[PATCH v5 13/17] mesh: do not allow pri/sec channel switch

Jouni Malinen j at w1.fi
Mon Jun 11 14:23:34 PDT 2018

On Mon, Jun 11, 2018 at 12:27:48PM -0700, Ben Greear wrote:
> On 06/11/2018 11:51 AM, Jouni Malinen wrote:
> >This is not about center frequency, but about matching primary
> >channels. Switching pri/sec channels does not really change the
> >frequency range. I'd assume it would be possible for the AP interface to
> >go through channel switch when the STA interface needs to move (or
> >connect initially), but sure, it may be simpler to hardcode channels and
> >pri/sec assignment. That last part, though, is likely to be
> >non-compliant with the standard requirements for co-existence..
> As far as I can tell, if a radio uses HT40- for an STA, it cannot simultaneously
> run HT40+ in AP mode on the same radio.

That sounds quite likely for many radio designs.

> In case you must connect an STA to an AP, that STA will negotiate a proper
> center freq and primary channel.  If you then want to start an AP vdev on the same
> radio, you must force it to not do pri/sec switch in order to have it reliably
> work.

Sure, no issues here; there's obviously an existing AP with the specific
pri/sec selection in radio range, so the co-existence rules allow the
local AP to be started with same parameters. This should not need any
additional changes.

> Another use case we have for test equipment:  If you want to run one vdev in
> HT20 mode and another in HT40 or HT80, then you must force the hostapds to all
> use the same primary channel as the HT20 AP or it will fail to work.

For test tools, it may be acceptable to be non-compliant with the

> I don't know enough about mesh or Peter's use to usefully comment more on
> this, but it sounds a lot like the issue I had with STA + AP.  I think you could,
> for instance, run MESH on one vdev and an AP on another in the same radio
> and end up with similar issues to what I had with STA + AP.

The commit messages certainly so not seem to point to any extra
constraints from virtual interfaces on the local device. What I really
want to see here is the exact use case that is being addressed so that I
can review whether it makes sense to provide exceptions to certain rules
or whether there are better ways of solving the issue (e.g., that STA+AP
example case using CSA on the AP).
Jouni Malinen                                            PGP id EFC895FA

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