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Date:   Fri, 21 Apr 2023 12:34:30 +0200
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Subject: Re: [PATCH v1] wifi: mac80211: Initialize EWMA fail avg to 1
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To:     Felix Fietkau <nbd@nbd.name>,
        Johannes Berg <johannes@sipsolutions.net>
CC:     Karthik M <quic_karm@quicinc.com>,
        <linux-wireless@vger.kernel.org>,
        Tamizh Chelvam Raja <quic_tamizhr@quicinc.com>
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From:   Benjamin Beichler <Benjamin.Beichler@uni-rostock.de>
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Am 21.04.2023 um 11:53 schrieb Felix Fietkau:
> =5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=
=5F=5F=5F=5F=5F=5F=5F
> Achtung! Externe E-Mail: Klicken Sie erst dann auf Links und Anh=C3=A4nge, 
> nachdem Sie die Vertrauensw=C3=BCrdigkeit der Absenderadresse gepr=C3=BCft h=
aben.
> =5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=
=5F=5F=5F=5F=5F=5F=5F
>
> On 21.04.23 11:35, Johannes Berg wrote:
>> To me, the first question is if there are potentially any users that are
>> =5Frelying=5F on the current behaviour. This seems unlikely though, looking
>> at the ~30 users, most sound like signal/rssi, packet sizes, etc.
>>
>> So let's say with the bug found here that prompted this patch, chances
>> are that there aren't any users that really want 0 to be special. I also
>> can't even really think of a reason for wanting that.
>>
>>
>> So then let's say we want to fix the existing code. I can think of these
>> possible ways:
>>
>> =C2=A0 * splitting off a bit for initialized from the unsigned long
>> =C2=A0=C2=A0=C2=A0 (which at least for 64-bit should be OK since presumably=
 most code
>> =C2=A0=C2=A0=C2=A0=C2=A0 using this will run on 32-bit systems too)
>> =C2=A0 * adding another value for it, e.g. making it u32 and adding a bool 
>> for
>> =C2=A0=C2=A0=C2=A0 "first value"
>> =C2=A0 * biasing the value, like Felix proposes, could be by 1 or -1 for
>> =C2=A0=C2=A0=C2=A0 example
You forgot the possibility to introduce a separate init function, which 
boils down to a shift with an assignment statement for code, and no 
further data memory cost. Even simply extending the current init 
function (which simply always set 0) would be enough.
>>
>> All of these have a memory cost, of course, though the first two are
>> data and the second code, so for things like stations the code exists
>> only once and the data multiple times. On 64-bit we can probably make
>> the first two not have a data memory cost though.
>>
>> As for biasing the value, couldn't that lead to a similar problem=3F It's
>> clearly less likely that the end of the range is reached rather than
>> zero, but still=3F
> I don't see how it can reduce the range in any way, since the bias is
> added to the fractional part. A range reduction would seem to imply
> having an average value that's bigger than the maximum allowed shifted
> input (top bits cut off), and I don't think that's possible.
>
It does not reduce the range, but it does not matter whether your 
internal state is 0 or 2^(-precision), the non-intuitive behavior stays 
the same.


> - Felix
>

-- 
M.Sc. Benjamin Beichler

Universit=C3=A4t Rostock, Fakult=C3=A4t f=C3=BCr Informatik und Elektrotechnik
Institut f=C3=BCr Angewandte Mikroelektronik und Datentechnik

University of Rostock, Department of CS and EE
Institute of Applied Microelectronics and CE

Albert-Einstein-Str. 26
18059 Rostock
Deutschland/Germany

phone: +49 (0) 381 498 - 7278
email: Benjamin.Beichler@uni-rostock.de
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