linux-kernel - Re: [PATCH v3 41/47] arm_mpam: Generate a configuration for min controls

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Message-ID: <9e01328f-7f28-4b43-aaf4-59d3ce80c268@nvidia.com>
Date: Fri, 30 Jan 2026 20:30:01 -0600
From: Shanker Donthineni <sdonthineni@...dia.com>
To: Ben Horgan <ben.horgan@....com>,
 Jonathan Cameron <jonathan.cameron@...wei.com>,
 "fenghuay@...dia.com" <fenghuay@...dia.com>
Cc: amitsinght@...vell.com, baisheng.gao@...soc.com,
 baolin.wang@...ux.alibaba.com, carl@...amperecomputing.com,
 dave.martin@....com, david@...nel.org, dfustini@...libre.com,
 gshan@...hat.com, james.morse@....com, kobak@...dia.com,
 lcherian@...vell.com, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, peternewman@...gle.com,
 punit.agrawal@....qualcomm.com, quic_jiles@...cinc.com,
 reinette.chatre@...el.com, rohit.mathew@....com,
 scott@...amperecomputing.com, tan.shaopeng@...itsu.com,
 xhao@...ux.alibaba.com, catalin.marinas@....com, will@...nel.org,
 corbet@....net, maz@...nel.org, oupton@...nel.org, joey.gouly@....com,
 suzuki.poulose@....com, kvmarm@...ts.linux.dev,
 Zeng Heng <zengheng4@...wei.com>
Subject: Re: [PATCH v3 41/47] arm_mpam: Generate a configuration for min
 controls

Hi Ben,

On 1/30/2026 8:17 AM, Ben Horgan wrote:
> External email: Use caution opening links or attachments
>
>
> Hi Fenghua, Jonathan,
>
> On 1/13/26 15:39, Jonathan Cameron wrote:
>> On Mon, 12 Jan 2026 16:59:08 +0000
>> Ben Horgan <ben.horgan@....com> wrote:
>>
>>> From: James Morse <james.morse@....com>
>>>
>>> MPAM supports a minimum and maximum control for memory bandwidth. The
>>> purpose of the minimum control is to give priority to tasks that are below
>>> their minimum value. Resctrl only provides one value for the bandwidth
>>> configuration, which is used for the maximum.
>>>
>>>
>>> Hence, I'll drop this patch, and update the mbw_min default to be 0xFFFF
>>> and for the value not to change even if mbw_max changes. I think this
>>> leaves us in the best position going forward without any heuristics that
>>> may come back to bite us later when proper support for a schema
>>> supporting mbw_min is added to resctrl.

Background: I previouslyshared original fix(seecodesnippet below) with 
James Morse
~2 years ago to address the errata, which explicitly recommends usinga 
5% gap for
mitigation of the Hardware issue (the problem described in commit text 
of T241-MPAM-4)

For some reason theoriginalimplementationwas splitinto two patches:
   - Generic change applicable toall chips
   - Specific fixfor Graceerrata T241-MPAM-4


Issue: Dropping this patch impacts[PATCH v3 45/47] forthe errata fix. If 
removalis
necessary, please mergethis changeinto the T241-MPAM-4-specific patch.

--- a/drivers/platform/mpam/mpam_devices.c
+++ b/drivers/platform/mpam/mpam_devices.c
@@ -1190,8 +1190,12 @@ static void mpam_reprogram_ris_partid(struct
mpam_msc_ris *ris, u16 partid, rprops->mbw_pbm_bits);
          }
-       if (mpam_has_feature(mpam_feat_mbw_min, rprops))
-               mpam_write_partsel_reg(msc, MBW_MIN, 0);
+       if (mpam_has_feature(mpam_feat_mbw_min, rprops)) {
+               if (mpam_has_feature(mpam_feat_mbw_max, cfg))
+                       mpam_write_partsel_reg(msc, MBW_MIN, cfg->mbw_min);
+               else
+                       mpam_write_partsel_reg(msc, MBW_MIN, 0);
+       }
         if (mpam_has_feature(mpam_feat_mbw_max, rprops)) {
                 if (mpam_has_feature(mpam_feat_mbw_max, cfg)) @@
-2332,6 +2336,31 @@ static int __write_config(void *arg)
          return 0;
   }
+static void mpam_extend_config(struct mpam_class *class, struct mpam_config *cfg)
+{
+       struct mpam_props *cprops = &class->props;
+       u32 min, delta;
+
+       /*
+        * MAX and MIN should be set together. If only one is provided,
+        * generate a configuration for the other. If only one control
+        * type is supported, the other value will be ignored.
+        *
+        * Resctrl can only configure the MAX.
+        *
+        * Parts affected by Nvidia's T241-MPAM-4 depend on this occurring,
+        * and recommend a 5% difference.
+        */
+       if (mpam_has_feature(mpam_feat_mbw_max, cfg) &&
+           !mpam_has_feature(mpam_feat_mbw_min, cfg)){
+               delta = ((5 * MPAMCFG_MBW_MAX_MAX) / 100) - 1;
+               min = max_t(s32, cfg->mbw_max - delta, BIT(cprops->bwa_wd));
+
+               cfg->mbw_min = max_t(s32, cfg->mbw_max - delta, BIT(16 - cprops->bwa_wd));
+               mpam_set_feature(mpam_feat_mbw_min, cfg);
+       }
+}

Shanker