netdev - Re: [PATCH v4 1/3] dsa: marvell: Provide per device information about max frame size

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Message-ID: <Y8Fno+svcnNY4h/8@shell.armlinux.org.uk>
Date:   Fri, 13 Jan 2023 14:16:03 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Lukasz Majewski <lukma@...x.de>
Cc:     Andrew Lunn <andrew@...n.ch>, Vladimir Oltean <olteanv@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/3] dsa: marvell: Provide per device information
 about max frame size

On Fri, Jan 06, 2023 at 11:16:49AM +0100, Lukasz Majewski wrote:
> Different Marvell DSA switches support different size of max frame
> bytes to be sent. This value corresponds to the memory allocated
> in switch to store single frame.
> 
> For example mv88e6185 supports max 1632 bytes, which is now in-driver
> standard value. On the other hand - mv88e6250 supports 2048 bytes.
> To be more interresting - devices supporting jumbo frames - use yet
> another value (10240 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.
> 
> This commit doesn't change the code functionality - it just provides
> the max frame size value explicitly - up till now it has been
> assigned depending on the callback provided by the IC driver
> (e.g. .set_max_frame_size, .port_set_jumbo_size).

I don't think this patch is correct.

One of the things that mv88e6xxx_setup_port() does when initialising
each port is:

        if (chip->info->ops->port_set_jumbo_size) {
                err = chip->info->ops->port_set_jumbo_size(chip, port, 10218);
                if (err)
                        return err;
        }

There is one implementation of this, which is mv88e6165_port_set_jumbo_size()
and that has the effect of setting port register 8 to the largest
size. So any chip that supports the port_set_jumbo_size() method will
be programmed on initialisation to support this larger size.

However, you seem to be listing e.g. the 88e6190 (if I'm interpreting
the horrid mv88e6xxx_table changes correctly) as having a maximum
frame size of 1522, but it implements this method, supports 10240, and
thus is programmed to support frames of that size rather than 1522.

-- 
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