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Date:   Wed, 26 Apr 2023 08:36:19 +0200
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Subject: Re: [RFC PATCH v3] average: change non-init state and add check for
 added values
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From:   Benjamin Beichler <Benjamin.Beichler@uni-rostock.de>
To:     Johannes Berg <johannes@sipsolutions.net>, <nbd@nbd.name>,
        <linux-wireless@vger.kernel.org>
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Am 26.04.2023 um 08:19 schrieb Benjamin Beichler:
> Am 25.04.2023 um 20:18 schrieb Johannes Berg:
>> =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
>> =C2=A0 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 haben.
>> =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 Mon, 2023-04-24 at 21:04 +0000, Benjamin Beichler wrote:
>>> The uninitialized state 0 involves the danger of reaching that state in
>>> normal operation. Since the weight=5Frcp value needs to be bigger than 
>>> one
>>> (otherwise no averaging takes place), the final shifting in add 
>>> ensures,
>>> that the internal state never reach ULONG=5FMAX. Therefore use this value
>>> to signal, that the ewma has no initial value.
>>>
>>> The add function needs to check, that val is not to big, otherwise 
>>> the new
>>> value can overflow the internal state, which results in unexpected 
>>> outputs.
>> This seems nice to me, FWIW.
> Okay. I would split up the patch into sensible parts and send it 
> later. You are also fine with the extra init function with explicit 
> init value=3F Of course this one also needs the check, that it really 
> fits into the internal state :-D
>>
>>> Move the compile time checks into a separate macro, as they are used
>>> multiple times and noise up the functions.
>> Why do we even have those multiple times=3F Should be enough to have them
>> once, since we always compile the static inline functions=3F
>
> Good question ... my assumption was, that the build-bug macro may not 
> work, if the function is unused and maybe optimized away, but I'm not 
> that familiar with those details. I'm more in favor with the 
> static=5Fasserts of C++, which have defined semantics. :-D
>
> It would be nice to put the check into the struct, as some zero size 
> bad member, but I found no example in the kernel, so maybe this is not 
> easily doable.
>
> One may argue, that we only put it in init and if the users do not use 
> init, we call it shit-in-shit-out, but I'm not really in favor of that.
>
> Anyone with more experiences on build time asserts in the kernel=3F
>
>
I just checked out of curiosity the kernel for static=5Fassert, and voil=C3=A0=
 
it already contains it as a wrapper to the C11 =5FStatic=5Fassert, which 
should work better than BUILD=5FBUG. I will transform it and remove the 
multiple insertions.

Benjamin

-- 
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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