"Overall" and per-chain signal/noise values (was Re: multi-antenna handling)

[Guy Harris <guy-FrUbXkNCsVf2fBVCVOL8/A@public.gmane.org>]

On Jun 25, 2014, at 11:20 PM, Johannes Berg <johannes-cdvu00un1VgdHxzADdlk8Q@public.gmane.org> wrote:

> 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.

About 4 1/2 years later, the ath9k driver plus the mac80211 code appears to provide the "overall" strength plus per-antenna strengths, at least as of the 4.17.10 kernel, so I guess it's somewhat of a *de facto* standard that there's an "overall" value.

Given that, this is probably worth documenting - but not requiring, in case some adapter+driver combination doesn't supply it.

Presumably the description would be that:

	if you see a signal strength indication, and *don't* see an antenna number before the end of the header or a Radiotap Namespace field, it's an "overall" value;

	otherwise, it's a per-antenna value for the specified antenna.

Parsers must not assume that there will or won't be an "overall" value if there are per-antenna values.

The "overall" value for a multi-antenna receiver could be described as device-dependent, which might suggest that, unless you know the device, you shouldn't try to interpret it.