Re: [RFC v2 3/4] mac80211: allow reservation of a running chanctx

[Luca Coelho <luca@coelho.fi>]

On Fri, 2014-02-28 at 15:07 +0100, Michal Kazior wrote:
> On 28 February 2014 14:41, Luca Coelho <luca@coelho.fi> wrote:
> > On Fri, 2014-02-28 at 13:56 +0100, Michal Kazior wrote:
> >> The check is simply I have in mind is simply:
> >>
> >> bool ieee80211_chanctx_needs_channel_change(struct ieee80211_local
> >> *local, struct ieee80211_chanctx *ctx) {
> >>  lockdep_assert_held(&local->chanctx_mtx);
> >>  rcu_read_lock();
> >>  list_for_each_entry_rcu(sdata, &local->interfaces, list) {
> >>   if (sdata->reserved_chanctx != ctx)
> >>    continue;
> >>   if (get_current_chanctx(sdata) == sdata->reserved_chanctx)
> >>    return true;
> >>  }
> >>  rcu_read_unlock();
> >>  return false;
> >> }
> >>
> >> IOW if there's a least one vif bound to given chanctx and the vif has
> >> both current and future chanctx the same, then the chanctx requires
> >> in-place channel change (and this matches your original condition
> >> (mode == RESERVED)).
> >>
> >> This should be future proof for multi-interface/channel.
> >
> > Okay, I get your point, it's not strictly necessary.  But this would be
> > needed in other places too, for example in the combinations check.  We
> > don't want to allow a new interface to join a chanctx that is going to
> > change.  In my merge between the combination check series and this one,
> > I have this: http://pastebin.coelho.fi/65603f9d06b28cb2.txt
> 
> Hmm.. Good point, but the snippet doesn't prevent new vifs from
> joining a chanctx that's going to change channel.

No, the prevention actually happens in ieee80211_find_chanctx() and it's
already part of this series.  I just wanted to point out that this is
needed in several different places.


> I'm also not quite sure if you need it in the combo check at all.
> Can't you just throw EBUSY when you try to assign a new vif to chanctx
> that's going to change channel?

A reserved channel context is taking up space, so new interfaces can't
rely on being able to use it.  Let's say a HW supports 1 chanctx and
there is one vif.  Now the vif wants to change channel and reserves its
chanctx to be changed later.  If a new vif needs a chanctx, it can't use
the one that is reserved, because it will be changed in the future.  So,
during the combination checks, we need to calculate the number of needed
chanctxs for the new vif to be added is 2, so it should fail.



> For multi-channel hw you could allow creating new chanctx (if there's
> enough channels in current combination) and make 2 chanctx that will
> be compatible in the future (and worry about merging them later), or
> you could deny that until reservation is finalized.

Yes, if you have enough chanctxs to use, it's not a problem.  But during
the count we need to consider the ones that will be changed (and are
thus marked as RESERVED) almost as an EXCLUSIVE chanctx, so they are
counted separately.  The difference between EXCLUSIVE and RESERVED, is
only that a RESERVED chanctx can have more than one vif (as long as all
of them have compatible future chandefs).


> > If I'd use the iteration function there would be a lot of iterations
> > going on.  Not sure that's a problem though.
> >
> > The advantages of your approach is that we need less moving parts (ie.
> > less stuff to save in sdata).  The advantage of using a new mode is that
> > it would require less code to run.
> 
> I'd rather not have to worry about memoizing variables and
> recalculating them when it's not strictly necessary (this isn't tx
> path). In both cases you have to worry about locking which I think is
> enough.

As I said, I don't have a preference.  Except that, being lazy, I'd
prefer not to change what I already did. :P

--
Luca.