arm_mpam: selecting hard or soft MBW_MAX limits

[Ping Li]

Hi Ben,

mbw_max support in the driver always programs MPAMCFG_MBW_MAX without
setting MPAMCFG_MBW_MAX.HARDLIM, so the limit is always soft: a
PARTID above its MAX contends with low priority for bandwidth beyond
the limit, but is not prevented from exceeding it. On our platform we
would like to be able to select the hard limit, where a PARTID above
its MAX is throttled even when the system is otherwise idle - soft
limits let a noisy task exceed its limit and disturb higher-priority
work when bandwidth contention appears.

MPAMF_MBW_IDR.MAXLIM reports whether the MSC implements a hard limit,
a soft limit, or both, but the driver does not read it today.

Is there a preferred way to expose this choice? The options we see:

  - a mount option or info flag, if the choice must be global
  - a per-resource or per-group resctrl knob
  - selecting hard limits by default whenever MAXLIM reports the MSC
    supports them

Thanks,
Ping