netdev - Re: [PATCH] net: dsa: mt7530: support setting MTU

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Message-ID: <CALW65jZYeWdK--aNZiTo0cZTqB2WwJa0r1cP-=Tw37Y6n6ra4g@mail.gmail.com>
Date:   Thu, 29 Oct 2020 16:19:50 +0800
From:   DENG Qingfang <dqfext@...il.com>
To:     Jonathan McDowell <noodles@...th.li>
Cc:     Vladimir Oltean <olteanv@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Sean Wang <sean.wang@...iatek.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        René van Dorst <opensource@...rst.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Murali Krishna Policharla <murali.policharla@...adcom.com>,
        Chris Packham <chris.packham@...iedtelesis.co.nz>
Subject: Re: [PATCH] net: dsa: mt7530: support setting MTU

On Thu, Oct 29, 2020 at 4:11 PM Jonathan McDowell <noodles@...th.li> wrote:
>
> On Thu, Oct 29, 2020 at 11:32:36AM +0800, DENG Qingfang wrote:
> > On Thu, Oct 29, 2020 at 2:31 AM Vladimir Oltean <olteanv@...il.com> wrote:
> > >
> > > On Thu, Oct 29, 2020 at 02:12:21AM +0800, DENG Qingfang wrote:
> ...
> > > > +static int
> > > > +mt7530_port_change_mtu(struct dsa_switch *ds, int port, int new_mtu)
> > > > +{
> > > > +     struct mt7530_priv *priv = ds->priv;
> > > > +     int length;
> > > > +
> > > > +     /* When a new MTU is set, DSA always set the CPU port's MTU to the largest MTU
> > > > +      * of the slave ports. Because the switch only has a global RX length register,
> > > > +      * only allowing CPU port here is enough.
> > > > +      */
> > >
> > > Good point, please tell that to Linus (cc) - I'm talking about
> > > e0b2e0d8e669 ("net: dsa: rtl8366rb: Roof MTU for switch"),
> >
> > And 6ae5834b983a ("net: dsa: b53: add MTU configuration support"),
> > 1baf0fac10fb ("net: dsa: mv88e6xxx: Use chip-wide max frame size for MTU"),
> > f58d2598cf70 ("net: dsa: qca8k: implement the port MTU callbacks")
> >
> > CC'd them as well.
>
> qca8k tracks and use the maximum provided mtu; perhaps that could be
> optimised by only allow the CPU port to be set but it feels a bit more
> future proof (e.g. if/when we support multiple CPU ports).

btw, there is a bug in your commit f58d2598cf70, in
qca8k_port_change_mtu the loop variable is not used inside the for
loop.

>
> > Also, the commit e0b2e0d8e669 states that the new_mtu parameter is L2
> > frame length instead of L2 payload. But according to my tests, it is
> > L2 payload (i.e. the same as the MTU shown in `ip link` or `ifconfig`.
> > Is that right?
>
> Certainly that's what I saw; qca8k sets the MTU to the provided MTU +
> ETH_HLEN + ETH_FCS_LEN.
>
> J.
>
> --
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