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Message-ID: <d6aeda30-152f-4dde-ba76-57f703f24b1c@arm.com>
Date: Tue, 5 Aug 2025 18:06:48 +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,
Rob Herring <robh@...nel.org>, Ben Horgan <ben.horgan@....com>,
Rohit Mathew <rohit.mathew@....com>,
Shanker Donthineni <sdonthineni@...dia.com>, Zeng Heng
<zengheng4@...wei.com>, Lecopzer Chen <lecopzerc@...dia.com>,
Carl Worth <carl@...amperecomputing.com>,
shameerali.kolothum.thodi@...wei.com,
D Scott Phillips OS <scott@...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>, Rex Nie <rex.nie@...uarmicro.com>,
Dave Martin <dave.martin@....com>, Koba Ko <kobak@...dia.com>,
Sudeep Holla <sudeep.holla@....com>
Subject: Re: [RFC PATCH 07/36] ACPI / PPTT: Find cache level by cache-id
Hi Jonathan,
On 16/07/2025 17:21, Jonathan Cameron wrote:
> On Fri, 11 Jul 2025 18:36:19 +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.
>>
>> acpi_count_levels() expects its levels parameter to be initialised to
>> zero as it passes it to acpi_find_cache_level() as starting_level.
>> The existing callers do this. Document it.
> A few suggestions inline. Mostly driven by the number of missing table
> puts I've seen in ACPI code. You don't have any missing here but with a
> bit of restructuring you can make that easy to see.
Sounds good,
>> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
>> index 13ca2eee3b98..f53748a5df19 100644
>> --- a/drivers/acpi/pptt.c
>> +++ b/drivers/acpi/pptt.c
>> @@ -912,3 +912,76 @@ int find_acpi_cpu_topology_hetero_id(unsigned int cpu)
>> return find_acpi_cpu_topology_tag(cpu, PPTT_ABORT_PACKAGE,
>> ACPI_PPTT_ACPI_IDENTICAL);
>> }
>> +
>> +/**
>> + * 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,
>> + *
>> + * 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, 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;
>> + acpi_status status;
>> + 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;
>> +
>> + status = acpi_get_table(ACPI_SIG_PPTT, 0, &table);
>> + if (ACPI_FAILURE(status)) {
>> + acpi_pptt_warn_missing();
>> + return -ENOENT;
>> + }
>> +
>> + if (table->revision < 3) {
> Maybe a unified exit path given all paths need to do
> acpi_put_table() and return either error or level.
>
> Or maybe it's time for some cleanup.h magic for acpi tables. I've
> been thinking about it for a while and mostly stuck on the name ;)
(Isn't that the hard bit?)
> (simpler suggestion follows)
>
> static struct acpi_table_header *acpi_get_table_ret(char *signature, u32 instance)
> {
> struct acpi_table_header *table;
> int status = acpi_get_table(signature, instance, &table);
>
> if (ACPI_FAILURE(status))
> return ERR_PTR(-ENOENT);
> return table;
> }
>
> DEFINE_FREE(acpi_table, struct acpi_table_header *, if (!IS_ERR(_T)) acpi_put_table(_T))
>
> Finally in here and loads of other places we avoid chance of missing an acpi_put_table
> and generally simplify the code a little.
>
> int find_acpi_cache_level_from_id(u32 cache_id)
> {
> u32 acpi_cpu_id;
> acpi_status status;
> 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);
>
> if (IS_ERR(table)
> return PTR_ERR(table);
>
> if (table->revision < 3)
> return -ENOENT;
>
> /*
> * If we found the cache first, we'd still need to walk from each CPU
> * to find the level...
> */
> 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;
> acpi_count_levels(table, cpu_node, &num_levels, 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) {
> acpi_put_table(table);
> return level;
> }
> }
> }
> return -ENOENT;
> }
>
>
> A less 'fun' alternative is pull some code out as a helper to make put the get and put
> near each other with no conditionals to confuse things.
I still find the cleanup stuff slightly sickening ... so lets use it some more.
Added to linux/acpi.h to make it easier to use elsewhere. I think the earlier patches in
this series are simple enough in this area its not worth changing them...
Thanks,
James
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