net/fec: Unable to get a permanent link when connecting to a PC

[to-fleischer at t-online.de]

I have an i.MX283 board with a KSZ8081 PHY that is connected to the controller
via RMII. The PHY's interrupt line is currently unused, but connected to the
controller. The board runs the linux version 3.17-rc5.
When I connect the board directly to a PC the ethernet link goes up and a second
later down. This is repeated again and again.
After some investigation I found that this is caused by the reset of the FEC at
the begin of fec_restart() that is called by fec_enet_adjust_link() if the link
is up.
Due to the reset the mode is set to MII. After switching back to RMII the PHY 
seems to restart the autonegotiation. Since this takes a while the next call of 
phy_state_machine() detects that the link is down.
It would work, if the reset in fec_restart is avoided for the case that the link
goes up and the fec is still stopped.

Its quite remarkable that this issue does not occur if I connect the board to a
switch or if I use the PHY's interrupt line.

What do you think?

Best Regards,
Torsten Fleischer