netdev - Re: mvpp2: incorrect max mtu?

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Message-ID: <20210514172138.GG12395@shell.armlinux.org.uk>
Date:   Fri, 14 May 2021 18:21:38 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Andrew Lunn <andrew@...n.ch>
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 04:35:12PM +0200, Andrew Lunn wrote:
> 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?

Oh, there's more values given elsewhere that disagree, see my reply to
Marcin. I would not be surprised if this was all "confused" about
what the proper value is. Certainly the comment you mention above
disagrees with what is in mvpp2.h

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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