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: <aYxieS3Pycm5UthH@e134344.arm.com>
Date: Wed, 11 Feb 2026 11:05:29 +0000
From: Ben Horgan <ben.horgan@....com>
To: Reinette Chatre <reinette.chatre@...el.com>
Cc: amitsinght@...vell.com, baisheng.gao@...soc.com,
	baolin.wang@...ux.alibaba.com, carl@...amperecomputing.com,
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	jonathan.cameron@...wei.com, kobak@...dia.com, lcherian@...vell.com,
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	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 Reinette,

On Tue, Feb 10, 2026 at 03:39:51PM -0800, Reinette Chatre wrote:
> 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.

Yes, not ideal but I don't think it's urgent to tidy this up.

> 
> > +			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.

Makes sense. This will make the code more robust.

> 
> > +		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

Thanks,

Ben