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

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Date:   Fri, 10 Apr 2020 10:49:15 +0800
From:   DENG Qingfang <dqfext@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     David Miller <davem@...emloft.net>, 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>,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH net-next] net: dsa: mt7530: enable jumbo frame

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

>
> >
> > On Fri, Apr 10, 2020 at 1:20 AM David Miller <davem@...emloft.net> wrote:
> >>
> >> From: DENG Qingfang <dqfext@...il.com>
> >> Date: Thu,  9 Apr 2020 23:54:09 +0800
> >>
> >>> +static void
> >>> +mt7530_set_jumbo(struct mt7530_priv *priv, u8 kilobytes)
> >>> +{
> >>> +     if (kilobytes > 15)
> >>> +             kilobytes = 15;
> >>  ...
> >>> +     /* Enable jumbo frame up to 15 KB */
> >>> +     mt7530_set_jumbo(priv, 15);
> >>
> >> You've made the test quite pointless, honestly.
>
> --
> Florian