Re: [PATCHv6 3/6] nl80211/cfg80211: add radar detection command/event

Zefir Kurtisi
2013-01-24T12:56:43Z

On 01/23/2013 01:49 PM, Simon Wunderlich wrote:
> Hey Zefir,
>
> On Mon, Jan 21, 2013 at 11:44:22AM +0100, Zefir Kurtisi wrote:
>> On 01/18/2013 10:54 PM, Johannes Berg wrote:
>>> On Thu, 2013-01-17 at 14:40 +0100, Simon Wunderlich wrote:
>>>
>>>> Actually there is no limit how long a channel is considered "available", at
>>>> least in ETSI. ETSI EN 301-893 v1.4.1 had a limit of 24 hours for that,
>>>> but that was removed in v1.5.1 and didn't re-appear since then (current is
>>>> v1.7.1).
>>>
>>> Huh indeed, I would have expected that to be there. It does have a
>>> non-occupancy time though (30 minutes), maybe we should implement that?
>>>
>> No, why be more restrictive than regulatory demands?
>>
>> With the recent incremental updates, a general note: Victor's initial approach was
>> to keep all logic in hostapd and minimize the modifications in mac by only
>> ensuring CAC times there. If NOP handling is also added to mac, we'd have
>> everything needed to handle DFS channel states available. With hostapd only left
>> to do the selection of the channel to switch to after a radar detection, it might
>> make sense to move everything down to mac. I understand that questioning the
>> design that late is not helpful, at the same time and since the initial path was
>> left, it might be worth considering.
>
> Actually questioning the design NOW is a good idea, we are already questioning it
> through the last patches and better bring up the issues now than later. We have started from the
> simple hostap-handles-everything approach and have seen some points are missing when
> we think about multi interfaces etc for the future (and implementation issues of course).
>
> If we move channel states (available/unavailable) already into the kernel space, we might
> as well check for other states (already doing CAC, not valid until). Maybe it's better like
> this than splitting the management over hostapd and cfg/mac80211?
>
As written in my other post today, managed master mode can't be supported (as long
as we can't override channel states), so for our needs we need a different approach.

Remarkably, the proof-of-concept I proposed as 'poor-man's solution' in Vancouver
two years ago turned out to be quite well usable. It goes like this:
*) hostapd: patched to allow DFS channels
*) ath9k: patched to enable radar detection on DFS-channels
*) channel states handled by a daemon in user space

Switching to AVAILABLE channel:
*) set up master immediately

Switching to non AVAILABLE channel:
*) set up monitor
*) wait CAC
*) set up master if no radar
*) else select different channel

Since I need to patch / bypass the master functionality to enable managed DFS
mode, I am naturally biased. Nevertheless, ideally everything should be in
cfg/mac, as long as there is no compelling reason to split it with hostapd.

>>> I'm also thinking with the next regdb format update we should allow
>>> specifying these timeouts etc. there.
>>>
>>> Does anyone have the relevant FCC rules? I can't find anything with
>>> google ...
>>>
>> The FCC 06-96 document (freely available, e.g.
>> http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-06-96A1.pdf) seems to be the
>> most recent one. Skimming over I did not find a requirement for the validity
>> period after CAC.
>>
>
> Thanks for pointing us to that doc. Couldn't find anything either, so I'd just
> skip any "validity" for now.
>
>>>> But we can move the CAC/timeout in the wdev and have keep a flag field in
>>>> the channel struct instead, marking the channel as available, unavailable, etc.
>>>>
>>>> What do you think?
>>>
>>> I think that would make sense. Probably available/unavailable and
>>> "non-occupancy until"?
>>>
>> At that stage, we would have half of all potential states (UNKNOWN, AVAILABLE,
>> OPERATING, UNAVAILABLE, USABLE, SCANNING) covered, so a current state per channel
>> and the time it was entered would give everything required for the complete state
>> machine in mac.
>
> Sounds good! Will look into this ...
>
State machine is trivial. If you need a reference implementation and are not
scared of C++ code, I can provide you mine.
> Thanks,
> Simon
>
Thanks for handling this,
Zefir