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

[Ben Greear <greearb@candelatech.com>]

On 02/24/2015 02:11 AM, Johannes Berg wrote:
> 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.

We could push more and more of this to user-space and let it decide whether and
how to forward or accept frames for particular radios.

But, to do that, we need the low-level settings sent to user-space
(such as current channel).  Encryption keys could be a future enhancement
here, so that we can do 'hardware' encryption in hwsim (and handle encrypt/decrypt
logic however we want in user-space).

For in-kernel testing (ie, no user-space hwsim tool), then other methods can be used,
or we can just ignore the trickier details.  My interest is primarily in user-space
hwsim tool at the moment.

Thanks,
Ben

-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com