[PATCH v3 45/47] arm_mpam: Add workaround for T241-MPAM-4

Ben Horgan
2026-01-12T17:03:58Z

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.

[ morse: Added as second quirk, adapted to use the new intermediate values
in mpam_extend_config() ]

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>
---
Changes since rfc:
MPAM_IIDR_NVIDIA_T421 -> MPAM_IIDR_NVIDIA_T241
Handling when reset_mbw_min is set
---
Documentation/arch/arm64/silicon-errata.rst | 2 ++
drivers/resctrl/mpam_devices.c | 34 +++++++++++++++++++--
drivers/resctrl/mpam_internal.h | 1 +
3 files changed, 34 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 bdf13a22d98f..884ca6a6d8f3 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 +1470,17 @@ 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;
+}
+
/* 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)
@@ -1508,10 +1525,15 @@ static void mpam_reprogram_ris_partid(struct mpam_msc_ris *ris, u16 partid,

if (mpam_has_feature(mpam_feat_mbw_min, rprops) &&
mpam_has_feature(mpam_feat_mbw_min, cfg)) {
- if (cfg->reset_mbw_min)
- mpam_write_partsel_reg(msc, MBW_MIN, 0);
- else
+ if (cfg->reset_mbw_min) {
+ u16 reset = 0;
+
+ if (mpam_has_quirk(T241_FORCE_MBW_MIN_TO_ONE, msc))
+ reset = mpam_wa_t241_force_mbw_min_to_one(rprops);
+ mpam_write_partsel_reg(msc, MBW_MIN, reset);
+ } else {
mpam_write_partsel_reg(msc, MBW_MIN, cfg->mbw_min);
+ }
}

if (mpam_has_feature(mpam_feat_mbw_max, rprops) &&
@@ -2570,6 +2592,12 @@ static void mpam_extend_config(struct mpam_class *class, struct mpam_config *cfg
cfg->mbw_min = max(min, min_hw_granule);
mpam_set_feature(mpam_feat_mbw_min, cfg);
}
+
+ if (mpam_has_quirk(T241_FORCE_MBW_MIN_TO_ONE, class) &&
+ cfg->mbw_min <= min_hw_granule) {
+ cfg->mbw_min = min_hw_granule + 1;
+ mpam_set_feature(mpam_feat_mbw_min, cfg);
+ }
}

static void mpam_reset_component_cfg(struct mpam_component *comp)
diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
index 9d15d37d4b5a..7b4566814945 100644
--- a/drivers/resctrl/mpam_internal.h
+++ b/drivers/resctrl/mpam_internal.h
@@ -224,6 +224,7 @@ 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