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

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Message-ID: <a3462502-5732-4eae-b0b3-04fc7f040372@arm.com>
Date: Thu, 9 Oct 2025 18:48:41 +0100
From: James Morse <james.morse@....com>
To: Jonathan Cameron <jonathan.cameron@...wei.com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-acpi@...r.kernel.org,
 D Scott Phillips OS <scott@...amperecomputing.com>,
 carl@...amperecomputing.com, lcherian@...vell.com,
 bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
 baolin.wang@...ux.alibaba.com, Jamie Iles <quic_jiles@...cinc.com>,
 Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com,
 dfustini@...libre.com, amitsinght@...vell.com,
 David Hildenbrand <david@...hat.com>, Dave Martin <dave.martin@....com>,
 Koba Ko <kobak@...dia.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 fenghuay@...dia.com, baisheng.gao@...soc.com, Rob Herring <robh@...nel.org>,
 Rohit Mathew <rohit.mathew@....com>, Rafael Wysocki <rafael@...nel.org>,
 Len Brown <lenb@...nel.org>, Lorenzo Pieralisi <lpieralisi@...nel.org>,
 Hanjun Guo <guohanjun@...wei.com>, Sudeep Holla <sudeep.holla@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Danilo Krummrich <dakr@...nel.org>, Ben Horgan <ben.horgan@....com>
Subject: Re: [PATCH v2 25/29] arm_mpam: Probe for long/lwd mbwu counters

Hi Jonathan,

On 12/09/2025 14:27, Jonathan Cameron wrote:
> On Wed, 10 Sep 2025 20:43:05 +0000
> James Morse <james.morse@....com> wrote:
>> From: Rohit Mathew <rohit.mathew@....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