linux-kernel - Re: [PATCH RFC net-next] net: phy: marvell: 88e1111: define gigabit features

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Message-ID: <aKnPAamqRIDS-5kP@shell.armlinux.org.uk>
Date: Sat, 23 Aug 2025 15:24:01 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Josua Mayer <josua@...id-run.com>
Cc: 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>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC net-next] net: phy: marvell: 88e1111: define gigabit
 features

On Sat, Aug 23, 2025 at 04:03:12PM +0200, Josua Mayer wrote:
> When connecting RJ45 SFP modules to Linux an ethernet phy is expected -
> and probed on the i2c bus when possible. Once the PHY probed, phylink
> populates the supported link modes for the netdev based on bmsr
> register bits set at the time (see phy_device.c: phy_probe).

No, phy*lib* does this.

> Marvell phy driver probe function only allocates memory, leaving actual
> configuration for config_init callback.
> This means the supported link modes of the netdev depend entirely on the
> power-on status of the phy bmsr register.
> 
> Certain Cisco SFP modules such as GLC-T and GLC-TE have invalid
> configuration at power-on: MII_M1111_HWCFG_MODE_COPPER_1000X_AN
> This means fiber with automatic negotiation to copper. As the module
> exhibits a physical RJ45 connector this configuration is wrong.
> As a consequence after power-on the bmsr does not set bits for 10/100
> modes.
> 
> During config_init marvell phy driver identifies the correct intended
> MII_M1111_HWCFG_MODE_SGMII_NO_CLK which means sgmii with automatic
> negotiation to copper, and configures the phy accordingly.
> 
> At this point the bmsr register correctly indicates support for 10/100
> link modes - however the netedev supported modes bitmask is never
> updated.
> 
> Hence the netdev fails to negotiate or link-up at 10/100
> speeds, limiting to 1000 links only.
> 
> Explicitly define features for 88e1111 phy to ensure that all supported
> modes are available at runtime even when phy power-on configuration was
> invalid.

So we have a PHY which changes what it's capable of depending on its
configuration, which gives us a chicken-and-egg problem when it comes
to working out whether a PHY (on a SFP module) can be used with a
MAC, because it's not clear from reading its abilities what it might
actually be capable of.

So yes, I think this is the right approach for the common case - but
if we really do have a PHY that's using 1000base-X -to- Copper mode,
this change will be wrong.

Reviewed-by: Russell King (Oracle) <rmk+kernel@...linux.org.uk>

Thanks!

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