Re: [PATCH net-next 2/2] net: dsa: Set the master device's MTU to account for DSA overheads

On Thu, Dec 06, 2018 at 12:21:31PM -0800, Florian Fainelli wrote:

On 12/6/18 2:36 AM, Andrew Lunn wrote:

quoted

DSA tagging of frames sent over the master interface to the switch
increases the size of the frame. Such frames can then be bigger than
the normal MTU of the master interface, and it may drop them. Use the
overhead information from the tagger to set the MTU of the master
device to include this overhead.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
---
 net/dsa/master.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/net/dsa/master.c b/net/dsa/master.c
index c90ee3227dea..42f525bc68e2 100644
--- a/net/dsa/master.c
+++ b/net/dsa/master.c

@@ -158,8 +158,24 @@ static void dsa_master_ethtool_teardown(struct net_device *dev)
 	cpu_dp->orig_ethtool_ops = NULL;
 }
 
+void dsa_master_set_mtu(struct net_device *dev, struct dsa_port *cpu_dp)
+{
+	unsigned int mtu = ETH_DATA_LEN + cpu_dp->tag_ops->overhead;
+	int err;
+
+	rtnl_lock();
+	if (mtu <= dev->max_mtu) {
+		err = dev_set_mtu(dev, mtu);
+		if (err)
+			netdev_dbg(dev, "Unable to set MTU to include for DSA overheads\n");
+	}

Would it make sense to warn the user that there might be
transmit/receive issues with the DSA tagging protocol if either
dev_set_mtu() fails or mtu > dev->max_mtu?

I thought about that. But we have setups which work today with the
standard MTU. The master might not implement the set_mtu op, or might
impose the standard MTU, but be quite happy to deal with our DSA
packets. So i wanted to make this a hint to the master device, not a
strong requirement.

Not that I think it matters too much to people because unbinding the
switch driver and expecting the CPU port to continue operating is
wishful thinking, but we should probably unwind that operation in
dsa_master_teardown(), right?

That would make sense.

David has already accepted the patchset, so i will add a followup
patch.

	Andrew