Re: [PATCH v2 24/45] arm_mpam: resctrl: Convert to/from MPAMs fixed-point formats
From: Jonathan Cameron <jonathan.cameron@huawei.com>
Date: 2026-01-06 11:55:29
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On Fri, 19 Dec 2025 18:11:26 +0000 Ben Horgan [off-list ref] 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. 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>
Perhaps it's just me, but I found this hard to check mostly because of the spec being very light when describing this (that I could find, maybe there is more somewhere else). So whilst I'll give a tag, I'd like ideally a little more documentation, particularly around the oddity of the left aligned field and max value of bwa_wd being 16. Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
quoted hunk ↗ jump to hunk
--- 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 bdbc5504964b..a20656e49edc 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.
This bit of the spec is really confusing. I particularly like BWA_WD being 6 bits but only allowed to take values up to 16. That took me a while to spot as I couldn't work out what this was supposed to do if that was 32. Anyhow this is a request for a bit more documentation for this function such as explaining the field is left aligned to bit 16.
+ * 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)
{