From: http://marc.info/?l=linux-netdev&m=135474171009246&w=2

The watchdog will fire if all these conditions are met:
1. The interface is up
2. A TX queue is stopped (normally because it is full)
3. No packets have been added to the queue in the last 5 seconds
4. The driver has not told the kernel that the device is unable to
transmit now (e.g. link is down).

Conditions 2 and 3 together normally mean that the TX queue has been
stopped for 5 seconds and therefore that few packets (not necessarily
none at all) have been completed in that time. The time taken for
individual packets to be completed is *not* considered.

This can happen due to:
a. Driver bug causing conditions 2 and 4 to be true during
reconfiguration
b. MAC blocked by a pause frame flood
c. IRQ handling is delayed by a long time (can happen due to excessive
serial logging)
d. Firmware bug causes driver to see link as up when it's not
e. Hardware fault (always a possibility)
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Item d should really be expanded to hardware/firmware/software bug.