linux-kernel - Re: [PATCH v3 03/29] ACPI / PPTT: Find cache level by cache-id

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Message-ID: <20251024151519.000058a8@huawei.com>
Date: Fri, 24 Oct 2025 15:15:19 +0100
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: James Morse <james.morse@....com>
CC: <linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-acpi@...r.kernel.org>, D Scott Phillips OS
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	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

On Fri, 17 Oct 2025 18:56:19 +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.
> 
> 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.

Fwiw that sentence doesn't make sense to me. Too many tables.


>  * 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.
Trivial comment only from me.  Ben's question on matching an ID against an
l1 instruction cache needs addressing (or ruling out as a 'won't fix')
though before an RB is appropriate.

>  /**
>   * 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);
>  	}
>  }
> +
> +/*

Smells like kernel doc.  Why not /** ?
Then can at least verify formatting etc.

> + * 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.
> + */