Re: [PATCH v2 43/45] arm_mpam: Add workaround for T241-MPAM-4

Jonathan Cameron
2026-01-06T15:20:19Z

On Fri, 19 Dec 2025 18:11:45 +0000
Ben Horgan <ben.horgan@arm.com> wrote:

> 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 cocern if the MPAM software allows to program
concern

> 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 MW_MAX in the absence of contention,
MBW_MAX

> 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 | 59 +++++++++++++++------
> drivers/resctrl/mpam_internal.h | 1 +
> 3 files changed, 46 insertions(+), 16 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 |
> +----------------+-----------------+-----------------+-----------------------------+