Re: [PATCH net-next 1/3] ethtool: (uapi) Add ETHTOOL_PHY_LOOPBACK to PHY tunables

quoted

If you really do what to do this, you should look at NETIF_F_LOOPBACK
and consider how this concept can be applied at the PHY, not the MAC.
But you need the full concept, so you see things like:

2: eth0: <LOOPBACK,BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
    link/ether 80:ee:73:83:60:27 brd ff:ff:ff:ff:ff:ff

Humm, i've no idea how you actually enable the MAC loopback
NETIF_F_LOOPBACK represents. I don't see anything with ip link set.

I am afraid you lost me on this, NETIF_F_LOOPBACK is a netdev_features_t
bit, so it tells ethtool that this is a potential feature to be turned
on with ethtool -K <iface>. 

Yep, i'm talking rubbish after jumping to a wrong conclusion.

The semantics of this loopack feature are
not defined AFAICT, but a reasonable behavior from the driver is to put
itself in a mode where packets send by a socket-level application are
looped through the Ethernet adapter itself. Whether this happens at the
DMA level, the MII signals, or somewhere in the PHY, is driver specific
unfortunately.

Yes, the interesting one might be forcedeth which writes to the PHY
BMCR_LOOPBACK | BMCR_FULLDPLX | BMCR_SPEED1000;
when the NETIF_F_LOOPBACK feature is set.

Maybe this could be generalised and made available for all MACs which
don't support MAC loopback?

What needs considering is the correct duplex and speed. We need to
ensure the MAC and PHY agree on this.
 

Now, there is another way to toggle a loopback for a given Ethernet
adapter which is to actually set IFF_LOOPBACK in dev->flags for this
interface. Some drivers seem to be able to properly react to that as
well, although I see no way this can be done looking at the iproute2 or
ifconfig man pages..

I prefer this one, since i expect this will set LOOPBACK in

2: eth0: <LOOPBACK,BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000

making it lot more obvious something funny is going on.

       Andrew