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

From: Stephen Hemminger <[email-masked]>
Date: Wed, 12 Dec 2012 10:54:48 -0800

On Wed, 12 Dec 2012 19:49:26 +0100
Jiri Pirko <[email-masked]> wrote:

> Wed, Dec 12, 2012 at 07:36:32PM CET, [email-masked] wrote:
> >On Wed, 12 Dec 2012 19:25:56 +0100
> >Jiri Pirko <[email-masked]> wrote:
> >
> >> Wed, Dec 12, 2012 at 07:12:08PM CET, [email-masked] wrote:
> >> >On Wed, 12 Dec 2012 19:10:17 +0100
> >> >Jiri Pirko <[email-masked]> wrote:
> >> >
> >> >> ># ip li show dev dummy0
> >> >> >12: dummy0: <NO-CARRIER,BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state DORMANT mode DORMANT   
> >> >> 
> >> >> if you mean this "NO-CARRIER"
> >> >> it has no direct relation with netif_carrier_ok().
> >> >
> >> >It is the same value (IFF_RUNNING) that is visible from user space.
> >> 
> >> static inline bool netif_carrier_ok(const struct net_device *dev)
> >> {
> >> 	        return !test_bit(__LINK_STATE_NOCARRIER, &dev->state);
> >> }
> >> 
> >> So netif_carrier[ok/on/off] are working with on __LINK_STATE_NOCARRIER
> >> bit. Not with IFF_RUNNING flag.
> >
> >What is the code path that you are worried about netif_carrier_ok being set or clear?
> >The interaction here is complex, and right now LINK_STATE_NOCARRIER is purely
> >controlled by the driver, your patch changes that, but before acking I want
> >to make sure why it is required.
> 
> This patchset would provide a possibility to set or clear the carrier
> from userspace. For dummy device it would serve for direct emulation
> of link fail.
> 
> Also for team deriver, that would serve for teamd (userspace part) to
> set the carrier actually on or off (in case of LACP runner for example
> this is required).
> 

You want to able to control the dummy device, so that you can test carrier
management in the team device. Another alternative is to use carrier control
on a virtual device. Vmware can do it, there were patches to do this with KVM/QEMU
not sure if they ever got incorporated.

Since this is a specific feature of the dummy device which is specialized for
testing, maybe it should just be done by adding device specific ioctl rather
than letting it creep in as a general facility.