linux-kernel - Re: [PATCH v4 15/41] arm_mpam: resctrl: Pick the caches we will use as resctrl resources

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Message-ID: <c8d53e59-5661-4ac2-bc8f-0661cde59967@intel.com>
Date: Tue, 10 Feb 2026 15:39:51 -0800
From: Reinette Chatre <reinette.chatre@...el.com>
To: Ben Horgan <ben.horgan@....com>
CC: <amitsinght@...vell.com>, <baisheng.gao@...soc.com>,
	<baolin.wang@...ux.alibaba.com>, <carl@...amperecomputing.com>,
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	<linux-doc@...r.kernel.org>, Shaopeng Tan <tan.shaopeng@...fujitsu.com>
Subject: Re: [PATCH v4 15/41] arm_mpam: resctrl: Pick the caches we will use
 as resctrl resources

Hi Ben,

On 2/3/26 1:43 PM, Ben Horgan wrote:
...
> +
>  static int mpam_resctrl_control_init(struct mpam_resctrl_res *res)
>  {
> -	/* TODO: initialise the resctrl resources */
> +	struct mpam_class *class = res->class;
> +	struct rdt_resource *r = &res->resctrl_res;
> +
> +	switch (r->rid) {
> +	case RDT_RESOURCE_L2:
> +	case RDT_RESOURCE_L3:
> +		r->alloc_capable = true;
> +		r->schema_fmt = RESCTRL_SCHEMA_BITMAP;
> +		r->cache.arch_has_sparse_bitmasks = true;
> +
> +		r->cache.cbm_len = class->props.cpbm_wd;
> +		/* mpam_devices will reject empty bitmaps */
> +		r->cache.min_cbm_bits = 1;
> +
> +		if (r->rid == RDT_RESOURCE_L2) {
> +			r->name = "L2";

This code is fine but highlights that resctrl fs should not let the
arch need to do this since this name is used as part of user interface.

> +			r->ctrl_scope = RESCTRL_L2_CACHE;
> +		} else {
> +			r->name = "L3";
> +			r->ctrl_scope = RESCTRL_L3_CACHE;
> +		}
> +
> +		/*
> +		 * Which bits are shared with other ...things...
> +		 * Unknown devices use partid-0 which uses all the bitmap
> +		 * fields. Until we configured the SMMU and GIC not to do this
> +		 * 'all the bits' is the correct answer here.
> +		 */
> +		r->cache.shareable_bits = resctrl_get_default_ctrl(r);

I would like to recommend one style change to set r->alloc_capable as the final
setting. This is the setting that informs resctrl fs that this resource needs
attention. The reason I recommend this to be done last is if/when a future
change adds some configuration here that may fail then it should not fail with
r->alloc_capable as true while partially initialized.

> +		break;
> +	default:
> +		return -EINVAL;
> +	}
>  
>  	return 0;
>  }
> @@ -324,7 +409,8 @@ int mpam_resctrl_setup(void)
>  		res->resctrl_res.rid = rid;
>  	}
>  
> -	/* TODO: pick MPAM classes to map to resctrl resources */
> +	/* Find some classes to use for controls */
> +	mpam_resctrl_pick_caches();
>  
>  	/* Initialise the resctrl structures from the classes */
>  	for_each_mpam_resctrl_control(res, rid) {

Reinette