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Date:   Thu, 24 Feb 2022 10:38:27 +0100
From:   Oleksij Rempel <o.rempel@pengutronix.de>
To:     Vladimir Oltean <olteanv@gmail.com>
Cc:     Woojung Huh <woojung.huh@microchip.com>,
        Andrew Lunn <andrew@lunn.ch>,
        Florian Fainelli <f.fainelli@gmail.com>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Vivien Didelot <vivien.didelot@gmail.com>,
        kernel@pengutronix.de, Jakub Kicinski <kuba@kernel.org>,
        UNGLinuxDriver@microchip.com,
        "David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH net-next v2 1/1] net: dsa: microchip: ksz9477: implement
 MTU configuration
Message-ID: <20220224093827.GC4594@pengutronix.de>
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On Thu, Feb 24, 2022 at 11:33:29AM +0200, Vladimir Oltean wrote:
> On Thu, Feb 24, 2022 at 05:59:36AM +0100, Oleksij Rempel wrote:
> > > Are you sure the unit of measurement is ok? My KSZ9477 documentation
> > > says this about register 0x0308:
> > > 
> > > Maximum Frame Length (MTU)
> > > Specifies the maximum transmission unit (MTU), which is the maximum
> > > frame payload size. Frames which exceed this maximum are truncated. This
> > > value can be set as high as 9000 (= 0x2328) if jumbo frame support is
> > > required.
> > > 
> > > "frame payload" to me means what MTU should mean. And ETH_HLEN +
> > > VLAN_HLEN + ETH_FCS_LEN isn't part of that meaning.
> > 
> > if I set this value to anything less then 1522, it breaks the NFS boot. Since
> > my NFS server is configured with MTU 1500, i tried to guess how
> > frame size is calculated on this chip.
> 
> Sad that Microchip engineers can't decide on whether the MTU register
> holds the "Maximum Frame Length" or the "maximum frame payload size".
> They said both to have themselves covered, you understand what you will,
> of course both are not right :)

¯\_(ツ)_/¯

> > > > +	/* Now we can configure default MTU value */
> > > > +	ret = regmap_update_bits(dev->regmap[1], REG_SW_MTU__2, REG_SW_MTU_MASK,
> > > > +				 VLAN_ETH_FRAME_LEN + ETH_FCS_LEN);
> > > 
> > > Why do you need this? Doesn't DSA call dsa_slave_create() ->
> > > dsa_slave_change_mtu(ETH_DATA_LEN) on probe?
> > 
> > This was my initial assumption as well, but cadence macb driver provides
> > buggy max MTU == -18. I hardcoded bigger MTU for now[1], but was not able to
> > find proper way to fix it. To avoid this kinds of regressions I decided
> > to keep some sane default configuration.
> > 
> > [1] - my workaround.
> > commit 5f8385e9641a383478a65f96ccee8fd992201f68
> > Author: Oleksij Rempel <linux@rempel-privat.de>
> > Date:   Mon Feb 14 14:41:06 2022 +0100
> > 
> >     WIP: net: macb: fix max mtu size
> >     
> >     The gem_readl(bp, JML) will return 0, so we get max_mtu size of -18,
> >     this is breaking MTU configuration for DSA
> >     
> >     Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
> > 
> > diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
> > index a363da928e8b..454d811991bb 100644
> > --- a/drivers/net/ethernet/cadence/macb_main.c
> > +++ b/drivers/net/ethernet/cadence/macb_main.c
> > @@ -4727,7 +4727,7 @@ static int macb_probe(struct platform_device *pdev)
> >  	/* MTU range: 68 - 1500 or 10240 */
> >  	dev->min_mtu = GEM_MTU_MIN_SIZE;
> >  	if (bp->caps & MACB_CAPS_JUMBO)
> > -		dev->max_mtu = gem_readl(bp, JML) - ETH_HLEN - ETH_FCS_LEN;
> > +		dev->max_mtu = 10240 - ETH_HLEN - ETH_FCS_LEN;
> >  	else
> >  		dev->max_mtu = ETH_DATA_LEN;
> 
> Yes, but the macb driver can be a DSA master for any switch, not just
> for ksz9477. Better to fix this differently.

Yes, it should be fixed. I just need some time to understand the proper
way to do so. For now, let's proceed with the ksz patch. Should I send
new version with some changes?

Regards,
Oleksij
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