linux-kernel - Re: [RFC PATCH 05/36] ACPI / PPTT: Add a helper to fill a cpumask from a processor container

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Message-ID: <3fe6b4a6-d27b-4530-9f76-eb6c0daabbde@arm.com>
Date: Fri, 25 Jul 2025 18:05:48 +0100
From: James Morse <james.morse@....com>
To: Ben Horgan <ben.horgan@....com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org
Cc: Rob Herring <robh@...nel.org>, 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 05/36] ACPI / PPTT: Add a helper to fill a cpumask
 from a processor container

Hi Ben,

On 23/07/2025 15:42, Ben Horgan wrote:
> On 7/11/25 19:36, James Morse wrote:
>> The PPTT describes CPUs and caches, as well as processor containers.
>> The ACPI table for MPAM describes the set of CPUs that can access an MSC
>> with the UID of a processor container.
>>
>> Add a helper to find the processor container by its id, then walk
>> the possible CPUs to fill a cpumask with the CPUs that have this
>> processor container as a parent.

>> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
>> index 54676e3d82dd..13619b1b821b 100644
>> --- a/drivers/acpi/pptt.c
>> +++ b/drivers/acpi/pptt.c
>> @@ -298,6 +298,99 @@ static struct acpi_pptt_processor *acpi_find_processor_node(struct
>> +/**
>> + * acpi_pptt_get_cpus_from_container() - Populate a cpumask with all CPUs in a
>> + *                                       processor containers
>> + * @acpi_cpu_id:    The UID of the processor container.
>> + * @cpus        The resulting CPU mask.
>> + *
>> + * Find the specified Processor Container, and fill @cpus with all the cpus
>> + * below it.
>> + *
>> + * Not all 'Processor' entries in the PPTT are either a CPU or a Processor
>> + * Container, they may exist purely to describe a Private resource. CPUs
>> + * have to be leaves, so a Processor Container is a non-leaf that has the
>> + * 'ACPI Processor ID valid' flag set.
>> + *
>> + * Return: 0 for a complete walk, or an error if the mask is incomplete.
>> + */
>> +int acpi_pptt_get_cpus_from_container(u32 acpi_cpu_id, cpumask_t *cpus)
>> +{
>> +    struct acpi_pptt_processor *cpu_node;
>> +    struct acpi_table_header *table_hdr;
>> +    struct acpi_subtable_header *entry;
>> +    bool leaf_flag, has_leaf_flag = false;
>> +    unsigned long table_end;
>> +    acpi_status status;
>> +    u32 proc_sz;
>> +    int ret = 0;
>> +
>> +    cpumask_clear(cpus);
>> +
>> +    status = acpi_get_table(ACPI_SIG_PPTT, 0, &table_hdr);
>> +    if (ACPI_FAILURE(status))
>> +        return 0;
>> +
>> +    if (table_hdr->revision > 1)
>> +        has_leaf_flag = true;
>> +
>> +    table_end = (unsigned long)table_hdr + table_hdr->length;
>> +    entry = ACPI_ADD_PTR(struct acpi_subtable_header, table_hdr,
>> +                 sizeof(struct acpi_table_pptt));
>> +    proc_sz = sizeof(struct acpi_pptt_processor);
>> +    while ((unsigned long)entry + proc_sz <= table_end) {
>> +        cpu_node = (struct acpi_pptt_processor *)entry;
>> +        if (entry->type == ACPI_PPTT_TYPE_PROCESSOR &&
>> +            cpu_node->flags & ACPI_PPTT_ACPI_PROCESSOR_ID_VALID) {
>> +            leaf_flag = cpu_node->flags & ACPI_PPTT_ACPI_LEAF_NODE;
>> +            if ((has_leaf_flag && !leaf_flag) ||
>> +                (!has_leaf_flag && !acpi_pptt_leaf_node(table_hdr, cpu_node))) {
>> +                if (cpu_node->acpi_processor_id == acpi_cpu_id)
>> +                    acpi_pptt_get_child_cpus(table_hdr, cpu_node, cpus);
>> +            }

> acpi_pptt_leaf_node() returns early based on the leaf flag so you can just rely on that
> here; remove has_leaf_flag and the corresponding extra logic.

Aha! I was only doing this to try and avoid that extra descent of the tree. I missed that
its already taken into account.


Thanks,

James