Re: multi-antenna handling

[Johannes Berg <johannes-cdvu00un1VgdHxzADdlk8Q@public.gmane.org>]

On Wed, 2014-06-25 at 19:07 -0700, Guy Harris wrote:
> On Jun 25, 2014, at 8:11 AM, Johannes Berg <johannes-cdvu00un1VgdHxzADdlk8Q@public.gmane.org> wrote:
> 
> > On Wed, 2014-06-25 at 09:09 -0400, Matteo Cypriani wrote:
> > 
> >> I observed the same on TP-Link TL-WDR3500 (which is also ath9k-based), and
> >> assumed the signal in the first namespace was an average of the two others.
> >> Should I understand this may be a wrong assumption?
> > 
> > Ah, there's a value in the first part, without per-antenna information -
> > that's the max of the per-antenna values.
> 
> In the capture for the ask.wireshark.org question, frame 2 has -67 dBm in the first part, -68 dBm for antenna 0, and -73 dBm for antenna 1; I don't know what version of the kernel the machine in question is running, so I don't know what version of the ath9k driver it's running, but that's not a max or min or even an average.
> 
> Perhaps somebody needs to gently remind the maintainers of the ath9k
> driver not to put an extra signal value into the radiotap header, lest
> they confuse users.

I think that's likely my fault :-)

>     Present flags
>         .... .... .... .... .... .... .... ...1 = TSFT: True
>         .... .... .... .... .... .... .... ..1. = Flags: True
>         .... .... .... .... .... .... .... .1.. = Rate: True
>         .... .... .... .... .... .... .... 1... = Channel: True
>         .... .... .... .... .... .... ...0 .... = FHSS: False
>         .... .... .... .... .... .... ..1. .... = dBm Antenna Signal: True
>         .... .... .... .... .... 0... .... .... = Antenna: False
[...]
>         .... .... .... .... .... .... ..1. .... = dBm Antenna Signal: True
>         .... .... .... .... .... 1... .... .... = Antenna: True
>         .... .... .... .... .... .... ..1. .... = dBm Antenna Signal: True
>         .... .... .... .... .... 1... .... .... = Antenna: True

>     SSI Signal: -67 dBm

>     SSI Signal: -68 dBm
>     Antenna: 0
>     SSI Signal: -73 dBm
>     Antenna: 1

We actually have one per-chain value, and one "overall" value, the
latter is what we report in the first radiotap namespace, the per-chain
ones in the latter namespaces that only have the signal and antenna
value (but obviously we could add anything else that made sense per
antenna)

Maybe we shouldn't be reporting the "overall" value? The calculation of
that seems to be driver dependent, our (the Intel) driver just takes the
max. If ath9k has a more specific calculation then I can see how that's
a bit confusing but OTOH it actually reports what much else of the stack
used for further work.

johannes