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

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Message-ID: <db799d84-53be-41ea-97a4-f11e03d7f517@arm.com>
Date: Tue, 6 Jan 2026 15:23:05 +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 14:04, Jonathan Cameron wrote:
> On Tue, 6 Jan 2026 11:33:44 +0000
> Ben Horgan <ben.horgan@....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@....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.
> 
> Do the resulting 'holes' in the values that are valid here cause
> trouble it isn't power of 2? 

Theoretically they are a problem, and if resctrl allocates them then we
would get invalid pmg values which can cause msc error interrupts.
However, I don't know of any platforms where pmg_max is more than 1.

Anyhow, I'll look into using multiplication in these rmid arch helpers
rather than just shifting to avoid leaving these holes.

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

-- 
Thanks,

Ben