1 files changed, 41 insertions, 20 deletions

diff --git a/include/linux/average.h b/include/linux/average.h
index d04aa58280de..7ddaf340d2ac 100644
--- a/include/linux/average.h
+++ b/include/linux/average.h
@@ -1,45 +1,66 @@
 #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 */