netdev - Re: bridge interface initial carrier state

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Date:	Mon, 11 Feb 2013 16:46:41 -0600
From:	Dan Williams <dcbw@...hat.com>
To:	Stephen Hemminger <stephen@...workplumber.org>
Cc:	netdev@...r.kernel.org
Subject: Re: bridge interface initial carrier state

On Mon, 2013-02-11 at 13:59 -0800, Stephen Hemminger wrote:
> On Mon, 11 Feb 2013 14:01:55 -0600
> Dan Williams <dcbw@...hat.com> wrote:
> 
> > Hi,
> > 
> > I'm wondering if the initial carrier state of 'on' is intentional for a
> > bridge without ports; immediately after adding ports, the carrier is
> > recalculated and depends on the combined state of each port's carrier
> > and STP forwarding state.  So a userspace program attempting to decide
> > whether the bridge was usable or not has to look at both (a) how many
> > ports are available and (b) bridge carrier state, instead of just
> > looking at the bridge carrier state.
> > 
> > Dan
> 
> Perhaps a future enhancement of bridge would be to use operstate flags
> to indicate lower layer down if there are no ports.

While we're at it, it appears that if a bridge port is added when it has
no carrier, nothing directly triggers br_port_state_selection() to
ensure that the bridge's carrier state is correct:

	if ((dev->flags & IFF_UP) && netif_carrier_ok(dev) &&
	    (br->dev->flags & IFF_UP))
		br_stp_enable_port(p);

Any reason why we can't run br_port_state_selection() unconditionally
when adding a new port?  When removing a port that gets run by
br_stp_disable_port(), which is somewhat asymmetrical.

When adding, we don't necessarily want to enable STP operation on the
port until it's ready, so that hunk above for br_stp_enable_port() is
probably just fine, but we should still probably be recalculating the
bridge's carrier when it gets its first port even if that port isn't yet
usable?  Or not?

Dan

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