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

[Jonathan Cameron <jonathan.cameron@huawei.com>]

On Tue, 6 Jan 2026 11:33:44 +0000
Ben Horgan <ben.horgan@arm.com> wrote:

> Hi Jonathan,
> 
> On 1/6/26 11:21, Jonathan Cameron wrote:
> > 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  
> 
> We only warn if (mpam_pmg_max + 1) is a power of 2 and so I'll keep this
> as it is, although yours is equivalent in the expected case.

Do the resulting 'holes' in the values that are valid here cause
trouble it isn't power of 2? 

> 
> >   
> >> +}  
> >   
> 
> Thanks,
> 
> Ben
> 
>