Ethernet frames can be extracted or injected to or from the device's
DDR memory. There is one channel for injection and one channel for
extraction. Each of these channels contain a linked list of DCBs which
contains DB. The DCB contains only 1 DB for both the injection and
extraction. Each DB contains a frame. Every time when a frame is received
or transmitted an interrupt is generated.

It is not possible to use both the FDMA and the manual
injection/extraction of the frames. Therefore the FDMA has priority over
the manual because of better performance values.

FDMA:
iperf -c 192.168.1.1
[  5]   0.00-10.02  sec   420 MBytes   352 Mbits/sec    0 sender
[  5]   0.00-10.03  sec   420 MBytes   351 Mbits/sec      receiver

iperf -c 192.168.1.1 -R
[  5]   0.00-10.01  sec   528 MBytes   442 Mbits/sec    0 sender
[  5]   0.00-10.00  sec   524 MBytes   440 Mbits/sec      receiver

Manual:
iperf -c 192.168.1.1
[  5]   0.00-10.02  sec  93.8 MBytes  78.5 Mbits/sec    0 sender
[  5]   0.00-10.03  sec  93.8 MBytes  78.4 Mbits/sec      receiver

ipers -c 192.168.1.1 -R
[  5]   0.00-10.03  sec   121 MBytes   101 Mbits/sec    0 sender
[  5]   0.00-10.01  sec   118 MBytes  99.0 Mbits/sec      receiver
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>