[PATCH v4 38/41] arm_mpam: Add workaround for T241-MPAM-4
Shaopeng Tan (Fujitsu)
tan.shaopeng at fujitsu.com
Thu Feb 12 23:02:59 PST 2026
Hello Ben, Fenghua
> From: Shanker Donthineni <sdonthineni at 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 at redhat.com>
> Tested-by: Shaopeng Tan <tan.shaopeng at jp.fujitsu.com>
> Signed-off-by: Shanker Donthineni <sdonthineni at nvidia.com>
> Signed-off-by: James Morse <james.morse at arm.com>
> Signed-off-by: Ben Horgan <ben.horgan at 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
>
>
More information about the linux-arm-kernel
mailing list