[PATCH net v1 2/2] lan743x: boost performance: limit PCIe bandwidth requirement

[Sven Van Asbroeck <thesven73@gmail.com>]

From: Sven Van Asbroeck <thesven73@gmail.com>

To support jumbo frames, each rx ring dma buffer is 9K in size.
But the chip only stores a single frame per dma buffer.

When the chip is working with the default 1500 byte MTU, a 9K
dma buffer goes from chip -> cpu per 1500 byte frame. This means
that to get 1G/s ethernet bandwidth, we need 6G/s PCIe bandwidth !

Fix by limiting the rx ring dma buffer size to the current MTU
size.

Tested with iperf3 on a freescale imx6 + lan7430, both sides
set to mtu 1500 bytes.

Before:
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-20.00  sec   483 MBytes   203 Mbits/sec    0
After:
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-20.00  sec  1.15 GBytes   496 Mbits/sec    0

And with both sides set to MTU 9000 bytes:
Before:
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-20.00  sec  1.87 GBytes   803 Mbits/sec   27
After:
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-20.00  sec  1.98 GBytes   849 Mbits/sec    0

Tested-by: Sven Van Asbroeck <thesven73@gmail.com> # lan7430
Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>
---

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

To: Bryan Whitehead <bryan.whitehead@microchip.com>
To: Microchip Linux Driver Support <UNGLinuxDriver@microchip.com>
To: "David S. Miller" <davem@davemloft.net>
To: Jakub Kicinski <kuba@kernel.org>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

 drivers/net/ethernet/microchip/lan743x_main.c | 21 ++++++++++++-------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/drivers/net/ethernet/microchip/lan743x_main.c b/drivers/net/ethernet/microchip/lan743x_main.c
index ebb5e0bc516b..2bded1c46784 100644
--- a/drivers/net/ethernet/microchip/lan743x_main.c
+++ b/drivers/net/ethernet/microchip/lan743x_main.c
@@ -1957,11 +1957,11 @@ static int lan743x_rx_next_index(struct lan743x_rx *rx, int index)
 
 static struct sk_buff *lan743x_rx_allocate_skb(struct lan743x_rx *rx)
 {
-	int length = 0;
+	struct net_device *netdev = rx->adapter->netdev;
 
-	length = (LAN743X_MAX_FRAME_SIZE + ETH_HLEN + 4 + RX_HEAD_PADDING);
-	return __netdev_alloc_skb(rx->adapter->netdev,
-				  length, GFP_ATOMIC | GFP_DMA);
+	return __netdev_alloc_skb(netdev,
+				  netdev->mtu + ETH_HLEN + 4 + RX_HEAD_PADDING,
+				  GFP_ATOMIC | GFP_DMA);
 }
 
 static int lan743x_rx_init_ring_element(struct lan743x_rx *rx, int index,
@@ -1969,9 +1969,10 @@ static int lan743x_rx_init_ring_element(struct lan743x_rx *rx, int index,
 {
 	struct lan743x_rx_buffer_info *buffer_info;
 	struct lan743x_rx_descriptor *descriptor;
-	int length = 0;
+	struct net_device *netdev = rx->adapter->netdev;
+	int length;
 
-	length = (LAN743X_MAX_FRAME_SIZE + ETH_HLEN + 4 + RX_HEAD_PADDING);
+	length = netdev->mtu + ETH_HLEN + 4 + RX_HEAD_PADDING;
 	descriptor = &rx->ring_cpu_ptr[index];
 	buffer_info = &rx->buffer_info[index];
 	buffer_info->skb = skb;
@@ -2157,8 +2158,8 @@ static int lan743x_rx_process_packet(struct lan743x_rx *rx)
 			int index = first_index;
 
 			/* multi buffer packet not supported */
-			/* this should not happen since
-			 * buffers are allocated to be at least jumbo size
+			/* this should not happen since buffers are allocated
+			 * to be at least the mtu size configured in the mac.
 			 */
 
 			/* clean up buffers */
@@ -2632,9 +2633,13 @@ static int lan743x_netdev_change_mtu(struct net_device *netdev, int new_mtu)
 	struct lan743x_adapter *adapter = netdev_priv(netdev);
 	int ret = 0;
 
+	if (netif_running(netdev))
+		return -EBUSY;
+
 	ret = lan743x_mac_set_mtu(adapter, new_mtu);
 	if (!ret)
 		netdev->mtu = new_mtu;
+
 	return ret;
 }
 
-- 
2.17.1