Am 21.04.2023 um 11:53 schrieb Felix Fietkau:
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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
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? 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.
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
Universität Rostock, Fakultät für Informatik und Elektrotechnik
Institut für 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@xxxxxxxxxxxxxx
www: http://www.imd.uni-rostock.de/
