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Date:   Fri, 21 Apr 2023 11:53:06 +0200
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To:     Johannes Berg <johannes@sipsolutions.net>,
        Benjamin Beichler <Benjamin.Beichler@uni-rostock.de>
Cc:     Karthik M <quic_karm@quicinc.com>, linux-wireless@vger.kernel.org,
        Tamizh Chelvam Raja <quic_tamizhr@quicinc.com>
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 <ad5febda1c32c0d84aa8ac509fefc5b31a8b7ed4.camel@sipsolutions.net>
From:   Felix Fietkau <nbd@nbd.name>
Subject: Re: [PATCH v1] wifi: mac80211: Initialize EWMA fail avg to 1
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On 21.04.23 11:35, Johannes Berg wrote:
> To me, the first question is if there are potentially any users that are
> _relying_ 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:
> 
>   * splitting off a bit for initialized from the unsigned long
>     (which at least for 64-bit should be OK since presumably most code
>      using this will run on 32-bit systems too)
>   * adding another value for it, e.g. making it u32 and adding a bool for
>     "first value"
>   * biasing the value, like Felix proposes, could be by 1 or -1 for
>     example
> 
> 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? It's
> clearly less likely that the end of the range is reached rather than
> zero, but still?
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.

- Felix