netdev - Re: [PATCH net-next v3 08/47] net: phylink: Support differing link speeds and interface speeds

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Date:   Mon, 18 Jul 2022 17:06:25 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Sean Anderson <sean.anderson@...o.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Madalin Bucur <madalin.bucur@....com>, netdev@...r.kernel.org,
        Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Alexandru Marginean <alexandru.marginean@....com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Vladimir Oltean <olteanv@...il.com>
Subject: Re: [PATCH net-next v3 08/47] net: phylink: Support differing link
 speeds and interface speeds

On Sat, Jul 16, 2022 at 10:06:01PM +0200, Andrew Lunn wrote:
> This seem error prone when new PHY_INTERFACE_MODES are added. I would
> prefer a WARN_ON_ONCE() in the default: so we get to know about such
> problems.
> 
> I'm also wondering if we need a sanity check here. I've seen quite a
> few boards a Fast Ethernet MAC, but a 1G PHY because they are
> cheap. In such cases, the MAC is supposed to call phy_set_max_speed()
> to indicate it can only do 100Mbs. PHY_INTERFACE_MODE_MII but a
> link_speed of 1G is clearly wrong. Are there other cases where we
> could have a link speed faster than what the interface mode allows?

Currently, phylink will deal with that situation - the MAC will report
that it only supports 10/100, and when the PHY is brought up, the
supported/advertisement masks will be restricted to those speeds.

> Bike shedding a bit, but would it be better to use host_side_speed and
> line_side_speed? When you say link_speed, which link are your
> referring to? Since we are talking about the different sides of the
> PHY doing different speeds, the naming does need to be clear.

Yes, we definitely need that clarification.

I am rather worried that we have drivers using ->speed today in their
mac_config and we're redefining what that means in this patch. Also,
the value that we pass to the *_link_up() calls appears to be the
phy <-> (pcs|mac) speed not the media speed. It's also ->speed and
->duplex that we report to the user in the "Link is Up" message,
which will be confusing if it always says 10G despite the media link
being e.g. 100M.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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