Re: multi-antenna handling

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

On Tue, 2014-06-24 at 18:53 -0700, Guy Harris wrote:

> > FWIW, I've now implemented this in Linux (in mac80211), although I start
> > a new radiotap namespace for each antenna, separate from the first one,
> > so you'll see
> > 
> > * any of: TSF, flags, rate, channel, MCS, a-mpdu, VHT, ...
> > (new radiotap namespace)
> > * ant signal: -40 dBm
> > * anntenna: 0
> > (new radiotap namespace)
> > * ant signal: -45 dBm
> > * anntenna: 1
> > ... for however many antennas/chains there are.
> 
> So is the idea that, in the first radiotap-namespace block, there will
> be no per-antenna information such as the antenna signal - the only
> per-antenna information will be in the subsequent namespace blocks?

Exactly. It would have seemed strange to include the per-antenna fields
in general since the other data isn't specific to an antenna. It also
shows nicer in wireshark this way :)

> Context: somebody asked on ask.wireshark.org about a capture from a
> D-Link router running OpenWRT; he claims that the router has 2
> antennas, but the capture has 3 antenna signal strength values, the
> first of which is in the first radiotap namespace:
> 
> 	http://ask.wireshark.org/questions/33994/why-there-are-3-rssi-values-in-the-radiotap-header-for-a-dual-antenna-wireless-card

That's a bit strange - might be a driver quirk. The code I wrote has to
rely on the driver, adding Felix who might know more, I don't know how
ath9k works in this regard.

johannes