[PATCH] net: ibm: emac: Use napi_gro_receive() for Rx packets

[Rosen Penev <rosenp@gmail.com>]

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

Assisted-by: Claude:Opus-4.7
Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
 drivers/net/ethernet/ibm/emac/core.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/ibm/emac/core.c b/drivers/net/ethernet/ibm/emac/core.c
index adac0b55936c..11d673a9f119 100644
--- a/drivers/net/ethernet/ibm/emac/core.c
+++ b/drivers/net/ethernet/ibm/emac/core.c
@@ -1738,7 +1738,6 @@ static inline int emac_rx_sg_append(struct emac_instance *dev, int slot)
 /* NAPI poll context */
 static int emac_poll_rx(void *param, int budget)
 {
-	LIST_HEAD(rx_list);
 	struct emac_instance *dev = param;
 	int slot = dev->rx_slot, received = 0;
 
@@ -1795,7 +1794,7 @@ static int emac_poll_rx(void *param, int budget)
 		skb->protocol = eth_type_trans(skb, dev->ndev);
 		emac_rx_csum(dev, skb, ctrl);
 
-		list_add_tail(&skb->list, &rx_list);
+		napi_gro_receive(&dev->mal->napi, skb);
 	next:
 		++dev->stats.rx_packets;
 	skip:
@@ -1839,8 +1838,6 @@ static int emac_poll_rx(void *param, int budget)
 		goto next;
 	}
 
-	netif_receive_skb_list(&rx_list);
-
 	if (received) {
 		DBG2(dev, "rx %d BDs" NL, received);
 		dev->rx_slot = slot;
-- 
2.54.0