netdev - Re: [PATCH net-next v2 1/1] net: dsa: microchip: ksz9477: implement MTU configuration

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Date:   Thu, 24 Feb 2022 05:59:36 +0100
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Woojung Huh <woojung.huh@...rochip.com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Vivien Didelot <vivien.didelot@...il.com>,
        kernel@...gutronix.de, Jakub Kicinski <kuba@...nel.org>,
        UNGLinuxDriver@...rochip.com,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net-next v2 1/1] net: dsa: microchip: ksz9477: implement
 MTU configuration

On Thu, Feb 24, 2022 at 01:38:33AM +0200, Vladimir Oltean wrote:
> On Wed, Feb 23, 2022 at 09:40:55AM +0100, Oleksij Rempel wrote:
> > This chips supports two ways to configure max MTU size:
> > - by setting SW_LEGAL_PACKET_DISABLE bit: if this bit is 0 allowed packed size
> >   will be between 64 and bytes 1518. If this bit is 1, it will accept
> >   packets up to 2000 bytes.
> > - by setting SW_JUMBO_PACKET bit. If this bit is set, the chip will
> >   ignore SW_LEGAL_PACKET_DISABLE value and use REG_SW_MTU__2 register to
> >   configure MTU size.
> > 
> > Current driver has disabled SW_JUMBO_PACKET bit and activates
> > SW_LEGAL_PACKET_DISABLE. So the switch will pass all packets up to 2000 without
> > any way to configure it.
> > 
> > By providing port_change_mtu we are switch to SW_JUMBO_PACKET way and will
> > be able to configure MTU up to ~9000.
> > 
> > Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
> > ---
> > changes v2:
> > - rename max_mtu to max_frame and new_mtu to frame_size
> > - use max() instead of if(>)
> > ---
> >  drivers/net/dsa/microchip/ksz9477.c     | 40 +++++++++++++++++++++++--
> >  drivers/net/dsa/microchip/ksz9477_reg.h |  4 +++
> >  drivers/net/dsa/microchip/ksz_common.h  |  1 +
> >  3 files changed, 43 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/net/dsa/microchip/ksz9477.c b/drivers/net/dsa/microchip/ksz9477.c
> > index 18ffc8ded7ee..5c5f78cb970e 100644
> > --- a/drivers/net/dsa/microchip/ksz9477.c
> > +++ b/drivers/net/dsa/microchip/ksz9477.c
> > @@ -11,6 +11,7 @@
> >  #include <linux/platform_data/microchip-ksz.h>
> >  #include <linux/phy.h>
> >  #include <linux/if_bridge.h>
> > +#include <linux/if_vlan.h>
> >  #include <net/dsa.h>
> >  #include <net/switchdev.h>
> >  
> > @@ -182,6 +183,33 @@ static void ksz9477_port_cfg32(struct ksz_device *dev, int port, int offset,
> >  			   bits, set ? bits : 0);
> >  }
> >  
> > +static int ksz9477_change_mtu(struct dsa_switch *ds, int port, int mtu)
> > +{
> > +	struct ksz_device *dev = ds->priv;
> > +	u16 frame_size, max_frame = 0;
> > +	int i;
> > +
> > +	frame_size = mtu + ETH_HLEN + VLAN_HLEN + ETH_FCS_LEN;
> 
> 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.

> > +
> > +	if (dsa_is_cpu_port(ds, port))
> > +		frame_size += KSZ9477_INGRESS_TAG_LEN;
> > +
> > +	/* Cache the per-port MTU setting */
> > +	dev->ports[port].max_frame = frame_size;
> > +
> > +	for (i = 0; i < dev->port_cnt; i++)
> > +		max_frame = max(max_frame, dev->ports[i].max_frame);
> > +
> > +	return regmap_update_bits(dev->regmap[1], REG_SW_MTU__2,
> > +				  REG_SW_MTU_MASK, max_frame);
> > +}
> > +
> > +static int ksz9477_max_mtu(struct dsa_switch *ds, int port)
> > +{
> > +	return KSZ9477_MAX_FRAME_SIZE - ETH_HLEN - ETH_FCS_LEN - VLAN_HLEN -
> > +		KSZ9477_INGRESS_TAG_LEN;
> > +}
> > +
> >  static int ksz9477_wait_vlan_ctrl_ready(struct ksz_device *dev)
> >  {
> >  	unsigned int val;
> > @@ -1412,8 +1440,14 @@ static int ksz9477_setup(struct dsa_switch *ds)
> >  	/* Do not work correctly with tail tagging. */
> >  	ksz_cfg(dev, REG_SW_MAC_CTRL_0, SW_CHECK_LENGTH, false);
> >  
> > -	/* accept packet up to 2000bytes */
> > -	ksz_cfg(dev, REG_SW_MAC_CTRL_1, SW_LEGAL_PACKET_DISABLE, true);
> > +	/* Enable REG_SW_MTU__2 reg by setting SW_JUMBO_PACKET */
> > +	ksz_cfg(dev, REG_SW_MAC_CTRL_1, SW_JUMBO_PACKET, true);
> > +
> > +	/* 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@...pel-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@...gutronix.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;
 

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