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

[Jonathan Cameron <jonathan.cameron@huawei.com>]

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         |
>  +----------------+-----------------+-----------------+-----------------------------+