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

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Message-ID: <316c5635-75af-4ebb-ad2c-fab0742beb20@arm.com>
Date: Tue, 6 Jan 2026 11:33:44 +0000
From: Ben Horgan <ben.horgan@....com>
To: Jonathan Cameron <jonathan.cameron@...wei.com>
Cc: amitsinght@...vell.com, baisheng.gao@...soc.com,
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Subject: Re: [PATCH v2 23/45] arm_mpam: resctrl: Add rmid index helpers

Hi Jonathan,

On 1/6/26 11:21, Jonathan Cameron wrote:
> 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

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.

> 
>> +}
> 

Thanks,

Ben