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Date:   Tue, 22 Mar 2022 15:03:36 +0100
From:   Ansuel Smith <>
To:     Vladimir Oltean <>
Cc:     Andrew Lunn <>,
        Vivien Didelot <>,
        Florian Fainelli <>,
        "David S. Miller" <>,
        Jakub Kicinski <>,
        Paolo Abeni <>,,
Subject: Re: [net-next PATCH 1/4] drivers: net: dsa: qca8k: drop MTU tracking
 from qca8k_priv

On Tue, Mar 22, 2022 at 03:55:35PM +0200, Vladimir Oltean wrote:
> On Tue, Mar 22, 2022 at 02:38:08PM +0100, Ansuel Smith wrote:
> > On Tue, Mar 22, 2022 at 01:58:12PM +0200, Vladimir Oltean wrote:
> > > On Tue, Mar 22, 2022 at 02:45:03AM +0100, Ansuel Smith wrote:
> > > > Drop the MTU array from qca8k_priv and use slave net dev to get the max
> > > > MTU across all user port. CPU port can be skipped as DSA already make
> > > > sure CPU port are set to the max MTU across all ports.
> > > > 
> > > > Signed-off-by: Ansuel Smith <>
> > > > ---
> > > 
> > > I hardly find this to be an improvement and I would rather not see such
> > > unjustified complexity in a device driver. What are the concrete
> > > benefits, size wise?
> > >
> > 
> > The main idea here is, if the value is already present and accessible,
> > why should we duplicate it? Tracking the MTU in this custom way already
> > caused some bugs (check the comment i'm removing). We both use standard
> > way to track ports MTU and we save some additional space. At the cost of
> > 2 additional checks are are not that much of a problem.
> Where is the bug?

There was a bug where we tracked the MTU with the FCS and L2 added and
then in the change_mtu code we added another time the FCS and L2 header
just because we used this custom way and nobody notice that we were adding
2 times the same headers. (it's now fixed but still it's a reason why
using standard way to track MTU would have prevented that)

> > Also from this I discovered that (at least on ipq806x that use stmmac)
> > when master needs to change MTU, stmmac complains that the interface is
> > up and it must be put down. Wonder if that's common across other drivers
> > or it's only specific to stmmac.
> I never had the pleasure of dealing with such DSA masters. I wonder why
> can't stmmac_change_mtu() check if netif_running(), call dev_close and
> set a bool, and at the end, if the bool was set, call dev_open back?

Oh ok so it's not standard that stmmac_change_mtu() just refuse to
change the MTU instead of put the interface down, change MTU and reopen
it... Fun stuff...

>From system side to change MTU to a new value (so lower MTU on any port
or set MTU to a higher value for one pot) I have to:
1. ifconfig eth0 down
2. ifconfig lan1 mtu 1600 up
3. ifconfig eth up

If I just ifconfig lan1 mtu 1600 up it's just rejected with stmmac


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