net: ibm: emac: Use napi_gro_receive() for Rx packets

net: ibm: emac: Use napi_gro_receive() for Rx packets

emac_poll_rx() already runs in NAPI context and TAH-equipped EMACs set
CHECKSUM_UNNECESSARY on verified frames, which lets GRO coalesce TCP
segments without a software checksum on the merge path. Replace the
per-poll rx_list batched with netif_receive_skb_list() with direct
napi_gro_receive() calls so the stack can merge segments into super-skbs
and skip a full traversal per packet -- a meaningful win on the slow
4xx-class CPUs this driver targets.

Small routing speed improvement tested on a Cisco Meraki MX60W:

Tested with iperf3

Before:

[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec   494 MBytes   414 Mbits/sec  839             sender
[  5]   0.00-10.04  sec   492 MBytes   411 Mbits/sec                  receiver

After:

[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec   510 MBytes   428 Mbits/sec  580             sender
[  5]   0.00-10.04  sec   508 MBytes   424 Mbits/sec                  receiver

Traffic to and from the router seems to be slow no matter what:

Tested with iperf3 --bidir

Before:

[ ID][Role] Interval           Transfer     Bitrate         Retr
[  8][TX-C]   0.00-10.00  sec   297 MBytes   249 Mbits/sec   35            sender
[  8][TX-C]   0.00-10.00  sec   293 MBytes   245 Mbits/sec                  receiver
[ 10][RX-C]   0.00-10.00  sec   184 MBytes   154 Mbits/sec    0            sender
[ 10][RX-C]   0.00-10.00  sec   184 MBytes   154 Mbits/sec                  receiver

After:

[ ID][Role] Interval           Transfer     Bitrate         Retr
[  8][TX-C]   0.00-10.00  sec   295 MBytes   248 Mbits/sec   31            sender
[  8][TX-C]   0.00-10.00  sec   294 MBytes   246 Mbits/sec                  receiver
[ 10][RX-C]   0.00-10.00  sec   181 MBytes   152 Mbits/sec    0            sender
[ 10][RX-C]   0.00-10.00  sec   181 MBytes   152 Mbits/sec                  receiver

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://patch.msgid.link/20260521215908.257118-1-rosenp@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>