linux-kernel - Re: [PATCH v2 23/45] arm_mpam: resctrl: Add rmid index helpers

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Message-ID: <20260106112115.00000201@huawei.com>
Date: Tue, 6 Jan 2026 11:21:15 +0000
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: Ben Horgan <ben.horgan@....com>
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Subject: Re: [PATCH v2 23/45] arm_mpam: resctrl: Add rmid index helpers

On Fri, 19 Dec 2025 18:11:25 +0000
Ben Horgan <ben.horgan@....com> wrote:

> From: James Morse <james.morse@....com>
> 
> Because MPAM's pmg aren't identical to RDT's rmid, resctrl handles some
> data structures by index. This allows x86 to map indexes to RMID, and MPAM
> to map them to partid-and-pmg.
> 
> Add the helpers to do this.
> 
> Signed-off-by: James Morse <james.morse@....com>
> Signed-off-by: Ben Horgan <ben.horgan@....com>
one comment inline.
I messed around with GENMASK + field_prep()/field_get() - new
versions of these with no need for runtime constant masks, but
it ended up as not that much more readable than what you have here.

Reviewed-by: Jonathan Cameron <jonathan.cameron@...wei.com>
> ---
> Changes since rfc:
> Use ~0U instead of ~0 in lhs of left shift
> ---
>  drivers/resctrl/mpam_resctrl.c | 28 ++++++++++++++++++++++++++++
>  include/linux/arm_mpam.h       |  3 +++
>  2 files changed, 31 insertions(+)
> 
> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
> index 4275b1a85887..bdbc5504964b 100644
> --- a/drivers/resctrl/mpam_resctrl.c
> +++ b/drivers/resctrl/mpam_resctrl.c
> @@ -120,6 +120,34 @@ u32 resctrl_arch_get_num_closid(struct rdt_resource *ignored)
>  	return mpam_partid_max + 1;
>  }
>  
> +u32 resctrl_arch_system_num_rmid_idx(void)
> +{
> +	u8 closid_shift = fls(mpam_pmg_max);
> +	u32 num_partid = resctrl_arch_get_num_closid(NULL);
> +
> +	return num_partid << closid_shift;

Given I think you restrict mpam_pmg_max to be power of 2 elsewhere,
doesn't this end up the same as something like
	return mpam_pmg_max * resctrl_arch_get_num_closid(NULL);
Maybe its worth keeping it in the form you have here as
it sort of provides documentation for how you pack those IDs

> +}