FEC ethernet issues [Was: PL310 errata workarounds]

[Russell King - ARM Linux]

On Mon, Mar 24, 2014 at 11:44:43PM +0000, Russell King - ARM Linux wrote:
> Now, you mention packet loss above.  Even on half-duplex networks, I
> would not expect there to be much packet loss because of the
> retransmissions after a collision, and 100Mbit networks handle this
> much better than 10Mbit.  It takes repeated collisions to cause a
> packet to actually be dropped (and a packet being dropped because
> of collisions should be logged by the transmitting host in its
> interface statistics.)
> 
> Another possibility is that the FEC isn't properly negotiating with
> the hub, and the two ends are ending up configured differently, or
> that it is negotiating properly but the FEC isn't being configured
> correctly.
> 
> Now that we know a little more about your setup, including that it
> seems to be one specific setup which is causing the problems, I'll
> do some more testing - I have an old Allied Telesys 10/100 hub here
> which I'll try putting the iMX6 on and re-run your tests.

PC --- 100Mbit hub --- iMX6S

iperf (2.0.5) -c PC -r reports:

iMX6 Tx: [  5]  0.0-10.0 sec  85.5 MBytes  71.5 Mbits/sec
iMX6 Rx: [  3]  0.0-10.0 sec   103 MBytes  86.2 Mbits/sec

However, there's a marked difference when the iMX6 sends - the
collision LED is almost permanently on at full brightness, whereas
when the PC is sending, it's almost always off.

I'm seeing lots of late collisions and CRC errors in the statistics
registers.

It looks to me like the iMX6's transmitter is blathering at any moment,
not taking any notice whether there's already a carrier on the RX side.
I'm not yet sure what to make of this yet.

-- 
FTTC broadband for 0.8mile line: now at 9.7Mbps down 460kbps up... slowly
improving, and getting towards what was expected from it.