Re: [patch net-next 0/4] net: allow to change carrier from userspace

On Thu, 13 Dec 2012 09:15:41 -0800
John Fastabend [off-list ref] wrote:

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That is what the operstate mechanism was for. Why did we build that mechanism
if it doesn't work from userspace.

Maybe the fix is to make setting linkstate also set carrier bits.

Hmm. You mean to call netif_carrier_on/off as a reaction to operstate
change? How exactly would you like to do that?

Thanks

Jiri

This would break existing applications and would not really be in the
spirit of the operstate mechanism as I read the documentation:

./Documentation/networking/operstates.txt
  63 IF_OPER_DORMANT (5):
  64  Interface is L1 up, but waiting for an external event, f.e. for a
  65  protocol to establish. (802.1X)

The L1 up is netif_carrier_on here.

Agreed. I am also not finding how to change carrier bits from the
current states.

We use this in user space when we do not want applications to start
using the link until we have negotiated and configured some link layer
attributes. To do this we set IFLA_LINKMODE and then use the
IF_OPER_DORMANT event to trigger the application eventually setting
IF_OPER_UP when the link layer negotiation is complete. If you take the
carrier down this breaks. In my case the protocol is LLDP but I think
there are other examples. This is basically the example Stephen already
gave.

I guess I still am missing why teamd doesn't just set IFLA_LINKMODE
then manage the operstate this way? Sure teamd would have to become a
bit smarter but it would save adding additional interfaces to the
kernel. In the LinkAgg case you could just pin the operstate down this
would also allow protocols to run under the linkagg over the LLC in
IEEE speak which I think is being discussed in the latest round of
LLDP/LinkAgg spec updates (I'll check on that later today).

Think about the ARP monitoring as an example. If the teamd (userlevel)
doesn't receive the reply packet, it should set the master interface 
as down in terms of carrier because there isn't a valid L1 from the
master interface perspective.

LACP needs the same action. If the ports can't move to DISTRIBUTING/
COLLECTING state, it should set master's carrier bit to down.

Even the lack of ports should keep the master with carrier down.

I am saying this because people are used to and there are scripts out
there using something like:
# ethtool <iface> | grep 'Link'
to react an interface failure.

Therefore, I am not seeing how setting operstate to down helps with the
current code.  Can ethtool report Link based on a logic between operstate
and netif_carrier_ok()? If operstate is down, it would report down regardless
of netif_carrier_ok() for instance.

Calling netif_carrier_off() also calls dev_deactivate() to stop everything.
So, if team sets operstate to down, the interface remains active sending/
receiving as far as I can see, right? Not sure if this would be a problem.

-- 
fbl