[RFC PATCH 15/38] arm_mpam: resctrl: Convert to/from MPAMs fixed-point formats

James Morse
2025-12-05T22:00:29Z

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.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
---
drivers/resctrl/mpam_resctrl.c | 41 ++++++++++++++++++++++++++++++++++
1 file changed, 41 insertions(+)

diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
index 19d70d00bbcc..55576d0caf12 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>
@@ -246,6 +247,46 @@ 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.
+ * 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)
{
--
2.39.5