Re: [PATCH v2 43/45] arm_mpam: Add workaround for T241-MPAM-4
From: Jonathan Cameron <jonathan.cameron@huawei.com>
Date: 2026-01-06 15:20:19
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On Fri, 19 Dec 2025 18:11:45 +0000 Ben Horgan [off-list ref] wrote:
From: Shanker Donthineni <redacted> 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
quoted hunk ↗ jump to hunk
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 <redacted> 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 | +----------------+-----------------+-----------------+-----------------------------+