Re: [PATCH v2 25/29] arm_mpam: Probe for long/lwd mbwu counters

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

On Wed, 10 Sep 2025 20:43:05 +0000
James Morse <james.morse@arm.com> wrote:

> From: Rohit Mathew <rohit.mathew@arm.com>
> 
> mpam v0.1 and versions above v1.0 support optional long counter for
> memory bandwidth monitoring. The MPAMF_MBWUMON_IDR register have fields
> indicating support for long counters. As of now, a 44 bit counter
> represented by HAS_LONG field (bit 30) and a 63 bit counter represented
> by LWD (bit 29) can be optionally integrated. Probe for these counters
> and set corresponding feature bits if any of these counters are present.
> 
> Signed-off-by: Rohit Mathew <rohit.mathew@arm.com>
> Signed-off-by: James Morse <james.morse@arm.com>
> Reviewed-by: Ben Horgan <ben.horgan@arm.com>

Hi Rohit, James.

I'd like a little more justification of the 'front facing' use for the first
feature bit.  To me that seems confusing but I may well be missing why
we can't have 3 exclusive features.

Jonathan

> ---
>  drivers/resctrl/mpam_devices.c  | 23 ++++++++++++++++++++++-
>  drivers/resctrl/mpam_internal.h |  9 +++++++++
>  2 files changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index eeb62ed94520..bae9fa9441dc 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -795,7 +795,7 @@ static void mpam_ris_hw_probe(struct mpam_msc_ris *ris)
>  				dev_err_once(dev, "Counters are not usable because not-ready timeout was not provided by firmware.");
>  		}
>  		if (FIELD_GET(MPAMF_MSMON_IDR_MSMON_MBWU, msmon_features)) {
> -			bool hw_managed;
> +			bool has_long, hw_managed;
>  			u32 mbwumon_idr = mpam_read_partsel_reg(msc, MBWUMON_IDR);
>  
>  			props->num_mbwu_mon = FIELD_GET(MPAMF_MBWUMON_IDR_NUM_MON, mbwumon_idr);
> @@ -805,6 +805,27 @@ static void mpam_ris_hw_probe(struct mpam_msc_ris *ris)
>  			if (FIELD_GET(MPAMF_MBWUMON_IDR_HAS_RWBW, mbwumon_idr))
>  				mpam_set_feature(mpam_feat_msmon_mbwu_rwbw, props);
>  
> +			/*
> +			 * Treat long counter and its extension, lwd as mutually
> +			 * exclusive feature bits. Though these are dependent
> +			 * fields at the implementation level, there would never
> +			 * be a need for mpam_feat_msmon_mbwu_44counter (long
> +			 * counter) and mpam_feat_msmon_mbwu_63counter (lwd)
> +			 * bits to be set together.
> +			 *
> +			 * mpam_feat_msmon_mbwu isn't treated as an exclusive
> +			 * bit as this feature bit would be used as the "front
> +			 * facing feature bit" for any checks related to mbwu
> +			 * monitors.

Why do we need such a 'front facing' bit?  Why isn't it sufficient just to
add a little helper or macro to find out if mbwu is turned on?

> +			 */
> +			has_long = FIELD_GET(MPAMF_MBWUMON_IDR_HAS_LONG, mbwumon_idr);
> +			if (props->num_mbwu_mon && has_long) {
> +				if (FIELD_GET(MPAMF_MBWUMON_IDR_LWD, mbwumon_idr))
> +					mpam_set_feature(mpam_feat_msmon_mbwu_63counter, props);
> +				else
> +					mpam_set_feature(mpam_feat_msmon_mbwu_44counter, props);
> +			}
> +
>  			/* Is NRDY hardware managed? */
>  			hw_managed = mpam_ris_hw_probe_hw_nrdy(ris, MBWU);
>  			if (hw_managed)
> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
> index 725c2aefa8a2..c190826dfbda 100644
> --- a/drivers/resctrl/mpam_internal.h
> +++ b/drivers/resctrl/mpam_internal.h
> @@ -158,7 +158,16 @@ enum mpam_device_features {
>  	mpam_feat_msmon_csu_capture,
>  	mpam_feat_msmon_csu_xcl,
>  	mpam_feat_msmon_csu_hw_nrdy,
> +
> +	/*
> +	 * Having mpam_feat_msmon_mbwu set doesn't mean the regular 31 bit MBWU
> +	 * counter would be used. The exact counter used is decided based on the
> +	 * status of mpam_feat_msmon_mbwu_44counter/mpam_feat_msmon_mbwu_63counter
> +	 * as well.
> +	 */
>  	mpam_feat_msmon_mbwu,
> +	mpam_feat_msmon_mbwu_44counter,
> +	mpam_feat_msmon_mbwu_63counter,
>  	mpam_feat_msmon_mbwu_capture,
>  	mpam_feat_msmon_mbwu_rwbw,
>  	mpam_feat_msmon_mbwu_hw_nrdy,