arm_mpam: resctrl: Convert to/from MPAMs fixed-point formats

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>
Tested-by: Jesse Chick <jessechick@os.amperecomputing.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>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>

1 files changed, 51 insertions, 0 deletions

diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
index 2331e6ddb814..240a06df2f07 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)
 {