linux-kernel - Re: [PATCH] net: phy: phy_ethtool_ksettings

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Message-ID: <20241202100334.454599a7@fedora.home>
Date: Mon, 2 Dec 2024 10:03:34 +0100
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: Nikita Yushchenko <nikita.yoush@...entembedded.com>
Cc: Andrew Lunn <andrew@...n.ch>, Heiner Kallweit <hkallweit1@...il.com>,
 Russell King <linux@...linux.org.uk>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
 <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, Michael Dege <michael.dege@...esas.com>,
 Christian Mardmoeller <christian.mardmoeller@...esas.com>, Dennis Ostermann
 <dennis.ostermann@...esas.com>
Subject: Re: [PATCH] net: phy: phy_ethtool_ksettings_set: Allow any
 supported speed

Hello Nikita,

On Mon,  2 Dec 2024 13:33:52 +0500
Nikita Yushchenko <nikita.yoush@...entembedded.com> wrote:

> When auto-negotiation is not used, allow any speed/duplex pair
> supported by the PHY, not only 10/100/1000 half/full.
> 
> This enables drivers to use phy_ethtool_set_link_ksettings() in their
> ethtool_ops and still support configuring PHYs for speeds above 1 GBps.
> 
> Also this will cause an error return on attempt to manually set
> speed/duplex pair that is not supported by the PHY.

There have been attempts to do the same thing before :

https://lore.kernel.org/netdev/1c55b353-ddaf-48f2-985c-5cb67bd5cb0c@lunn.ch/

It seems that 1G and above require autoneg to properly work. The 802.3
spec for 2.5G/5G (126.6.1 Support for Auto-Negotiation) does say :

  All 2.5GBASE-T and 5GBASE-T PHYs shall provide support for
  Auto-Negotiation (Clause 28) and shall be capable of operating as
  MASTER or SLAVE.

[...]

  Auto-Negotiation is performed as part of the initial set-up of the
  link, and allows the PHYs at each end to advertise their capabilities
  (speed, PHY type, half or full duplex) and to automatically
  select the operating   mode for communication on the link.
  Auto-Negotiation signaling is used for the following primary purposes
  for 2.5GBASE-T and 5GBASE-T:

    a) To negotiate that the PHY is capable of supporting
       2.5GBASE-T or 5GBASE-T transmission.

    b) To determine the MASTER-SLAVE relationship between
       the PHYs at each end of the link.

Looking at this it does seem that autoneg should stay enabled when
operating at other speeds than 10/100/1000, at least in BaseT.

What's your use-case to need >1G fixed-settings link ?

Thanks,

Maxime