netdev - [PATCH net-next] net: ixp4xx_eth: Specify min/max MTU

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Message-Id: <20230923-ixp4xx-eth-mtu-v1-1-9e88b908e1b2@linaro.org>
Date: Sat, 23 Sep 2023 20:38:22 +0200
From: Linus Walleij <linus.walleij@...aro.org>
To: Krzysztof Halasa <khalasa@...p.pl>, 
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Linus Walleij <linus.walleij@...aro.org>
Subject: [PATCH net-next] net: ixp4xx_eth: Specify min/max MTU

As we don't specify the MTU in the driver, the framework
will fall back to 1500 bytes and this doesn't work very
well when we try to attach a DSA switch:

  eth1: mtu greater than device maximum
  ixp4xx_eth c800a000.ethernet eth1: error -22 setting
  MTU to 1504 to include DSA overhead

I checked the developer docs and the hardware can actually
do really big frames, so update the driver accordingly.

Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
---
 drivers/net/ethernet/xscale/ixp4xx_eth.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/net/ethernet/xscale/ixp4xx_eth.c b/drivers/net/ethernet/xscale/ixp4xx_eth.c
index 3b0c5f177447..8f40287c8d58 100644
--- a/drivers/net/ethernet/xscale/ixp4xx_eth.c
+++ b/drivers/net/ethernet/xscale/ixp4xx_eth.c
@@ -24,6 +24,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/dmapool.h>
 #include <linux/etherdevice.h>
+#include <linux/if_vlan.h>
 #include <linux/io.h>
 #include <linux/kernel.h>
 #include <linux/net_tstamp.h>
@@ -1488,6 +1489,13 @@ static int ixp4xx_eth_probe(struct platform_device *pdev)
 	ndev->dev.dma_mask = dev->dma_mask;
 	ndev->dev.coherent_dma_mask = dev->coherent_dma_mask;
 
+	/* Maximum frame size is 16320 bytes and includes VLAN and
+	 * ethernet headers. See "IXP400 Software Programmer's Guide"
+	 * section 10.3.2, page 161.
+	 */
+	ndev->min_mtu = ETH_MIN_MTU;
+	ndev->max_mtu = 16320 - VLAN_ETH_HLEN;
+
 	netif_napi_add_weight(ndev, &port->napi, eth_poll, NAPI_WEIGHT);
 
 	if (!(port->npe = npe_request(NPE_ID(port->id))))

---
base-commit: 0bb80ecc33a8fb5a682236443c1e740d5c917d1d
change-id: 20230923-ixp4xx-eth-mtu-c041d7efe932

Best regards,
-- 
Linus Walleij <linus.walleij@...aro.org>