Re: [PATCH v2 25/29] arm_mpam: Probe for long/lwd mbwu counters

James Morse
2025-10-09T17:48:49Z

Hi Jonathan,

On 12/09/2025 14:27, Jonathan Cameron wrote:
> On Wed, 10 Sep 2025 20:43:05 +0000
> James Morse <james.morse@arm.com> wrote:
>> From: Rohit Mathew <rohit.mathew@arm.com>
>>
>> mpam v0.1 and versions above v1.0 support optional long counter for
>> memory bandwidth monitoring. The MPAMF_MBWUMON_IDR register have fields
>> indicating support for long counters. As of now, a 44 bit counter
>> represented by HAS_LONG field (bit 30) and a 63 bit counter represented
>> by LWD (bit 29) can be optionally integrated. Probe for these counters
>> and set corresponding feature bits if any of these counters are present.

> I'd like a little more justification of the 'front facing' use for the first
> feature bit. To me that seems confusing but I may well be missing why
> we can't have 3 exclusive features.
>> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
>> index eeb62ed94520..bae9fa9441dc 100644
>> --- a/drivers/resctrl/mpam_devices.c
>> +++ b/drivers/resctrl/mpam_devices.c
>> @@ -795,7 +795,7 @@ static void mpam_ris_hw_probe(struct mpam_msc_ris *ris)
>> dev_err_once(dev, "Counters are not usable because not-ready timeout was not provided by firmware.");
>> }
>> if (FIELD_GET(MPAMF_MSMON_IDR_MSMON_MBWU, msmon_features)) {
>> - bool hw_managed;
>> + bool has_long, hw_managed;
>> u32 mbwumon_idr = mpam_read_partsel_reg(msc, MBWUMON_IDR);
>>
>> props->num_mbwu_mon = FIELD_GET(MPAMF_MBWUMON_IDR_NUM_MON, mbwumon_idr);
>> @@ -805,6 +805,27 @@ static void mpam_ris_hw_probe(struct mpam_msc_ris *ris)
>> if (FIELD_GET(MPAMF_MBWUMON_IDR_HAS_RWBW, mbwumon_idr))
>> mpam_set_feature(mpam_feat_msmon_mbwu_rwbw, props);
>>
>> + /*
>> + * Treat long counter and its extension, lwd as mutually
>> + * exclusive feature bits. Though these are dependent
>> + * fields at the implementation level, there would never
>> + * be a need for mpam_feat_msmon_mbwu_44counter (long
>> + * counter) and mpam_feat_msmon_mbwu_63counter (lwd)
>> + * bits to be set together.
>> + *
>> + * mpam_feat_msmon_mbwu isn't treated as an exclusive
>> + * bit as this feature bit would be used as the "front
>> + * facing feature bit" for any checks related to mbwu
>> + * monitors.

> Why do we need such a 'front facing' bit? Why isn't it sufficient just to
> add a little helper or macro to find out if mbwu is turned on?

(I read Rohit's front-facing as top-level).

I think Rohit thought it would be simpler - there is one feature enum that gets passed in
from the resctrl glue code saying "I want to read a bandwidth counter", because there
is only one, and it doesn't care what size. I think Rohit didn't want to touch that code!

As that is really a separate concept, I think its worth handling explicitly:
mpam_feat_msmon_mbwu means there are counters, and mpam_feat_msmon_mbwu_{31,44,63}counter
say which ones are supported.

The helper you suggest an then pick which one is best.

Thanks,

James