Hi Ben,
On 3/14/26 12:45 AM, Ben Horgan wrote:
> From: Dave Martin <Dave.Martin@arm.com>
>
> MPAM uses a fixed-point formats for some hardware controls. Resctrl
> provides the bandwidth controls as a percentage. Add helpers to convert
> between these.
>
> Ensure bwa_wd is at most 16 to make it clear higher values have no meaning.
>
> Tested-by: Gavin Shan <gshan@redhat.com>
> Tested-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
> Tested-by: Peter Newman <peternewman@google.com>
> Tested-by: Zeng Heng <zengheng4@huawei.com>
> Tested-by: Punit Agrawal <punit.agrawal@oss.qualcomm.com>
> Reviewed-by: Zeng Heng <zengheng4@huawei.com>
> Reviewed-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
> Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> Signed-off-by: James Morse <james.morse@arm.com>
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>
> ---
> Changes since v2:
> Ensure bwa_wd is at most 16 (moved from patch 40: arm_mpam: Generate a
> configuration for min controls)
> Expand comments
> ---
> drivers/resctrl/mpam_devices.c | 7 +++++
> drivers/resctrl/mpam_resctrl.c | 51 ++++++++++++++++++++++++++++++++++
> 2 files changed, 58 insertions(+)
>
One nitpick below, but this looks good to me in either way.
Reviewed-by: Gavin Shan <gshan@redhat.com>
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 0e5e24ef60fe..0c97f7708722 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -713,6 +713,13 @@ static void mpam_ris_hw_probe(struct mpam_msc_ris *ris)
> mpam_set_feature(mpam_feat_mbw_part, props);
>
> props->bwa_wd = FIELD_GET(MPAMF_MBW_IDR_BWA_WD, mbw_features);
> +
> + /*
> + * The BWA_WD field can represent 0-63, but the control fields it
> + * describes have a maximum of 16 bits.
> + */
> + props->bwa_wd = min(props->bwa_wd, 16);
> +
16 may deserve a definition for it since it's a constant value and referred
for multiple times in this patch, if we need to give this series another
respin :-)
> if (props->bwa_wd && FIELD_GET(MPAMF_MBW_IDR_HAS_MAX, mbw_features))
> mpam_set_feature(mpam_feat_mbw_max, props);
>
> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
> index cab3e9ccb5c7..adaec522c1a1 100644
> --- a/drivers/resctrl/mpam_resctrl.c
> +++ b/drivers/resctrl/mpam_resctrl.c
> @@ -10,6 +10,7 @@
> #include <linux/errno.h>
> #include <linux/limits.h>
> #include <linux/list.h>
> +#include <linux/math.h>
> #include <linux/printk.h>
> #include <linux/rculist.h>
> #include <linux/resctrl.h>
> @@ -242,6 +243,56 @@ static bool cache_has_usable_cpor(struct mpam_class *class)
> return class->props.cpbm_wd <= 32;
> }
>
> +/*
> + * Each fixed-point hardware value architecturally represents a range
> + * of values: the full range 0% - 100% is split contiguously into
> + * (1 << cprops->bwa_wd) equal bands.
> + *
> + * Although the bwa_bwd fields have 6 bits the maximum valid value is 16
> + * as it reports the width of fields that are at most 16 bits. When
> + * fewer than 16 bits are valid the least significant bits are
> + * ignored. The implied binary point is kept between bits 15 and 16 and
> + * so the valid bits are leftmost.
> + *
> + * See ARM IHI0099B.a "MPAM system component specification", Section 9.3,
> + * "The fixed-point fractional format" for more information.
> + *
> + * Find the nearest percentage value to the upper bound of the selected band:
> + */
> +static u32 mbw_max_to_percent(u16 mbw_max, struct mpam_props *cprops)
> +{
> + u32 val = mbw_max;
> +
> + val >>= 16 - cprops->bwa_wd;
> + val += 1;
> + val *= MAX_MBA_BW;
> + val = DIV_ROUND_CLOSEST(val, 1 << cprops->bwa_wd);
> +
> + return val;
> +}
> +
> +/*
> + * Find the band whose upper bound is closest to the specified percentage.
> + *
> + * A round-to-nearest policy is followed here as a balanced compromise
> + * between unexpected under-commit of the resource (where the total of
> + * a set of resource allocations after conversion is less than the
> + * expected total, due to rounding of the individual converted
> + * percentages) and over-commit (where the total of the converted
> + * allocations is greater than expected).
> + */
> +static u16 percent_to_mbw_max(u8 pc, struct mpam_props *cprops)
> +{
> + u32 val = pc;
> +
> + val <<= cprops->bwa_wd;
> + val = DIV_ROUND_CLOSEST(val, MAX_MBA_BW);
> + val = max(val, 1) - 1;
> + val <<= 16 - cprops->bwa_wd;
> +
> + return val;
> +}
> +
> /* Test whether we can export MPAM_CLASS_CACHE:{2,3}? */
> static void mpam_resctrl_pick_caches(void)
> {
Thanks,
Gavin