netdev - Re: [PATCH net-next] net: dsa: mt7530: enable jumbo frame

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Date:   Fri, 10 Apr 2020 13:46:45 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     DENG Qingfang <dqfext@...il.com>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        René van Dorst <opensource@...rst.com>,
        John Crispin <john@...ozen.org>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Sean Wang <sean.wang@...iatek.com>,
        Weijie Gao <weijie.gao@...iatek.com>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@...ts.infradead.org>
Subject: Re: [PATCH net-next] net: dsa: mt7530: enable jumbo frame

Hi Qingfang,

On Fri, 10 Apr 2020 at 05:51, DENG Qingfang <dqfext@...il.com> wrote:
>
> On Fri, Apr 10, 2020 at 10:27 AM Florian Fainelli <f.fainelli@...il.com> wrote:
> >
> >
> >
> > On 4/9/2020 7:19 PM, DENG Qingfang wrote:
> > > So, since nothing else uses the mt7530_set_jumbo function, should I
> > > remove the function and just add a single rmw to mt7530_setup?
> >
> > (please do not top-post on netdev)
> >
> > There is a proper way to support the MTU configuration for DSA switch
> > drivers which is:
> >
> >         /*
> >          * MTU change functionality. Switches can also adjust their MRU
> > through
> >          * this method. By MTU, one understands the SDU (L2 payload) length.
> >          * If the switch needs to account for the DSA tag on the CPU
> > port, this
> >          * method needs to to do so privately.
> >          */
> >         int     (*port_change_mtu)(struct dsa_switch *ds, int port,
> >                                    int new_mtu);
> >         int     (*port_max_mtu)(struct dsa_switch *ds, int port);
>
> MT7530 does not support configuring jumbo frame per-port
> The register affects globally
>
> >
> > --
> > Florian

This is a bit more tricky, but I think you can still deal with it
using the port_change_mtu functionality. Basically it is only a
problem when the other ports are standalone - otherwise the
dsa_bridge_mtu_normalization function should kick in.
So if you implement port_change_mtu, you should do something along the lines of:

for (i = 0; i < MT7530_NUM_PORTS; i++) {
    struct net_device *slave;

    if (!dsa_is_user_port(ds, i))
        continue;

    slave = ds->ports[i].slave;

    slave->mtu = new_mtu;
}

to update the MTU known by the stack for all net devices.

Hope this helps,
-Vladimir