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

Jonathan Cameron
2026-01-06T11:21:21Z

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

> From: James Morse <james.morse@arm.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@arm.com>
> Signed-off-by: Ben Horgan <ben.horgan@arm.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@huawei.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

> +}