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Message-ID: <baea7e2b-7f09-4054-a1fc-31cbd349e65e@arm.com>
Date: Mon, 20 Oct 2025 11:34:38 +0100
From: Ben Horgan <ben.horgan@....com>
To: James Morse <james.morse@....com>, linux-kernel@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, linux-acpi@...r.kernel.org
Cc: 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,
Jonathan Cameron <jonathan.cameron@...wei.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>, Jeremy Linton <jeremy.linton@....com>,
Gavin Shan <gshan@...hat.com>
Subject: Re: [PATCH v3 03/29] ACPI / PPTT: Find cache level by cache-id
Hi James,
On 10/17/25 19:56, 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@....com>
> Reviewed-by: Fenghua Yu <fenghuay@...dia.com>
> Reviewed-by: Gavin Shan <gshan@...hat.com>
> Tested-by: Fenghua Yu <fenghuay@...dia.com>
> ---
> Changes sinec v2:
> * Search all caches, not just unified caches. This removes the need to count
> the caches first, but means a failure to find the table walks the table
> three times for different cache types.
> * Fixed return value of the no-acpi stub.
> * Punctuation typo in a comment,
> * Keep trying to parse the table even if a bogus CPU is encountered.
> * Specified CPUs share caches with other CPUs.
>
> Changes since v1:
> * Droppeed the cleanup based table freeing, use acpi_get_pptt() instead.
> * Removed a confusing comment.
> * Clarified the kernel doc.
>
> Changes since RFC:
> * acpi_count_levels() now returns a value.
> * Converted the table-get stuff to use Jonathan's cleanup helper.
> * Dropped Sudeep's Review tag due to the cleanup change.
> ---
> drivers/acpi/pptt.c | 82 ++++++++++++++++++++++++++++++++++++++++++++
> include/linux/acpi.h | 5 +++
> 2 files changed, 87 insertions(+)
>
> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
> index 63c3a344c075..50c8f2a3c927 100644
> --- a/drivers/acpi/pptt.c
> +++ b/drivers/acpi/pptt.c
> @@ -350,6 +350,27 @@ static struct acpi_pptt_cache *acpi_find_cache_node(struct acpi_table_header *ta
> return found;
> }
>
> +static struct acpi_pptt_cache *
> +acpi_find_any_type_cache_node(struct acpi_table_header *table_hdr,
> + u32 acpi_cpu_id, unsigned int level,
> + struct acpi_pptt_processor **node)
> +{
> + struct acpi_pptt_cache *cache;
> +
> + cache = acpi_find_cache_node(table_hdr, acpi_cpu_id, CACHE_TYPE_UNIFIED,
> + level, node);
> + if (cache)
> + return cache;
> +
> + cache = acpi_find_cache_node(table_hdr, acpi_cpu_id, CACHE_TYPE_DATA,
> + level, node);
> + if (cache)
> + return cache;
> +
> + return acpi_find_cache_node(table_hdr, acpi_cpu_id, CACHE_TYPE_INST,
> + level, node);
> +}
> +
> /**
> * update_cache_properties() - Update cacheinfo for the given processor
> * @this_leaf: Kernel cache info structure being updated
> @@ -903,3 +924,64 @@ 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 cache
> + *
> + * Determine the level relative to any CPU for the 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 caches that are peers.
> + *
> + * The PPTT table must be rev 3 or later.
> + *
> + * If one CPU's L2 is shared with another CPU 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)
> +{
> + int level, cpu;
> + u32 acpi_cpu_id;
> + 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)
> + continue;
> +
> + /* Start at 1 for L1 */
> + level = 1;
> + cache = acpi_find_any_type_cache_node(table, acpi_cpu_id, level,
> + &cpu_node);
> + while (cache) {
> + cache_v1 = ACPI_ADD_PTR(struct acpi_pptt_cache_v1,
> + cache, sizeof(*cache));
> +
> + if (cache->flags & ACPI_PPTT_CACHE_ID_VALID &&
> + cache_v1->cache_id == cache_id)
> + return level;
> +
> + level++;
If there is more than one type of cache at a given level only one is
checked. For example, if there is an L1 data cache and an L1 instruction
cache then the L1 instruction cache will never be considered.
> + cache = acpi_find_any_type_cache_node(table, acpi_cpu_id,
> + level, &cpu_node);
> + }
> + }
> +
> + return -ENOENT;
> +}
Thanks,
Ben
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