linux-kernel - Re: [PATCH v3 08/47] arm64: mpam: Advertise the CPUs MPAM limits to the driver

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Message-ID: <7c224083-595b-432f-9147-1e8067df634c@redhat.com>
Date: Mon, 19 Jan 2026 14:37:46 +0800
From: Gavin Shan <gshan@...hat.com>
To: Ben Horgan <ben.horgan@....com>
Cc: amitsinght@...vell.com, baisheng.gao@...soc.com,
 baolin.wang@...ux.alibaba.com, carl@...amperecomputing.com,
 dave.martin@....com, david@...nel.org, dfustini@...libre.com,
 fenghuay@...dia.com, james.morse@....com, jonathan.cameron@...wei.com,
 kobak@...dia.com, lcherian@...vell.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 peternewman@...gle.com, punit.agrawal@....qualcomm.com,
 quic_jiles@...cinc.com, reinette.chatre@...el.com, rohit.mathew@....com,
 scott@...amperecomputing.com, sdonthineni@...dia.com,
 tan.shaopeng@...itsu.com, xhao@...ux.alibaba.com, catalin.marinas@....com,
 will@...nel.org, corbet@....net, maz@...nel.org, oupton@...nel.org,
 joey.gouly@....com, suzuki.poulose@....com, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v3 08/47] arm64: mpam: Advertise the CPUs MPAM limits to
 the driver

Hi Ben,

On 1/13/26 12:58 AM, Ben Horgan wrote:
> From: James Morse <james.morse@....com>
> 
> Requestors need to populate the MPAM fields for any traffic they send on
> the interconnect. For the CPUs these values are taken from the
> corresponding MPAMy_ELx register. Each requestor may have a limit on the
> largest PARTID or PMG value that can be used. The MPAM driver has to
> determine the system-wide minimum supported PARTID and PMG values.
> 
> To do this, the driver needs to be told what each requestor's limit is.
> 
> CPUs are special, but this infrastructure is also needed for the SMMU and
> GIC ITS. Call the helper to tell the MPAM driver what the CPUs can do.
> 
> The return value can be ignored by the arch code as it runs well before the
> MPAM driver starts probing.
> 
> Reviewed-by: Jonathan Cameron <jonathan.cameron@...wei.com>
> Signed-off-by: James Morse <james.morse@....com>
> Signed-off-by: Ben Horgan <ben.horgan@....com>
> ---
>   arch/arm64/kernel/mpam.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
> 
> diff --git a/arch/arm64/kernel/mpam.c b/arch/arm64/kernel/mpam.c
> index 9866d2ca0faa..e6feff2324ac 100644
> --- a/arch/arm64/kernel/mpam.c
> +++ b/arch/arm64/kernel/mpam.c
> @@ -3,6 +3,7 @@
>   
>   #include <asm/mpam.h>
>   
> +#include <linux/arm_mpam.h>
>   #include <linux/jump_label.h>
>   #include <linux/percpu.h>
>   
> @@ -11,3 +12,14 @@ DEFINE_PER_CPU(u64, arm64_mpam_default);
>   DEFINE_PER_CPU(u64, arm64_mpam_current);
>   
>   u64 arm64_mpam_global_default;
> +
> +static int __init arm64_mpam_register_cpus(void)
> +{
> +	u64 mpamidr = read_sanitised_ftr_reg(SYS_MPAMIDR_EL1);
> +	u16 partid_max = FIELD_GET(MPAMIDR_EL1_PARTID_MAX, mpamidr);
> +	u8 pmg_max = FIELD_GET(MPAMIDR_EL1_PMG_MAX, mpamidr);
> +
> +	return mpam_register_requestor(partid_max, pmg_max);

mpam_register_requestor() is exposed until CONFIG_ARM64_MPAM_DRIVER is set.
CONFIG_ARM64_MPAM_DRIVER and CONFIG_ARM64_MPAM can be different until PATCH[39/47]
is applied. So we need PATCH[39/47] to be applied prior to this patch so that
mpam_register_requestor() is always existing and exposed.

> +}
> +/* Must occur before mpam_msc_driver_init() from subsys_initcall() */
> +arch_initcall(arm64_mpam_register_cpus)

Thanks,
Gavin