linux-kernel - Re: [RFC PATCH 08/38] arm_mpam: resctrl: Pick the caches we will use as resctrl resources

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Message-ID: <c0139cdf-9263-4051-938d-cb86233399b0@arm.com>
Date: Tue, 9 Dec 2025 15:57:03 +0000
From: Ben Horgan <ben.horgan@....com>
To: James Morse <james.morse@....com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.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@...nel.org>, 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>, Gavin Shan
 <gshan@...hat.com>, rohit.mathew@....com, reinette.chatre@...el.com,
 Punit Agrawal <punit.agrawal@....qualcomm.com>
Subject: Re: [RFC PATCH 08/38] arm_mpam: resctrl: Pick the caches we will use
 as resctrl resources

Hi James,

On 12/5/25 21:58, James Morse wrote:
> Systems with MPAM support may have a variety of control types at any
> point of their system layout. We can only expose certain types of
> control, and only if they exist at particular locations.
> 
> Start with the well-know caches. These have to be depth 2 or 3
> and support MPAM's cache portion bitmap controls, with a number
> of portions fewer than resctrl's limit.
> 
> Signed-off-by: James Morse <james.morse@....com>
> ---
>  drivers/resctrl/mpam_resctrl.c | 91 +++++++++++++++++++++++++++++++++-
>  1 file changed, 89 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
> index 320cebbd37ce..ceaf11af4fc1 100644
> --- a/drivers/resctrl/mpam_resctrl.c
> +++ b/drivers/resctrl/mpam_resctrl.c
> @@ -60,10 +60,96 @@ struct rdt_resource *resctrl_arch_get_resource(enum resctrl_res_level l)
>  	return &mpam_resctrl_controls[l].resctrl_res;
>  }
>  
> +static bool cache_has_usable_cpor(struct mpam_class *class)
> +{
> +	struct mpam_props *cprops = &class->props;
> +
> +	if (!mpam_has_feature(mpam_feat_cpor_part, cprops))
> +		return false;
> +
> +	/* resctrl uses u32 for all bitmap configurations */
> +	return (class->props.cpbm_wd <= 32);
> +}

cpbm_wd > 32 is not support properly in mpam_devices.c (e.g. reset and
config value are limited to 32 bits.) and so we should consider just not
adding the feature to the class in that case.

Thanks,

Ben