netdev - Re: [RFC] increase MDIO i2c poll timeout gradually (including patch)

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Message-ID: <cbc4a620-36d3-409b-a248-a2b4add0016a@lunn.ch>
Date: Fri, 19 Sep 2025 19:04:45 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Janpieter Sollie <janpieter.sollie@...elmail.de>
Cc: netdev@...r.kernel.org, Russell King <rmk+kernel@...linux.org.uk>,
	Heiner Kallweit <hkallweit1@...il.com>
Subject: Re: [RFC] increase MDIO i2c poll timeout gradually (including patch)

On Fri, Sep 19, 2025 at 03:52:55PM +0200, Janpieter Sollie wrote:
> Hello everyone,

Please ensure you Cc: the correct Maintainers.

./scripts/get_maintainer.pl drivers/net/phy/sfp.c 
Russell King <linux@...linux.org.uk> (maintainer:SFF/SFP/SFP+ MODULE SUPPORT)
Andrew Lunn <andrew@...n.ch> (maintainer:ETHERNET PHY LIBRARY)
Heiner Kallweit <hkallweit1@...il.com> (maintainer:ETHERNET PHY LIBRARY)
"David S. Miller" <davem@...emloft.net> (maintainer:NETWORKING DRIVERS)
Eric Dumazet <edumazet@...gle.com> (maintainer:NETWORKING DRIVERS)
Jakub Kicinski <kuba@...nel.org> (maintainer:NETWORKING DRIVERS)
Paolo Abeni <pabeni@...hat.com> (maintainer:NETWORKING DRIVERS)
netdev@...r.kernel.org (open list:SFF/SFP/SFP+ MODULE SUPPORT)
linux-kernel@...r.kernel.org (open list)

> I tested a SFP module where the i2c bus is "unstable" at best.

Please tell us more about the hardware.

Also, what speed do you have the I2C bus running at? Have you tried
different clock-frequency values to slow down the I2C bus? Have you
checked the pull-up resistors? I2C problems are sometimes due to too
strong pull-ups.

> A good question may be: is this approach sufficient to close the gap between
> "high performance" equipment having a stable i2c bus and they do not want to wait,
> and embedded equipment (the device I tested on was a BPI-R4) where every milliwatt counts?

Does your board actually confirm to the standards? I2C busses should
be able to run at 100KHz, as defined by the standard. Also, the SFP
standards define modules should work at 100KHz. And counting every
milliwatt makes no sense when you are supposed to be able to deliver
3.3V at 300mA, i.e. 1 Watt, to the module.

> Should this be fixed at another point in the initialization process (eg: not
> probing ridiculously all phy ids)?

Unfortunately, MDIO over I2C is not standardised. So we have no idea
what address the PHY will be using, so we need to look at them all. If
you have an SFF, not an SFP, it might be possible to do some
optimisation.

	Andrew