netdev - Re: [PATCH net-next 3/3] net: phylink: add phylink_sfp_select_interface

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Message-ID: <CAKgT0UdVW6_hewR7zNzMd_h7b_Lm_SHdt72yVhc7cLHcfFxuYQ@mail.gmail.com>
Date: Wed, 9 Jul 2025 08:37:51 -0700
From: Alexander Duyck <alexander.duyck@...il.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Maxime Chevallier <maxime.chevallier@...tlin.com>, Andrew Lunn <andrew@...n.ch>, 
	Heiner Kallweit <hkallweit1@...il.com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org, 
	Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next 3/3] net: phylink: add phylink_sfp_select_interface_speed()

On Wed, Jul 2, 2025 at 12:18 PM Russell King (Oracle)
<linux@...linux.org.uk> wrote:
>
> On Wed, Jul 02, 2025 at 11:07:52AM -0700, Alexander Duyck wrote:
> > On Wed, Jul 2, 2025 at 6:37 AM Russell King (Oracle)
> > <linux@...linux.org.uk> wrote:
> > >
> > > On Wed, Jul 02, 2025 at 03:14:26PM +0200, Maxime Chevallier wrote:
> > > > On Wed, 02 Jul 2025 10:44:34 +0100
> > > > "Russell King (Oracle)" <rmk+kernel@...linux.org.uk> wrote:
> > > >
> > > > > Add phylink_sfp_select_interface_speed() which attempts to select the
> > > > > SFP interface based on the ethtool speed when autoneg is turned off.
> > > > > This allows users to turn off autoneg for SFPs that support multiple
> > > > > interface modes, and have an appropriate interface mode selected.
> > > > >
> > > > > Signed-off-by: Russell King (Oracle) <rmk+kernel@...linux.org.uk>
> > > >
> > > > Reviewed-by: Maxime Chevallier <maxime.chevallier@...tlin.com>
> > > >
> > > > I don't have any hardware to perform relevant tests on this :(
> > >
> > > Me neither, I should've said. I'd like to see a t-b from
> > > Alexander Duyck who originally had the problem before this is
> > > merged.
> >
> > It will probably be several days before I can get around to testing it
> > since I am slammed with meetings most of the next two days, then have
> > a holiday weekend coming up.
>
> I, too, have a vacation - from tomorrow for three weeks. I may dip in
> and out of kernel emails during that period, but it depends what
> happens each day.

So I was able to go in and test it. I ended up just running the
testing in QEMU w/ my patch set that currently enables QSFP support.
>From what I can tell it appears to be mostly working. Before when I
tried to alter the speed to go from 100G to 50G it wouldn't change.
After your patch set it appears to change, although I am noticing a
slight difference from the default config.

So by default we come up in the 100G w/ the QSFP configuration:
[root@...alhost fbnic]# ethtool enp1s0
Settings for enp1s0:
        Supported ports: [  ]
        Supported link modes:   50000baseCR/Full
                                100000baseCR2/Full
        Supported pause frame use: Symmetric Receive-only
        Supports auto-negotiation: No
        Supported FEC modes: RS
        Advertised link modes:  100000baseCR2/Full
        Advertised pause frame use: Symmetric Receive-only
        Advertised auto-negotiation: No
        Advertised FEC modes: RS
        Link partner advertised link modes:  100000baseCR2/Full
        Link partner advertised pause frame use: No
        Link partner advertised auto-negotiation: No
        Link partner advertised FEC modes: RS
        Speed: 100000Mb/s
        Duplex: Full
        Auto-negotiation: off
        Port: Other
        PHYAD: 0
        Transceiver: internal
        Link detected: yes

I then change the speed to 50G and it links back up after a few
seconds, however the "Advertised link modes" goes from
"100000baseCR2/Full" to "Not reported" as shown here:
[root@...alhost fbnic]# ethtool -s enp1s0 speed 50000
[root@...alhost fbnic]# ethtool enp1s0
Settings for enp1s0:
        Supported ports: [  ]
        Supported link modes:   50000baseCR/Full
                                100000baseCR2/Full
        Supported pause frame use: Symmetric Receive-only
        Supports auto-negotiation: No
        Supported FEC modes: RS
        Advertised link modes:  Not reported
        Advertised pause frame use: Symmetric Receive-only
        Advertised auto-negotiation: No
        Advertised FEC modes: RS
        Link partner advertised link modes:  100000baseCR2/Full
        Link partner advertised pause frame use: No
        Link partner advertised auto-negotiation: No
        Link partner advertised FEC modes: RS
        Speed: 50000Mb/s
        Duplex: Full
        Auto-negotiation: off
        Port: Other
        PHYAD: 0
        Transceiver: internal
        Link detected: yes

So all-in-all it is an improvement over the previous behavior although
there may still need to be some work done to improve the consistency
so that it more closely matches up with what happens when you
initially configure the interface.