Re: [PATCH v6 37/40] arm_mpam: Add workaround for T241-MPAM-4

[Gavin Shan <gshan@redhat.com>]

On 3/14/26 12:46 AM, Ben Horgan 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 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.
> 
> Clear the feature MBW_MIN feature from the class to ensure we don't
> accidentally change behaviour when resctrl adds support for a MBW_MIN
> interface.
> 
> Tested-by: Gavin Shan <gshan@redhat.com>
> Tested-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
> Reviewed-by: Zeng Heng <zengheng4@huawei.com>
> Reviewed-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
> Reviewed-by: Fenghua Yu <fenghuay@nvidia.com>
> 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>
> ---
> [ morse: Added as second quirk, adapted to use the new intermediate values
> in mpam_extend_config() ]
> 
> Changes since rfc:
> MPAM_IIDR_NVIDIA_T421 -> MPAM_IIDR_NVIDIA_T241
> Handling when reset_mbw_min is set
> 
> Changes since v3:
> Move the 5% gap policy back here
> Clear mbw_min feature in class
> 
> Changes since v5:
> Calculate min from max when resetting
> ---
>   Documentation/arch/arm64/silicon-errata.rst |  2 +
>   drivers/resctrl/mpam_devices.c              | 55 +++++++++++++++++++--
>   drivers/resctrl/mpam_internal.h             |  1 +
>   3 files changed, 55 insertions(+), 3 deletions(-)
> 

Reviewed-by: Gavin Shan <gshan@redhat.com>