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

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Date:   Thu, 29 Oct 2020 11:32:36 +0800
From:   DENG Qingfang <dqfext@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     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>,
        Jonathan McDowell <noodles@...th.li>,
        Chris Packham <chris.packham@...iedtelesis.co.nz>
Subject: Re: [PATCH] net: dsa: mt7530: support setting MTU

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:
> > MT7530/7531 has a global RX packet length register, which can be used
> > to set MTU.
> >
> > Signed-off-by: DENG Qingfang <dqfext@...il.com>
> > ---
>
> Reviewed-by: Vladimir Oltean <olteanv@...il.com>
>
> Also, please format your patches with --subject-prefix="PATCH net-next"
> in the future. Jakub installed some patchwork scripts that "guess" the
> tree based on the commit message, but maybe sometimes they might fail:
>
> https://patchwork.ozlabs.org/project/netdev/patch/e5fdcddeda21884a21162e441d1e8a04994f2825.1603837679.git.pavana.sharma@digi.com/
>
> >  drivers/net/dsa/mt7530.c | 36 ++++++++++++++++++++++++++++++++++++
> >  drivers/net/dsa/mt7530.h | 12 ++++++++++++
> >  2 files changed, 48 insertions(+)
> >
> > diff --git a/drivers/net/dsa/mt7530.c b/drivers/net/dsa/mt7530.c
> > index de7692b763d8..7764c66a47c9 100644
> > --- a/drivers/net/dsa/mt7530.c
> > +++ b/drivers/net/dsa/mt7530.c
> > @@ -1021,6 +1021,40 @@ mt7530_port_disable(struct dsa_switch *ds, int port)
> >       mutex_unlock(&priv->reg_mutex);
> >  }
> >
> > +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.

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?

>
> > +     if (!dsa_is_cpu_port(ds, port))
> > +             return 0;
> > +
> > +     /* RX length also includes Ethernet header, MTK tag, and FCS length */
> > +     length = new_mtu + ETH_HLEN + MTK_HDR_LEN + ETH_FCS_LEN;
> > +     if (length <= 1522)
> > +             mt7530_rmw(priv, MT7530_GMACCR, MAX_RX_PKT_LEN_MASK, MAX_RX_PKT_LEN_1522);
> > +     else if (length <= 1536)
> > +             mt7530_rmw(priv, MT7530_GMACCR, MAX_RX_PKT_LEN_MASK, MAX_RX_PKT_LEN_1536);
> > +     else if (length <= 1552)
> > +             mt7530_rmw(priv, MT7530_GMACCR, MAX_RX_PKT_LEN_MASK, MAX_RX_PKT_LEN_1552);
> > +     else
> > +             mt7530_rmw(priv, MT7530_GMACCR, MAX_RX_JUMBO_MASK | MAX_RX_PKT_LEN_MASK,
> > +                     MAX_RX_JUMBO(DIV_ROUND_UP(length, 1024)) | MAX_RX_PKT_LEN_JUMBO);
> > +
> > +     return 0;
> > +}
> > +
> > +static int
> > +mt7530_port_max_mtu(struct dsa_switch *ds, int port)
> > +{
> > +     return MT7530_MAX_MTU;
> > +}
> > +
> >  static void
> >  mt7530_stp_state_set(struct dsa_switch *ds, int port, u8 state)
> >  {
> > @@ -2519,6 +2553,8 @@ static const struct dsa_switch_ops mt7530_switch_ops = {
> >       .get_sset_count         = mt7530_get_sset_count,
> >       .port_enable            = mt7530_port_enable,
> >       .port_disable           = mt7530_port_disable,
> > +     .port_change_mtu        = mt7530_port_change_mtu,
> > +     .port_max_mtu           = mt7530_port_max_mtu,
> >       .port_stp_state_set     = mt7530_stp_state_set,
> >       .port_bridge_join       = mt7530_port_bridge_join,
> >       .port_bridge_leave      = mt7530_port_bridge_leave,