realtek: RTL93xx: Make use of correct ring size counters

realtek: RTL93xx: Make use of correct ring size counters

The receive path of the RTL93xx SoCs is currently discarding packets
in software. Analysis gives the following explanation:

- RX ring size registers are setup with the full software ring size
- When packets are received the packet counter registers are increased
- After RX processing the counter registers are changed the wrong way
- From then SOC is allowed to receive more packets than software allows
- Overflow interrupts are fired
- As a reaction to that the software drops packets

Change the processing as follows:

- Setup ring size registers with a headroom of 2 buffers
- Decrease the counter registers with the real work done

With this change no more overflow interrupts occur because the SoC
disables the queues before they can overflow or hit a buffer that is
still owned by the CPU.

Benchmark from single stream iperf3 run, with server process running
on ZyXEL XGS1210 (RTL930x).

iperf3 run before

-----------------------------------------------------------
Server listening on 5201 (test #1)
-----------------------------------------------------------
Accepted connection from 192.168.2.86, port 54412
[  5] local 192.168.2.71 port 5201 connected to 192.168.2.86 port 54418
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec   384 KBytes  3.14 Mbits/sec
[  5]   1.00-2.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   2.00-3.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   3.00-4.01   sec  5.12 MBytes  42.8 Mbits/sec
[  5]   4.01-5.00   sec  11.4 MBytes  95.8 Mbits/sec
[  5]   5.00-6.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   6.00-7.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   7.00-8.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   8.00-9.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   9.00-10.00  sec  0.00 Bytes  0.00 bits/sec

iperf3 run after

-----------------------------------------------------------
Server listening on 5201 (test #1)
-----------------------------------------------------------
Accepted connection from 192.168.2.86, port 55228
[  5] local 192.168.2.71 port 5201 connected to 192.168.2.86 port 55232
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec  22.8 MBytes   191 Mbits/sec
[  5]   1.00-2.01   sec  25.4 MBytes   211 Mbits/sec
[  5]   2.01-3.00   sec  25.4 MBytes   215 Mbits/sec
[  5]   3.00-4.01   sec  26.5 MBytes   220 Mbits/sec
[  5]   4.01-5.00   sec  26.2 MBytes   222 Mbits/sec
[  5]   5.00-6.00   sec  26.9 MBytes   225 Mbits/sec
[  5]   6.00-7.00   sec  27.0 MBytes   226 Mbits/sec
[  5]   7.00-8.01   sec  26.9 MBytes   224 Mbits/sec
[  5]   8.01-9.00   sec  26.5 MBytes   223 Mbits/sec
[  5]   9.00-10.00  sec  26.8 MBytes   225 Mbits/sec
[  5]  10.00-10.02  sec   640 KBytes   224 Mbits/sec

Signed-off-by: Markus Stockhausen <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19960
Signed-off-by: Robert Marko <[email protected]>