[RFC PATCH 1/6] arm_mpam: Clean up config update checks in mpam_apply_config()

[Dave Martin <Dave.Martin@arm.com>]

In mpam_apply_config(), a simple memcmp() test is used to check
whether the config passed by the caller is already installed or
not.

This check will never find a match except (very occasionally) by
accident, since the component's version of the struct contains
things that the caller won't pass or doesn't know (such as the
garbage collection record).  There might also be random padding.

This may result in MSCs being reprogrammed unnecessarily.

Instead, only compare fields that the caller specified.  If
anything is present in the caller's config and doesn't match the
installed config, paste it across.  If nothing was pasted across
then the MSC reprogramming step is skipped (as the previous code
attempted to do).

Signed-off-by: Dave Martin <Dave.Martin@arm.com>

---

There are comments in the code suggesting a change of this sort.
I may or may not have gone in the right direction with this, and
I have only tried to clean up the behaviour rather than optimising.

No attempt is made to skip unnecessary MSC register updates if the MSC
reprogramming goes ahead.

NOT well tested, yet.
---
 drivers/platform/arm64/mpam/mpam_devices.c | 23 ++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/drivers/platform/arm64/mpam/mpam_devices.c b/drivers/platform/arm64/mpam/mpam_devices.c
index 9463045c0011..41962dd1bb68 100644
--- a/drivers/platform/arm64/mpam/mpam_devices.c
+++ b/drivers/platform/arm64/mpam/mpam_devices.c
@@ -3120,6 +3120,29 @@ static void mpam_extend_config(struct mpam_class *class, struct mpam_config *cfg
 	}
 }
 
+#define maybe_update_config(cfg, feature, newcfg, member, changes) do { \
+	if (mpam_has_feature(feature, newcfg) &&			\
+	    (newcfg)->member != (cfg)->member) {			\
+		(cfg)->member = (newcfg)->member;			\
+		mpam_set_feature(feature, cfg);				\
+									\
+		(changes) |= (feature);					\
+	}								\
+} while (0)
+
+static mpam_features_t mpam_update_config(struct mpam_config *cfg,
+					  const struct mpam_config *newcfg)
+{
+	mpam_features_t changes = 0;
+
+	maybe_update_config(cfg, mpam_feat_cpor_part, newcfg, cpbm, changes);
+	maybe_update_config(cfg, mpam_feat_mbw_part, newcfg, mbw_pbm, changes);
+	maybe_update_config(cfg, mpam_feat_mbw_max, newcfg, mbw_max, changes);
+	maybe_update_config(cfg, mpam_feat_mbw_min, newcfg, mbw_min, changes);
+
+	return changes;
+}
+
 /* TODO: split into write_config/sync_config */
 /* TODO: add config_dirty bitmap to drive sync_config */
 int mpam_apply_config(struct mpam_component *comp, u16 partid,
-- 
2.34.1