linux-kernel - Re: [PATCH net-next v5] net: sfp: extend SMBus support

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Message-ID: <5e7c71f6-80dd-408b-a346-888e6febf07a@lunn.ch>
Date: Fri, 16 Jan 2026 15:07:31 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Jonas Jelonek <jelonek.jonas@...il.com>
Cc: Maxime Chevallier <maxime.chevallier@...tlin.com>,
	Russell King <linux@...linux.org.uk>,
	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,
	Bjørn Mork <bjorn@...k.no>
Subject: Re: [PATCH net-next v5] net: sfp: extend SMBus support

On Fri, Jan 16, 2026 at 02:43:47PM +0100, Jonas Jelonek wrote:
> Hi,
> 
> On 16.01.26 14:23, Maxime Chevallier wrote:
> > I think Russell pointed it out, but I was also wondering the same.
> > How do we deal with controllers that cannot do neither block nor
> > single-byte, i.e. that can only do word access ?
> >
> > We can't do transfers that have an odd length. And there are some,
> > see sfp_cotsworks_fixup_check() for example.
> >
> > Maybe these smbus controller don't even exist, but I think we should
> > anyway have some log saying that this doesn't work, either at SFP
> > access time, or at init time.
> 
> I tried to guard that in the sfp_i2c_configure() right now. The whole path
> to allow SMBus transfers is only allowed if there's at least byte access. For
> exactly the reason that we need byte access in case of odd lengths.

Is there a use case for odd lengths? Apart from 1.

> This of course rules out any controllers which just can do word access.

There are some PHYs embedded within SFPs which kill the bus if you do
anything but 1 byte access. There is a quirk for it. We should refuse
to drive the SFP if we have such an SFP and an I2C bus that can only
do words.

	Andrew