...amperecomputing.com>,
 carl@...amperecomputing.com, lcherian@...vell.com,
 bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
 baolin.wang@...ux.alibaba.com, Jamie Iles <quic_jiles@...cinc.com>,
 Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com,
 dfustini@...libre.com, amitsinght@...vell.com,
 David Hildenbrand <david@...hat.com>, Dave Martin <dave.martin@....com>,
 Koba Ko <kobak@...dia.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 fenghuay@...dia.com, baisheng.gao@...soc.com, Rob Herring <robh@...nel.org>,
 Rohit Mathew <rohit.mathew@....com>, Rafael Wysocki <rafael@...nel.org>,
 Len Brown <lenb@...nel.org>, Lorenzo Pieralisi <lpieralisi@...nel.org>,
 Hanjun Guo <guohanjun@...wei.com>, Sudeep Holla <sudeep.holla@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Danilo Krummrich <dakr@...nel.org>
Subject: Re: [PATCH v2 04/29] ACPI / PPTT: Add a helper to fill a cpumask from
 a cache_id

Hi Jonathan,

On 11/09/2025 12:06, Jonathan Cameron wrote:
> On Wed, 10 Sep 2025 20:42:44 +0000
> James Morse <james.morse@....com> 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.

> Why for this case does it makes sense to not just use acpi_get_pptt()?
> 
> Also you don't introduce the acpi_get_table_reg() helper until patch 6.

I missed fixing this one up. That's done now.


>> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
>> index c5f2a51d280b..c379a9952b00 100644
>> --- a/drivers/acpi/pptt.c
>> +++ b/drivers/acpi/pptt.c
>> @@ -966,3 +966,62 @@ 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
> 
> Similar comment to previous patch. If we are going to make this unified only
> can we reflect that in the function name.  I worry this will get reused
> and that restriction will surprise.

I agree - the unified restriction turns out only to be of interest to archaeologists.
I've ripped it out.



>> + * @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;
>> +
>> +	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 (WARN_ON_ONCE(!cpu_node))
>> +			continue;

I'm not sure why this one is a WARN_ON_ONCE() and the other isn't - both mean the PPTT
table is missing CPUs, but this looks like leftover debug. I'll drop it.


>> +		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) makes more sense to me.

Yup, I've done that in the previous one It's not otherwise done in this file - lets see if
someone cares strongly the other way.


>> +
>> +			if (cache->flags & ACPI_PPTT_CACHE_ID_VALID &&
>> +			    cache_v1->cache_id == cache_id)
>> +				cpumask_set_cpu(cpu, cpus);
>> +		}
>> +	}
>> +
>> +	return 0;
>> +}

Thanks,

James