[PATCH v3 22/47] arm_mpam: resctrl: Convert to/from MPAMs fixed-point formats

Ben Horgan
2026-01-12T17:01:52Z

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.

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(+)

diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
index 0dd7f613f7a3..c2127570cf37 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);
+
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 8112bcb85e73..71227d072d46 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>
@@ -223,6 +224,56 @@ 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)
{
--
2.43.0