netdev - Re: mvpp2: incorrect max mtu?

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Date:   Fri, 14 May 2021 16:35:12 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Stefan Chulski <stefanc@...vell.com>,
        Marcin Wojtas <mw@...ihalf.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: mvpp2: incorrect max mtu?

On Fri, May 14, 2021 at 02:00:18PM +0100, Russell King (Oracle) wrote:
> Hi all,
> 
> While testing out the 10G speeds on my Macchiatobin platforms, the first
> thing I notice is that they only manage about 1Gbps at a MTU of 1500.
> As expected, this increases when the MTU is increased - a MTU of 9000
> works, and gives a useful performance boost.
> 
> Then comes the obvious question - what is the maximum MTU.
> 
> #define MVPP2_BM_JUMBO_FRAME_SIZE       10432   /* frame size 9856 */
> 
> So, one may assume that 9856 is the maximum. However:
> 
> # ip li set dev eth0 mtu 9888
> # ip li set dev eth0 mtu 9889
> Error: mtu greater than device maximum.

Hi Russell

It all seems inconsistent:

https://elixir.bootlin.com/linux/latest/source/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c#L6879

	/* MTU range: 68 - 9704 */
	dev->min_mtu = ETH_MIN_MTU;
	/* 9704 == 9728 - 20 and rounding to 8 */
	dev->max_mtu = MVPP2_BM_JUMBO_PKT_SIZE;

Maybe this comment is correct, the code is now wrong, and the MAX MTU
should be 9704?

      Andrew