Re: [PATCH v4 38/41] arm_mpam: Add workaround for T241-MPAM-4

Shaopeng Tan (Fujitsu)
2026-02-13T07:03:02Z

Hello Ben, Fenghua

> From: Shanker Donthineni <sdonthineni@nvidia.com>
>
> In the T241 implementation of memory-bandwidth partitioning, in the absence
> of contention for bandwidth, the minimum bandwidth setting can affect the
> amount of achieved bandwidth. Specifically, the achieved bandwidth in the
> absence of contention can settle to any value between the values of
> MPAMCFG_MBW_MIN and MPAMCFG_MBW_MAX.  Also, if MPAMCFG_MBW_MIN is set
> zero (below 0.78125%), once a core enters a throttled state, it will never
> leave that state.
>
> The first issue is not a concern if the MPAM software allows to program
> MPAMCFG_MBW_MIN through the sysfs interface. This patch ensures program
> MBW_MIN=1 (0.78125%) whenever MPAMCFG_MBW_MIN=0 is programmed.
>
> In the scenario where the resctrl doesn't support the MBW_MIN interface via
> sysfs, to achieve bandwidth closer to MBW_MAX in the absence of contention,
> software should configure a relatively narrow gap between MBW_MIN and
> MBW_MAX. The recommendation is to use a 5% gap to mitigate the problem.

I have a question regarding the MBW_MIN values.
Are there any cases where the sum of all MBW_MIN values from different control groups exceeds 100%?
And if so, is it acceptable for it to exceed 100%?"

Best regards,
Shaopeng TAN

> Clear the feature MBW_MIN feature from the class to ensure we don't
> accidentally change behaviour when resctrl adds support for a MBW_MIN
> interface.
>
> Tested-by: Gavin Shan <gshan@redhat.com>
> Tested-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
> Signed-off-by: Shanker Donthineni <sdonthineni@nvidia.com>
> Signed-off-by: James Morse <james.morse@arm.com>
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>
> ---
> [ morse: Added as second quirk, adapted to use the new intermediate values
> in mpam_extend_config() ]
>
> Changes since rfc:
> MPAM_IIDR_NVIDIA_T421 -> MPAM_IIDR_NVIDIA_T241
> Handling when reset_mbw_min is set
>
> Changes since v3:
> Move the 5% gap policy back here
> Clear mbw_min feature in class
> ---
>  Documentation/arch/arm64/silicon-errata.rst |  2 +
>  drivers/resctrl/mpam_devices.c              | 50 +++++++++++++++++++--
>  drivers/resctrl/mpam_internal.h             |  1 +
>  3 files changed, 50 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/arch/arm64/silicon-errata.rst b/Documentation/arch/arm64/silicon-errata.rst
> index 4e86b85fe3d6..b18bc704d4a1 100644
> --- a/Documentation/arch/arm64/silicon-errata.rst
> +++ b/Documentation/arch/arm64/silicon-errata.rst
> @@ -248,6 +248,8 @@ stable kernels.
>  +----------------+-----------------+-----------------+-----------------------------+
>  | NVIDIA         | T241 MPAM       | T241-MPAM-1     | N/A                         |
>  +----------------+-----------------+-----------------+-----------------------------+
> +| NVIDIA         | T241 MPAM       | T241-MPAM-4     | N/A                         |
> ++----------------+-----------------+-----------------+-----------------------------+
>  +----------------+-----------------+-----------------+-----------------------------+
>  | Freescale/NXP  | LS2080A/LS1043A | A-008585        | FSL_ERRATUM_A008585         |
>  +----------------+-----------------+-----------------+-----------------------------+
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 76c8cfcef3c0..37a105d95d66 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -679,6 +679,12 @@ static const struct mpam_quirk mpam_quirks[] = {
>          .iidr_mask  = MPAM_IIDR_MATCH_ONE,
>          .workaround = T241_SCRUB_SHADOW_REGS,
>          },
> +       {
> +       /* NVIDIA t241 erratum T241-MPAM-4 */
> +       .iidr       = MPAM_IIDR_NVIDIA_T241,
> +       .iidr_mask  = MPAM_IIDR_MATCH_ONE,
> +       .workaround = T241_FORCE_MBW_MIN_TO_ONE,
> +       },
>          { NULL } /* Sentinel */
>  };
>
> @@ -1464,6 +1470,31 @@ static void mpam_quirk_post_config_change(struct mpam_msc_ris *ris, u16 partid,
>                  mpam_apply_t241_erratum(ris, partid);
>  }
>
> +static u16 mpam_wa_t241_force_mbw_min_to_one(struct mpam_props *props)
> +{
> +       u16 max_hw_value, min_hw_granule, res0_bits;
> +
> +       res0_bits = 16 - props->bwa_wd;
> +       max_hw_value = ((1 << props->bwa_wd) - 1) << res0_bits;
> +       min_hw_granule = ~max_hw_value;
> +
> +       return min_hw_granule + 1;
> +}
> +
> +static u16 mpam_wa_t241_calc_min_from_max(struct mpam_config *cfg)
> +{
> +       u16 val = 0;
> +
> +       if (mpam_has_feature(mpam_feat_mbw_max, cfg)) {
> +               u16 delta = ((5 * MPAMCFG_MBW_MAX_MAX) / 100) - 1;
> +
> +               if (cfg->mbw_max > delta)
> +                       val = cfg->mbw_max - delta;
> +       }
> +
> +       return val;
> +}
> +
>  /* Called via IPI. Call while holding an SRCU reference */
>  static void mpam_reprogram_ris_partid(struct mpam_msc_ris *ris, u16 partid,
>                                        struct mpam_config *cfg)
> @@ -1506,9 +1537,19 @@ static void mpam_reprogram_ris_partid(struct mpam_msc_ris *ris, u16 partid,
>                          mpam_write_partsel_reg(msc, MBW_PBM, cfg->mbw_pbm);
>          }
>
> -       if (mpam_has_feature(mpam_feat_mbw_min, rprops) &&
> -           mpam_has_feature(mpam_feat_mbw_min, cfg))
> -               mpam_write_partsel_reg(msc, MBW_MIN, 0);
> +       if (mpam_has_feature(mpam_feat_mbw_min, rprops)) {
> +               u16 val = 0;
> +
> +               if (mpam_has_quirk(T241_FORCE_MBW_MIN_TO_ONE, msc)) {
> +                       u16 min = mpam_wa_t241_force_mbw_min_to_one(rprops);
> +
> +                       val = mpam_wa_t241_calc_min_from_max(cfg);
> +                       if (val < min)
> +                               val = min;
> +               }
> +
> +               mpam_write_partsel_reg(msc, MBW_MIN, val);
> +       }
>
>          if (mpam_has_feature(mpam_feat_mbw_max, rprops) &&
>              mpam_has_feature(mpam_feat_mbw_max, cfg)) {
> @@ -2304,6 +2345,9 @@ static void mpam_enable_merge_class_features(struct mpam_component *comp)
>
>          list_for_each_entry(vmsc, &comp->vmsc, comp_list)
>                  __class_props_mismatch(class, vmsc);
> +
> +       if (mpam_has_quirk(T241_FORCE_MBW_MIN_TO_ONE, class))
> +               mpam_clear_feature(mpam_feat_mbw_min, &class->props);
>  }
>
>  /*
> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
> index 6b832f2100d9..f6bf462058d9 100644
> --- a/drivers/resctrl/mpam_internal.h
> +++ b/drivers/resctrl/mpam_internal.h
> @@ -220,6 +220,7 @@ struct mpam_props {
>  /* Workaround bits for msc->quirks */
>  enum mpam_device_quirks {
>          T241_SCRUB_SHADOW_REGS,
> +       T241_FORCE_MBW_MIN_TO_ONE,
>          MPAM_QUIRK_LAST
>  };
>
> --
> 2.43.0
>
>