[RFC PATCH 37/38] arm_mpam: Add workaround for T241-MPAM-6

James Morse
2025-12-05T22:01:59Z

From: Shanker Donthineni <sdonthineni@nvidia.com>

The registers MSMON_MBWU_L and MSMON_MBWU return the number of
requests rather than the number of bytes transferred.

Bandwidth resource monitoring is performed at the last level cache,
where each request arrive in 64Byte granularity. The current
implementation returns the number of transactions received at the
last level cache but does not provide the value in bytes. Scaling
by 64 gives an accurate byte count to match the MPAM specification
for the MSMON_MBWU and MSMON_MBWU_L registers. This patch fixes
the issue by reporting the actual number of bytes instead of the
number of transactions from __ris_msmon_read().

Signed-off-by: Shanker Donthineni <sdonthineni@nvidia.com>
Signed-off-by: James Morse <james.morse@arm.com>
---
Documentation/arch/arm64/silicon-errata.rst | 2 ++
drivers/resctrl/mpam_devices.c | 24 +++++++++++++++++++--
drivers/resctrl/mpam_internal.h | 1 +
3 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/Documentation/arch/arm64/silicon-errata.rst b/Documentation/arch/arm64/silicon-errata.rst
index b18bc704d4a1..e810b2a8f40e 100644
--- a/Documentation/arch/arm64/silicon-errata.rst
+++ b/Documentation/arch/arm64/silicon-errata.rst
@@ -250,6 +250,8 @@ stable kernels.
+----------------+-----------------+-----------------+-----------------------------+
| NVIDIA | T241 MPAM | T241-MPAM-4 | N/A |
+----------------+-----------------+-----------------+-----------------------------+
+| NVIDIA | T241 MPAM | T241-MPAM-6 | 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 5ba0aa703807..c17a6fdea982 100644
--- a/drivers/resctrl/mpam_devices.c
+++ b/drivers/resctrl/mpam_devices.c
@@ -684,6 +684,12 @@ static const struct mpam_quirk mpam_quirks[] = {
.iidr_mask = MPAM_IIDR_MATCH_ONE,
.workaround = T241_FORCE_MBW_MIN_TO_ONE,
},
+ {
+ /* NVIDIA t241 erratum T241-MPAM-6 */
+ .iidr = MPAM_IIDR_NVIDIA_T421,
+ .iidr_mask = MPAM_IIDR_MATCH_ONE,
+ .workaround = T241_MBW_COUNTER_SCALE_64,
+ },
{ NULL }, /* Sentinel */
};

@@ -1140,7 +1146,7 @@ static void write_msmon_ctl_flt_vals(struct mon_read *m, u32 ctl_val,
}
}

-static u64 mpam_msmon_overflow_val(enum mpam_device_features type)
+static u64 __mpam_msmon_overflow_val(enum mpam_device_features type)
{
/* TODO: implement scaling counters */
switch (type) {
@@ -1155,6 +1161,17 @@ static u64 mpam_msmon_overflow_val(enum mpam_device_features type)
}
}

+static u64 mpam_msmon_overflow_val(enum mpam_device_features type,
+ struct mpam_msc *msc)
+{
+ u64 overflow_val = __mpam_msmon_overflow_val(type);
+
+ if (mpam_has_quirk(T241_MBW_COUNTER_SCALE_64, msc))
+ overflow_val *= 64;
+
+ return overflow_val;
+}
+
static void __ris_msmon_read(void *arg)
{
u64 now;
@@ -1245,13 +1262,16 @@ static void __ris_msmon_read(void *arg)
now = FIELD_GET(MSMON___VALUE, now);
}

+ if (mpam_has_quirk(T241_MBW_COUNTER_SCALE_64, msc))
+ now *= 64;
+
if (nrdy)
break;

mbwu_state = &ris->mbwu_state[ctx->mon];

if (overflow)
- mbwu_state->correction += mpam_msmon_overflow_val(m->type);
+ mbwu_state->correction += mpam_msmon_overflow_val(m->type, msc);

/*
* Include bandwidth consumed before the last hardware reset and
diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
index 01882f0acee2..108a8373901c 100644
--- a/drivers/resctrl/mpam_internal.h
+++ b/drivers/resctrl/mpam_internal.h
@@ -224,6 +224,7 @@ struct mpam_props {
enum mpam_device_quirks {
T241_SCRUB_SHADOW_REGS,
T241_FORCE_MBW_MIN_TO_ONE,
+ T241_MBW_COUNTER_SCALE_64,
MPAM_QUIRK_LAST,
};

--
2.39.5