[PATCH net v1 1/2] lan743x: improve performance: fix rx_napi_poll/interrupt ping-pong

[Sven Van Asbroeck]

From: Sven Van Asbroeck <thesven73@xxxxxxxxx>

Even if the rx ring is completely full, and there is more rx data
waiting on the chip, the rx napi poll fn will never run more than
once - it will always immediately bail out and re-enable interrupts.
Which results in ping-pong between napi and interrupt.

This defeats the purpose of napi, and is bad for performance.

Fix by addressing two separate issues:

1. Ensure the rx napi poll fn always updates the rx ring tail
   when returning, even when not re-enabling interrupts.

2. Up to half of elements in a full rx ring are extension
   frames, which do not generate any skbs. Limit the default
   napi weight to the smallest no. of skbs that can be generated
   by a full rx ring.

Tested-by: Sven Van Asbroeck <thesven73@xxxxxxxxx> # lan7430
Signed-off-by: Sven Van Asbroeck <thesven73@xxxxxxxxx>
---

Tree: git://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git # 905b2032fa42

To: Bryan Whitehead <bryan.whitehead@xxxxxxxxxxxxx>
To: Microchip Linux Driver Support <UNGLinuxDriver@xxxxxxxxxxxxx>
To: "David S. Miller" <davem@xxxxxxxxxxxxx>
To: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: Andrew Lunn <andrew@xxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx

 drivers/net/ethernet/microchip/lan743x_main.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/microchip/lan743x_main.c b/drivers/net/ethernet/microchip/lan743x_main.c
index 87b6c59a1e03..ebb5e0bc516b 100644
--- a/drivers/net/ethernet/microchip/lan743x_main.c
+++ b/drivers/net/ethernet/microchip/lan743x_main.c
@@ -2260,10 +2260,11 @@ static int lan743x_rx_napi_poll(struct napi_struct *napi, int weight)
 				  INT_BIT_DMA_RX_(rx->channel_number));
 	}
 
+done:
 	/* update RX_TAIL */
 	lan743x_csr_write(adapter, RX_TAIL(rx->channel_number),
 			  rx_tail_flags | rx->last_tail);
-done:
+
 	return count;
 }
 
@@ -2405,9 +2406,15 @@ static int lan743x_rx_open(struct lan743x_rx *rx)
 	if (ret)
 		goto return_error;
 
+	/* up to half of elements in a full rx ring are
+	 * extension frames. these do not generate skbs.
+	 * to prevent napi/interrupt ping-pong, limit default
+	 * weight to the smallest no. of skbs that can be
+	 * generated by a full rx ring.
+	 */
 	netif_napi_add(adapter->netdev,
 		       &rx->napi, lan743x_rx_napi_poll,
-		       rx->ring_size - 1);
+		       (rx->ring_size - 1) / 2);
 
 	lan743x_csr_write(adapter, DMAC_CMD,
 			  DMAC_CMD_RX_SWR_(rx->channel_number));
-- 
2.17.1