average: change to declare precision, not factor

Declaring the factor is counter-intuitive, and people are prone
to using small(-ish) values even when that makes no sense.

Change the DECLARE_EWMA() macro to take the fractional precision,
in bits, rather than a factor, and update all users.

While at it, add some more documentation.
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/virtio_net.c

@@ -51,7 +51,7 @@ module_param(gso, bool, 0444);
  * at once, the weight is chosen so that the EWMA will be insensitive to short-
  * term, transient changes in packet size.
  */
-DECLARE_EWMA(pkt_len, 1, 64)
+DECLARE_EWMA(pkt_len, 0, 64)
 
 /* With mergeable buffers we align buffer address and use the low bits to
  * encode its true size. Buffer size is up to 1 page so we need to align to

drivers/net/wireless/ath/ath5k/ath5k.h

@@ -1252,7 +1252,7 @@ struct ath5k_statistics {
 #define ATH5K_TXQ_LEN_MAX	(ATH_TXBUF / 4)		/* bufs per queue */
 #define ATH5K_TXQ_LEN_LOW	(ATH5K_TXQ_LEN_MAX / 2)	/* low mark */
 
-DECLARE_EWMA(beacon_rssi, 1024, 8)
+DECLARE_EWMA(beacon_rssi, 10, 8)
 
 /* Driver state associated with an instance of a device */
 struct ath5k_hw {

drivers/net/wireless/ralink/rt2x00/rt2x00.h

@@ -257,7 +257,7 @@ struct link_qual {
 	int tx_failed;
 };
 
-DECLARE_EWMA(rssi, 1024, 8)
+DECLARE_EWMA(rssi, 10, 8)
 
 /*
  * Antenna settings about the currently active link.

include/linux/average.h

 #ifndef _LINUX_AVERAGE_H
 #define _LINUX_AVERAGE_H
 
-/* Exponentially weighted moving average (EWMA) */
+/*
+ * Exponentially weighted moving average (EWMA)
+ *
+ * This implements a fixed-precision EWMA algorithm, with both the
+ * precision and fall-off coefficient determined at compile-time
+ * and built into the generated helper funtions.
+ *
+ * The first argument to the macro is the name that will be used
+ * for the struct and helper functions.
+ *
+ * The second argument, the precision, expresses how many bits are
+ * used for the fractional part of the fixed-precision values.
+ *
+ * The third argument, the weight reciprocal, determines how the
+ * new values will be weighed vs. the old state, new values will
+ * get weight 1/weight_rcp and old values 1-1/weight_rcp. Note
+ * that this parameter must be a power of two for efficiency.
+ */
 
-#define DECLARE_EWMA(name, _factor, _weight)				\
+#define DECLARE_EWMA(name, _precision, _weight_rcp)			\
 	struct ewma_##name {						\
 		unsigned long internal;					\
 	};								\
 	static inline void ewma_##name##_init(struct ewma_##name *e)	\
 	{								\
-		BUILD_BUG_ON(!__builtin_constant_p(_factor));		\
-		BUILD_BUG_ON(!__builtin_constant_p(_weight));		\
-		BUILD_BUG_ON_NOT_POWER_OF_2(_factor);			\
-		BUILD_BUG_ON_NOT_POWER_OF_2(_weight);			\
+		BUILD_BUG_ON(!__builtin_constant_p(_precision));	\
+		BUILD_BUG_ON(!__builtin_constant_p(_weight_rcp));	\
+		/*							\
+		 * Even if you want to feed it just 0/1 you should have	\
+		 * some bits for the non-fractional part...		\
+		 */							\
+		BUILD_BUG_ON((_precision) > 30);			\
+		BUILD_BUG_ON_NOT_POWER_OF_2(_weight_rcp);		\
 		e->internal = 0;					\
 	}								\
 	static inline unsigned long					\
 	ewma_##name##_read(struct ewma_##name *e)			\
 	{								\
-		BUILD_BUG_ON(!__builtin_constant_p(_factor));		\
-		BUILD_BUG_ON(!__builtin_constant_p(_weight));		\
-		BUILD_BUG_ON_NOT_POWER_OF_2(_factor);			\
-		BUILD_BUG_ON_NOT_POWER_OF_2(_weight);			\
-		return e->internal >> ilog2(_factor);			\
+		BUILD_BUG_ON(!__builtin_constant_p(_precision));	\
+		BUILD_BUG_ON(!__builtin_constant_p(_weight_rcp));	\
+		BUILD_BUG_ON((_precision) > 30);			\
+		BUILD_BUG_ON_NOT_POWER_OF_2(_weight_rcp);		\
+		return e->internal >> (_precision);			\
 	}								\
 	static inline void ewma_##name##_add(struct ewma_##name *e,	\
 					     unsigned long val)		\
 	{								\
 		unsigned long internal = ACCESS_ONCE(e->internal);	\
-		unsigned long weight = ilog2(_weight);			\
-		unsigned long factor = ilog2(_factor);			\
+		unsigned long weight_rcp = ilog2(_weight_rcp);		\
+		unsigned long precision = _precision;			\
 									\
-		BUILD_BUG_ON(!__builtin_constant_p(_factor));		\
-		BUILD_BUG_ON(!__builtin_constant_p(_weight));		\
-		BUILD_BUG_ON_NOT_POWER_OF_2(_factor);			\
-		BUILD_BUG_ON_NOT_POWER_OF_2(_weight);			\
+		BUILD_BUG_ON(!__builtin_constant_p(_precision));	\
+		BUILD_BUG_ON(!__builtin_constant_p(_weight_rcp));	\
+		BUILD_BUG_ON((_precision) > 30);			\
+		BUILD_BUG_ON_NOT_POWER_OF_2(_weight_rcp);		\
 									\
 		ACCESS_ONCE(e->internal) = internal ?			\
-			(((internal << weight) - internal) +		\
-				(val << factor)) >> weight :		\
-			(val << factor);				\
+			(((internal << weight_rcp) - internal) +	\
+				(val << precision)) >> weight_rcp :	\
+			(val << precision);				\
 	}
 
 #endif /* _LINUX_AVERAGE_H */

net/batman-adv/types.h

@@ -402,7 +402,7 @@ struct batadv_gw_node {
 	struct rcu_head rcu;
 };
 
-DECLARE_EWMA(throughput, 1024, 8)
+DECLARE_EWMA(throughput, 10, 8)
 
 /**
  * struct batadv_hardif_neigh_node_bat_v - B.A.T.M.A.N. V private neighbor

net/mac80211/ieee80211_i.h

@@ -428,7 +428,7 @@ struct ieee80211_sta_tx_tspec {
 	bool downgraded;
 };
 
-DECLARE_EWMA(beacon_signal, 16, 4)
+DECLARE_EWMA(beacon_signal, 4, 4)
 
 struct ieee80211_if_managed {
 	struct timer_list timer;

net/mac80211/sta_info.h

@@ -372,7 +372,7 @@ struct mesh_sta {
 	unsigned int fail_avg;
 };
 
-DECLARE_EWMA(signal, 1024, 8)
+DECLARE_EWMA(signal, 10, 8)
 
 struct ieee80211_sta_rx_stats {
 	unsigned long packets;