linux-kernel - Re: [PATCH net-next v2 1/2] net: phy: sfp: Add support for SMBus module access

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Message-ID: <Z74GlXKiExICQ6Rd@shell.armlinux.org.uk>
Date: Tue, 25 Feb 2025 18:06:13 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Maxime Chevallier <maxime.chevallier@...tlin.com>
Cc: Andrew Lunn <andrew@...n.ch>, davem@...emloft.net,
	Jakub Kicinski <kuba@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Heiner Kallweit <hkallweit1@...il.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, thomas.petazzoni@...tlin.com,
	Florian Fainelli <f.fainelli@...il.com>,
	Köry Maincent <kory.maincent@...tlin.com>,
	Simon Horman <horms@...nel.org>,
	Romain Gantois <romain.gantois@...tlin.com>,
	Antoine Tenart <atenart@...nel.org>,
	Marek Behún <kabel@...nel.org>,
	Sean Anderson <sean.anderson@...ux.dev>,
	Bjørn Mork <bjorn@...k.no>
Subject: Re: [PATCH net-next v2 1/2] net: phy: sfp: Add support for SMBus
 module access

On Tue, Feb 25, 2025 at 02:56:17PM +0100, Maxime Chevallier wrote:
> > > +	while (len) {
> > > +		ret = i2c_smbus_xfer(sfp->i2c, bus_addr, 0,
> > > +				     I2C_SMBUS_READ, dev_addr,
> > > +				     I2C_SMBUS_BYTE_DATA, &smbus_data);
> > > +		if (ret < 0)
> > > +			return ret;  
> > 
> > Isn't this the wrong order? You should do the upper byte first, then
> > the lower?
> 
> You might be correct. As I have been running that code out-of-tree for
> a while, I was thinking that surely I'd have noticed if this was
> wrong, however there are only a few cases where we actually write to
> SFP :
> 
>  - sfp_modify_u8(...) => one-byte write
>  - in sfp_cotsworks_fixup_check(...) there are 2 writes : one 1-byte
> write and a 3-bytes write.
> 
> As I don't have any cotsworks SFP, then it looks like having the writes
> mis-ordered would have stayed un-noticed on my side as I only
> stressed the 1 byte write path...

This Cotsworks module is not a SFP. It's a solder-on SFF module.

-- 
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