Re: [RFC] mac80211_hwsim: notify user-space about channel change.

[Johannes Berg <johannes@sipsolutions.net>]

On Mon, 2015-02-23 at 09:43 -0800, Ben Greear wrote:

> > This seems a bit strange - don't we already tag packets with the
> > frequency? Why would you need the channel change separately? What does
> > that even mean? Depending on how you use this it could entirely break
> > off-channel operation, for example.
> 
> I was thinking about passive scans.  In that case, we would not always get a packet transmitted
> when the channel changes?

Ah, well, ok. However, hwsim doesn't actually really have a concept of
the 'current channel', for example in the offchannel code it just
temporarily listens on two channels ... so that's not very good for a
more realistic implementation :)

> I was thinking user-space would mimic a real radio that can only listen on
> one channel at once (can any real radios actually listen on two channels at once?)

No, real radios cannot do that (not really anyway - I guess if it was
VHT80 it's really already listening on 4 channels but ...)

> So, if we are off-channel, and pkt arrives for the 'main' channel, then
> a real radio should drop it, right?

Yes.

> Of course, if user-space does not care, then it can simply ignore the channel-change
> logic so I think this would be backwards compat with existing hwsim user-space apps.

Sure. But given the murky concept of channel change, and not going to PS
for off-channel in hwsim etc. I think this would need a bit more design
rather than just exposing the mac80211 channel change. Additionally,
with chanctx support that won't even be invoked for example.

johannes