Re: [PATCH RFC v1 2/6] arm_mpam: resctrl: probe MPAMF_MBW_IDR MAX_LIM and hardlim_rw

[Richard Cheng <icheng@nvidia.com>]

On Mon, Nov 09, 2026 at 01:29:33PM +0800, Fenghua Yu wrote:
> Before resctrl can expose MB_MAXHLM, the driver must discover
> whether the platform supports HARDLIM read/write and what
> MPAMF_MBW_IDR.MAX_LIM reports. MSCs on the same system may differ;
> inconsistent merge would expose controls on hardware that cannot honour
> them.
> 
> Read mbw_max_lim from MPAMF_MBW_IDR when MBW_MAX is present, derive
> mpam_feat_mbw_max_hardlim_rw when soft and hard limiting are both
> supported, merge capability across MSCs, and keep the merged view in
> sync via mpam_props_sync_mbw_max_hardlim_rw().
> 
> Signed-off-by: Fenghua Yu <fenghuay@nvidia.com>
> ---
>  drivers/resctrl/mpam_devices.c  | 40 ++++++++++++++++++++++++++++++++-
>  drivers/resctrl/mpam_internal.h |  7 ++++++
>  2 files changed, 46 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index cb6096e9d2cf..1e56e8f3858e 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -830,6 +830,19 @@ static bool mpam_ris_hw_probe_csu_nrdy(struct mpam_msc_ris *ris)
>  	return (!can_set || !can_clear);
>  }
>  
> +/* Align mpam_feat_mbw_max_hardlim_rw with MPAMF_MBW_IDR.MAX_LIM and mbw_max. */
> +static void mpam_props_sync_mbw_max_hardlim_rw(struct mpam_props *props)
> +{
> +	if (!mpam_has_feature(mpam_feat_mbw_max, props)) {
> +		mpam_clear_feature(mpam_feat_mbw_max_hardlim_rw, props);
> +		return;
> +	}
> +	if (props->mbw_max_lim == 0)
> +		mpam_set_feature(mpam_feat_mbw_max_hardlim_rw, props);
> +	else
> +		mpam_clear_feature(mpam_feat_mbw_max_hardlim_rw, props);
> +}
> +
>  static void mpam_ris_hw_probe(struct mpam_msc_ris *ris)
>  {
>  	int err;
> @@ -877,6 +890,8 @@ static void mpam_ris_hw_probe(struct mpam_msc_ris *ris)
>  	if (FIELD_GET(MPAMF_IDR_HAS_MBW_PART, ris->idr)) {
>  		u32 mbw_features = mpam_read_partsel_reg(msc, MBW_IDR);
>  
> +		props->mbw_max_lim = 0;
> +
>  		/* portion bitmap resolution */
>  		props->mbw_pbm_bits = FIELD_GET(MPAMF_MBW_IDR_BWPBM_WD, mbw_features);
>  		if (props->mbw_pbm_bits &&
> @@ -891,14 +906,18 @@ static void mpam_ris_hw_probe(struct mpam_msc_ris *ris)
>  		 */
>  		props->bwa_wd = min(props->bwa_wd, 16);
>  
> -		if (props->bwa_wd && FIELD_GET(MPAMF_MBW_IDR_HAS_MAX, mbw_features))
> +		if (props->bwa_wd && FIELD_GET(MPAMF_MBW_IDR_HAS_MAX, mbw_features)) {
>  			mpam_set_feature(mpam_feat_mbw_max, props);
> +			props->mbw_max_lim = FIELD_GET(MPAMF_MBW_IDR_MAX_LIM, mbw_features);
> +		}
>  
>  		if (props->bwa_wd && FIELD_GET(MPAMF_MBW_IDR_HAS_MIN, mbw_features))
>  			mpam_set_feature(mpam_feat_mbw_min, props);
>  
>  		if (props->bwa_wd && FIELD_GET(MPAMF_MBW_IDR_HAS_PROP, mbw_features))
>  			mpam_set_feature(mpam_feat_mbw_prop, props);
> +
> +		mpam_props_sync_mbw_max_hardlim_rw(props);
>  	}
>  
>  	/* Priority partitioning */
> @@ -2252,12 +2271,31 @@ static void __props_mismatch(struct mpam_props *parent,
>  	if (alias && !mpam_has_bwa_wd_feature(parent) &&
>  	    mpam_has_bwa_wd_feature(child)) {
>  		parent->bwa_wd = child->bwa_wd;
> +		parent->mbw_max_lim = child->mbw_max_lim;
> +		if (mpam_has_feature(mpam_feat_mbw_max_hardlim_rw, child))
> +			mpam_set_feature(mpam_feat_mbw_max_hardlim_rw, parent);
> +		else
> +			mpam_clear_feature(mpam_feat_mbw_max_hardlim_rw, parent);
>  	} else if (MISMATCHED_HELPER(parent, child, mpam_has_bwa_wd_feature,
>  				     bwa_wd, alias)) {
>  		pr_debug("took the min bwa_wd\n");
>  		parent->bwa_wd = min(parent->bwa_wd, child->bwa_wd);
>  	}
>  
> +	if (CAN_MERGE_FEAT(parent, child, mpam_feat_mbw_max, alias)) {
> +		parent->mbw_max_lim = child->mbw_max_lim;
> +		if (mpam_has_feature(mpam_feat_mbw_max_hardlim_rw, child))
> +			mpam_set_feature(mpam_feat_mbw_max_hardlim_rw, parent);
> +		else
> +			mpam_clear_feature(mpam_feat_mbw_max_hardlim_rw, parent);
> +	} else if (MISMATCHED_FEAT(parent, child, mpam_feat_mbw_max,
> +				   mbw_max_lim, alias)) {
> +		pr_debug("%s mbw_max_lim mismatch, clearing mbw_max\n", __func__);
> +		mpam_clear_feature(mpam_feat_mbw_max, parent);
> +		parent->mbw_max_lim = 0;
> +		mpam_props_sync_mbw_max_hardlim_rw(parent);

Hi Fenghua,

For alias=True, the existing bitmap_or() at the end of __props_mismatch() appears
to undo this rejection by copying the child's feature bits back into parent.

With both inputs advertising MBW_MAX and having matching bwa_wd, a small harness
using the actual merge functions produced:

  parent.MAX_LIM  child.MAX_LIM  resulting mbw_max  hardlim_rw
         0              1                1             0
         1              0                1             1

The resulting mbw_max_lim is 0 in both cases, thus the same pair of capabilities produces
different writable-HARDLIM support depending on merge order, and MBW_MAX remains enabled
despite the mismatch.

Rerunning mpam_props_sync_mbw_max_hardlim_rw() after the OR would not preserve the
rejection, mbw_max has been restored and mbw_max_lim was reset to 0, which means soft+hard
support.

Best regards,
Richard Cheng.

> +	}
> +
>  	if (alias && !mpam_has_cmax_wd_feature(parent) && mpam_has_cmax_wd_feature(child)) {
>  		parent->cmax_wd = child->cmax_wd;
>  	} else if (MISMATCHED_HELPER(parent, child, mpam_has_cmax_wd_feature,
> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
> index 1c9e0e8b78a9..6be32b2cb0ab 100644
> --- a/drivers/resctrl/mpam_internal.h
> +++ b/drivers/resctrl/mpam_internal.h
> @@ -172,6 +172,7 @@ enum mpam_device_features {
>  	mpam_feat_mbw_part,
>  	mpam_feat_mbw_min,
>  	mpam_feat_mbw_max,
> +	mpam_feat_mbw_max_hardlim_rw,
>  	mpam_feat_mbw_prop,
>  	mpam_feat_intpri_part,
>  	mpam_feat_intpri_part_0_low,
> @@ -203,6 +204,11 @@ struct mpam_props {
>  	u16			dspri_wd;
>  	u16			num_csu_mon;
>  	u16			num_mbwu_mon;
> +	/*
> +	 * MPAMF_MBW_IDR.MAX_LIM [1:0] when mpam_feat_mbw_max; else 0.
> +	 * 0 = soft+hard, 1 = soft only, 2 = hard only, 3 = reserved.
> +	 */
> +	u8			mbw_max_lim;
>  
>  /*
>   * Kunit tests use memset() to set up feature combinations that should be
> @@ -585,6 +591,7 @@ static inline void mpam_resctrl_teardown_class(struct mpam_class *class) { }
>  
>  /* MPAMF_MBW_IDR - MPAM features memory bandwidth partitioning ID register */
>  #define MPAMF_MBW_IDR_BWA_WD		GENMASK(5, 0)
> +#define MPAMF_MBW_IDR_MAX_LIM		GENMASK(9, 8)
>  #define MPAMF_MBW_IDR_HAS_MIN		BIT(10)
>  #define MPAMF_MBW_IDR_HAS_MAX		BIT(11)
>  #define MPAMF_MBW_IDR_HAS_PBM		BIT(12)
> -- 
> 2.53.0
>