Having the EWMA parameters stored in the runtime struct imposes
memory requirements for the constant values that could just be
inlined in the code. This particularly makes sense if there are
a lot of such structs, for example in mac80211 in the station
table where each station has a number of these in an array, and
there can be many stations.

Provide a macro DECLARE_EWMA() that declares the necessary struct
and inline functions to access it with the parameters hard-coded;
using this also means the user no longer needs to 'select AVERAGE'
as it's entirely self-contained.

In the mac80211 case, on x86-64, this actually slightly *reduces*
code size, while also saving 80 bytes of runtime memory per sta.
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>

Using bitshifts instead of division and multiplication should improve
performance. That requires weight and factor to be powers of two, but i think
this is something we can live with.

Thanks to Peter Zijlstra for the improved formula!
Signed-off-by: NBruno Randolf <br1@einfach.org>

--

v2:	use log2.h functions
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

This adds generic functions for calculating Exponentially Weighted Moving
Averages (EWMA). This implementation makes use of a structure which keeps the
EWMA parameters and a scaled up internal representation to reduce rounding
errors.

The original idea for this implementation came from the rt2x00 driver
(rt2x00link.c). I would like to use it in several places in the mac80211 and
ath5k code and I hope it can be useful in many other places in the kernel code.
Signed-off-by: NBruno Randolf <br1@einfach.org>
Reviewed-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>