Re: [PATCH v2 13/29] arm_mpam: Probe the hardware features resctrl supports

James Morse
2025-09-29T17:45:14Z

Hi Ben,

On 11/09/2025 16:37, Ben Horgan wrote:
> On 9/10/25 21:42, James Morse wrote:
>> Expand the probing support with the control and monitor types
>> we can use with resctrl.

>> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
>> index 4cc44d4e21c4..5ae5d4eee8ec 100644
>> --- a/drivers/resctrl/mpam_internal.h
>> +++ b/drivers/resctrl/mpam_internal.h
>> @@ -112,6 +112,55 @@ static inline void mpam_mon_sel_lock_init(struct mpam_msc *msc)
>> raw_spin_lock_init(&msc->_mon_sel_lock);
>> }
>>
>> +/*
>> + * When we compact the supported features, we don't care what they are.
>> + * Storing them as a bitmap makes life easy.
>> + */
>> +typedef u16 mpam_features_t;
>> +
>> +/* Bits for mpam_features_t */
>> +enum mpam_device_features {
>> + mpam_feat_ccap_part = 0,
>> + mpam_feat_cpor_part,
>> + mpam_feat_mbw_part,
>> + mpam_feat_mbw_min,
>> + mpam_feat_mbw_max,
>> + mpam_feat_mbw_prop,
>> + mpam_feat_msmon,
>> + mpam_feat_msmon_csu,
>> + mpam_feat_msmon_csu_capture,
>> + mpam_feat_msmon_csu_hw_nrdy,
>> + mpam_feat_msmon_mbwu,
>> + mpam_feat_msmon_mbwu_capture,
>> + mpam_feat_msmon_mbwu_rwbw,
>> + mpam_feat_msmon_mbwu_hw_nrdy,
>> + mpam_feat_msmon_capt,
>> + MPAM_FEATURE_LAST,
>> +};

> I added a garbled comment about this for v1. What I was trying to say is
> that I don't think this quite matches what resctrl supports. For
> instance, I don't think mpam_feat_ccap_part matches a resctrl feature.

Ah - right. I thought you meant something was removed later.
Looks like I thought something could be emulated with CCAP, but that turns out not to be
true because it doesn't have an implicit isolation property, which the
resctrl:bitmap-from-userspace requires.
(I think rwbw was a later addition to the architecture and I added it to the wrong patch).

I'll move that, _prop and _rwbw to the later patch. The split is fairly arbitrary - it was
just somewhere to split an otherwise large patch, and does help determine if a bug is
going to be visible to user-space or not.

_capt can go completely. Last I heard no-one was interested in firmware descriptions of
how the capture hardware can be triggered. I suspect no-one has done anything with it.

Thanks,

James