Most (but not all) of the drivers in this patch support more than one Ethernet speed and duplex. The most common are 10 and 100 Mbit, and half and full duplex, though the newest NICs support 1000 Mbit (gigabit) Ethernet as well.
All of the drivers let you specify speed=XX and duplex=Z, where XX is 10, 100 or 1000, and Z is 0 (zero) for half-duplex, and 1 (one) for full-duplex. Generally most 10 Mbit links are half-duplex (to old hubs or repeaters, which is the original Xerox blue-book Ethernet) and most 100 Mbit links are full-duplex (switches with point-to-point connections). However, for maximum confusion, you will occasionally see 10 Mbit/full-duplex, and 100 Mbit/half-duplex, but not very often.
If you don't specify speed and duplex, the driver attempts to auto-negotiate the speed and duplex to the fastest possible, by an IEEE specification. Most Ethernet hardware produced in the last few years is compliant with the IEEE specification, but there is some older hardware around that isn't.
In the absence of auto-negotiation, the PHY can figure out the speed pretty easily. However, the duplex is another matter. If auto-negotiation isn't supported, the remote device is assumed to be older and thus half-duplex, not full-duplex.
The moral of the story is that 99% of the time you shouldn't specify speed and duplex; the auto-negotiation should automatically figure it out for you. If you run the nicinfo utility, it will tell you what the auto-negotiated speed and duplex is, and most of the time, it will be correct.
So, you should specify speed and duplex to the network driver only if you have older, perhaps broken or nonstandard Ethernet hardware, and you manually control both ends of the Ethernet link. For example, a managed hub or a cross-over cable.