Re: [RFC v3] cfg80211: VHT regulatory

[Johannes Berg <johannes@sipsolutions.net>]

On Mon, 2012-09-24 at 16:24 -0700, Luis R. Rodriguez wrote:

> > Why would we want to restrict bandwidth use to the band we're operating
> > in at all? Fundamentally, the question, in terms of regulatory, is:
> >
> >  "Can I use a XY MHz around XYZ MHz?"
> >
> > It isn't really "Can I use VHT 80 on channel 6".
> 
> True, but we won't enable HT on non-HT cards, and so on. As much as we
> can assume code is non IEEE specific cfg80211 builds on IEEE framework
> and adding support to HT-foo or HT-bar to any band would require some
> changes today.

Not really, most cards also don't even assume that you only do 40 MHz on
a channel that it's permitted on. The hardware will (mostly) be happy to
do HT40- on channel 44, even if the 802.11 spec says that you should do
HT40+ only on that channel.

> > I think it's only superficially easier since we define regulatory rules
> > in terms of spectrum usage, and not in terms of IEEE channels.
> 
> I disagree here. Superficially it would indeed make things easier for
> IEEE usage but in practice also it would make run time checks faster.
> With a dynamic run time checker you are looking at a higher complex
> operation at channel change time. That is what I am concerned about
> ultimately.

You've got to be kidding? Runtime cost of this is negligible, we only
execute the test when we switch channels which happens very
infrequently.

> > Well, since the regulatory framework is now part of cfg80211 the line is
> > a bit blurred here,
> 
> Well I consider regulatory code as code not on the Linux kernel and
> cfg80211 regulatory code as IEEE 802.11 regulatory code. The regdb can
> surely still be used on other subsystems, and surely can be shared
> between them, but today the only user is IEEE 802.11

That's not really true though since the entire regdb handling is also
part of cfg80211, so I think we should at least pretend to think about
other spectrum use (or admit we don't care about other technologies,
which is probably closer to the truth). Regardless though, if userspace
gives us the right information for use of very wide channels, then the
internal checks must fall to checking the spectrum permissions anyway,
instead of iterating the list of possible 802.11 channels.

> > On the contrary, I think that if we frame the question
> > above in terms of 802.11 when asking the regulatory framework, then
> > we'll most likely end up with a regulatory framework that is 802.11
> > specific.
> 
> Agreed but I think cfg80211 regulatory code already is 802.11
> specific, but not the userspace stuff. If we have a reason to make and
> separate the cfg80211 regulatory code to be non-802.11 specific I'm
> fine by that, but what's the use case so far and for what gain?

Ok so basically you do want to say that this in-kernel code doesn't care
about other technologies, and if you want to use the regulatory database
for other things you have to separate out the current code into "db
handling" and "permission checking". I'm fine with that.

Still though, I fail to see how it helps? If you start from a database
that has just frequency ranges & information about those ranges, you
have to make a determination based on this information, for the channels
you want to use.

I'm proposing that we make that determination when we want to use a
channel, and don't really worry about the 802.11 rules, we can enforce
those in wpa_supplicant/hostapd as well, for example, like we actually
do today.

What you're proposing (as far as I understand) is that we iterate the
list of possible channels according to the 802.11 rules, and then make
*the same* determination up-front.

I fail to see the big difference that would make the latter easier,
since it falls back to the same decision algorithm.

johannes