netdev - Re: Ethtool query: Reset advertised speed modes if speed value is not passed in "set_link

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Message-ID: <fb8b2333-cde2-4ec4-9382-f3a563954d06@lunn.ch>
Date: Fri, 8 Mar 2024 14:56:30 +0100
From: Andrew Lunn <andrew@...n.ch>
To: "Sagar Dhoot (QUIC)" <quic_sdhoot@...cinc.com>
Cc: "mkubecek@...e.cz" <mkubecek@...e.cz>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"Nagarjuna Chaganti (QUIC)" <quic_nchagant@...cinc.com>,
	"Priya Tripathi (QUIC)" <quic_ppriyatr@...cinc.com>
Subject: Re: Ethtool query: Reset advertised speed modes if speed value is
 not passed in "set_link_ksettings"

On Fri, Mar 08, 2024 at 06:33:00AM +0000, Sagar Dhoot (QUIC) wrote:
> Hi Andrew,
> 
> Thanks for the quick response. Maybe I have put up a confusing scenario.
> 
> Let me rephrase with autoneg on.
> 
> 1. "ethtool eth_interface"
> 2. "ethtool -s eth_interface speed 25000 autoneg on"

phylib will ignore speed if autoneg is on

https://elixir.bootlin.com/linux/latest/source/drivers/net/phy/phy.c#L1044

It only copies speed/duplex when autoneg is off.

	if (autoneg == AUTONEG_DISABLE) {
		phydev->speed = speed;
		phydev->duplex = duplex;
	}

When configuring the PHY:

https://elixir.bootlin.com/linux/latest/source/drivers/net/phy/phy_device.c#L2234

	if (AUTONEG_ENABLE != phydev->autoneg)
		return genphy_setup_forced(phydev);

So we only force the PHY when autoneg is off, and it is
genphy_setup_forced() which uses this speed/duplex.

If you are trying to influence the link mode while autoneg is
enabled, you should be using advertise N

          advertise N
                  Sets the speed and  duplex  advertised  by  autonegotiation.
                  The  argument is a hexadecimal value using one or a combina‐
                  tion of the following values:
                  0x001                          10baseT Half
                  0x002                          10baseT Full
                  0x100000000000000000000000     10baseT1L Full
                  0x8000000000000000000000000    10baseT1S Full
                  0x10000000000000000000000000   10baseT1S Half
                  0x20000000000000000000000000   10baseT1S_P2MP Half
                  0x004                          100baseT Half
                  0x008                          100baseT Full

I think the hexadecimal part is out of date and you can now also use
symbolic names.

Another thing to think about. What does the speed here actually mean:

> 2. "ethtool -s eth_interface speed 25000 autoneg on"

phylib assumes it is the baseT speed:

        ETHTOOL_LINK_MODE_10baseT_Half_BIT      = 0,
        ETHTOOL_LINK_MODE_10baseT_Full_BIT      = 1,
        ETHTOOL_LINK_MODE_100baseT_Half_BIT     = 2,
        ETHTOOL_LINK_MODE_100baseT_Full_BIT     = 3,
        ETHTOOL_LINK_MODE_1000baseT_Half_BIT    = 4,
        ETHTOOL_LINK_MODE_1000baseT_Full_BIT    = 5,

However, what does 25000 mean? We currently don't have a
25000BaseT. So how do you decide which of the following to select:

        ETHTOOL_LINK_MODE_25000baseCR_Full_BIT  = 31,
        ETHTOOL_LINK_MODE_25000baseKR_Full_BIT  = 32,
        ETHTOOL_LINK_MODE_25000baseSR_Full_BIT  = 33,

This is where advertise N is much better, an actual linkmode, not just
a speed.

	 Andrew