average.h

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/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _LINUX_AVERAGE_H
#define _LINUX_AVERAGE_H
 
#include <linux/bug.h>
#include <linux/compiler.h>
#include <linux/log2.h>
 
/*
 * 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, _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(_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(_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 = READ_ONCE(e->internal);    \
        unsigned long weight_rcp = ilog2(_weight_rcp);      \
        unsigned long precision = _precision;           \
                                    \
        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);       \
                                    \
        WRITE_ONCE(e->internal, internal ?          \
            (((internal << weight_rcp) - internal) +    \
                (val << precision)) >> weight_rcp : \
            (val << precision));                \
    }
 
#endif /* _LINUX_AVERAGE_H */