netdev - Re: [PATCH v2 1/3] dsa: marvell: Provide per device information about max frame size

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Date:   Fri, 16 Dec 2022 09:24:35 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Lukasz Majewski <lukma@...x.de>
Cc:     Andrew Lunn <andrew@...n.ch>, Vladimir Oltean <olteanv@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Russell King <linux@...linux.org.uk>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] dsa: marvell: Provide per device information about
 max frame size

On Fri, Dec 16, 2022 at 5:05 AM Lukasz Majewski <lukma@...x.de> wrote:
>
> Hi Alexander,
>
> > On Thu, 2022-12-15 at 15:45 +0100, Lukasz Majewski wrote:
> > > Different Marvell DSA switches support different size of max frame
> > > bytes to be sent.
> > >
> > > For example mv88e6185 supports max 1632 bytes, which is now
> > > in-driver standard value. On the other hand - mv88e6250 supports
> > > 2048 bytes.
> > >
> > > As this value is internal and may be different for each switch IC,
> > > new entry in struct mv88e6xxx_info has been added to store it.
> > >
> > > Signed-off-by: Lukasz Majewski <lukma@...x.de>
> > > ---
> > > Changes for v2:
> > > - Define max_frame_size with default value of 1632 bytes,
> > > - Set proper value for the mv88e6250 switch SoC (linkstreet) family
> > > ---
> > >  drivers/net/dsa/mv88e6xxx/chip.c | 13 ++++++++++++-
> > >  drivers/net/dsa/mv88e6xxx/chip.h |  1 +
> > >  2 files changed, 13 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/net/dsa/mv88e6xxx/chip.c
> > > b/drivers/net/dsa/mv88e6xxx/chip.c index 2ca3cbba5764..7ae4c389ce50
> > > 100644 --- a/drivers/net/dsa/mv88e6xxx/chip.c
> > > +++ b/drivers/net/dsa/mv88e6xxx/chip.c
> > > @@ -3093,7 +3093,9 @@ static int mv88e6xxx_get_max_mtu(struct
> > > dsa_switch *ds, int port) if (chip->info->ops->port_set_jumbo_size)
> > >             return 10240 - VLAN_ETH_HLEN - EDSA_HLEN -
> > > ETH_FCS_LEN; else if (chip->info->ops->set_max_frame_size)
> > > -           return 1632 - VLAN_ETH_HLEN - EDSA_HLEN -
> > > ETH_FCS_LEN;
> > > +           return (chip->info->max_frame_size  - VLAN_ETH_HLEN
> > > +                   - EDSA_HLEN - ETH_FCS_LEN);
> > > +
> > >     return 1522 - VLAN_ETH_HLEN - EDSA_HLEN - ETH_FCS_LEN;
> > >  }
> > >
> > >
> >
> > Is there any specific reason for triggering this based on the
> > existance of the function call?
>
> This was the original code in this driver.
>
> This value (1632 or 2048 bytes) is SoC (family) specific.
>
> By checking which device defines set_max_frame_size callback, I could
> fill the chip->info->max_frame_size with 1632 value.
>
> > Why not just replace:
> >       else if (chip->info->ops->set_max_frame_size)
> > with:
> >       else if (chip->info->max_frame_size)
> >
>
> I think that the callback check is a bit "defensive" approach -> 1522B
> is the default value and 1632 (or 10240 - jumbo) can be set only when
> proper callback is defined.
>
> > Otherwise my concern is one gets defined without the other leading to
> > a future issue as 0 - extra headers will likely wrap and while the
> > return value may be a signed int, it is usually stored in an unsigned
> > int so it would effectively uncap the MTU.
>
> Please correct me if I misunderstood something:
>
> The problem is with new mv88eXXXX devices, which will not provide
> max_frame_size information to their chip->info struct?
>
> Or is there any other issue?

That was mostly my concern. I was adding a bit of my own defensive
programming in the event that somebody forgot to fill out the
chip->info. If nothing else it might make sense to add a check to
verify that the max_frame_size is populated before blindly using it.
So perhaps you could do something similar to the max_t approach I had
called out earlier but instead of applying it on the last case you
could apply it for the "set_max_frame_size" case with 1632 being the
minimum and being overwritten by 2048 if it is set in max_frame_size.