diff --git a/sys/compat/linuxkpi/common/include/linux/average.h b/sys/compat/linuxkpi/common/include/linux/average.h
new file mode 100644
index 000000000000..4291bf2d85be
--- /dev/null
+++ b/sys/compat/linuxkpi/common/include/linux/average.h
@@ -0,0 +1,92 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2020-2021 The FreeBSD Foundation
+ *
+ * This software was developed by Björn Zeeb under sponsorship from
+ * the FreeBSD Foundation.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+#ifndef __LKPI_LINUX_AVERAGE_H
+#define __LKPI_LINUX_AVERAGE_H
+
+#include <sys/param.h>
+#include <sys/systm.h>
+#include <sys/types.h>
+#include <linux/log2.h>
+
+/* EWMA stands for Exponentially Weighted Moving Average. */
+/*
+ * Z_t = d X_t + (1 - d) * Z_(t-1); 0 < d <= 1, t >= 1; Roberts (1959).
+ * t  : observation number in time.
+ * d  : weight for current observation.
+ * Xt : observations over time.
+ * Zt : EWMA value after observation t.
+ *
+ * wmba_*_read seems to return up-to [u]long values; have to deal with 32/64bit.
+ * According to the ath5k.h change log this seems to be a fix-(_p)recision impl.
+ * assert 2/4 bits for frac.
+ * Also all (_d) values seem to be pow2 which simplifies maths (shift by
+ * d = ilog2(_d) instead of doing division (d = 1/_d)).  Keep it this way until
+ * we hit the CTASSERT.
+ */
+
+#define	DECLARE_EWMA(_name, _p, _d)						\
+										\
+	CTASSERT((sizeof(unsigned long) <= 4) ? (_p < 30) : (_p < 60));		\
+	CTASSERT(_d > 0 && powerof2(_d));					\
+										\
+	struct ewma_ ## _name {							\
+		unsigned long zt;						\
+	};									\
+										\
+	static __inline void							\
+	ewma_ ## _name ## _init(struct ewma_ ## _name *ewma)			\
+	{									\
+		/* No target (no historical data). */				\
+		ewma->zt = 0;							\
+	}									\
+										\
+	static __inline void							\
+	ewma_ ## _name ## _add(struct ewma_ ## _name *ewma, unsigned long x)	\
+	{									\
+		unsigned long ztm1 = ewma->zt;	/* Z_(t-1). */			\
+		int d = ilog2(_d);						\
+										\
+		if (ewma->zt == 0)						\
+			ewma->zt = x << (_p);					\
+		else								\
+			ewma->zt = ((x << (_p)) >> d) +				\
+			    (((ztm1 << d) - ztm1) >> d);			\
+	}									\
+										\
+	static __inline unsigned long						\
+	ewma_ ## _name ## _read(struct ewma_ ## _name *ewma)			\
+	{									\
+		return (ewma->zt >> (_p));					\
+	}									\
+
+#endif /* __LKPI_LINUX_AVERAGE_H */