linux-kernel - Re: [PATCH net-next v4] net: sfp: add SMBus I2C block support

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Message-ID: <466efdd2-ffe2-4d2e-b964-decde3d6369b@bootlin.com>
Date: Fri, 9 Jan 2026 16:51:06 +0100
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: Jonas Jelonek <jelonek.jonas@...il.com>,
 Russell King <linux@...linux.org.uk>, 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>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 Bjørn Mork <bjorn@...k.no>
Subject: Re: [PATCH net-next v4] net: sfp: add SMBus I2C block support

Hi,

On 09/01/2026 11:13, Jonas Jelonek wrote:
> Commit 7662abf4db94 ("net: phy: sfp: Add support for SMBus module access")
> added support for SMBus-only controllers for module access. However,
> this is restricted to single-byte accesses and has the implication that
> hwmon is disabled (due to missing atomicity of 16-bit accesses) and
> warnings are printed.
> 
> There are probably a lot of SMBus-only I2C controllers out in the wild
> which support block reads. Right now, they don't work with SFP modules.
> This applies - amongst others - to I2C/SMBus-only controllers in Realtek
> longan and mango SoCs.
> 
> Downstream in OpenWrt, a patch similar to the abovementioned patch is
> used for current LTS kernel 6.12. However, this uses byte-access for all
> kinds of access and thus disregards the atomicity for wider access.
> 
> Introduce read/write SMBus I2C block operations to support SMBus-only
> controllers with appropriate support for block read/write. Those
> operations are used for all accesses if supported, otherwise the
> single-byte operations will be used. With block reads, atomicity for
> 16-bit reads as required by hwmon is preserved and thus, hwmon can be
> used.
> 
> The implementation requires the I2C_FUNC_SMBUS_I2C_BLOCK to be
> supported as it relies on reading a pre-defined amount of bytes.
> This isn't intended by the official SMBus Block Read but supported by
> several I2C controllers/drivers.
> 
> Support for word access is not implemented due to issues regarding
> endianness.

This patch should probably be accompanied with a similar addition to the
mdio-i2c driver. for now, we only support full-featured I2C adapters, or
single-byte smbus, nothing in-between :(

Do you have something like this in the pipe ? not that this blocks this
particular patch, however I think that Russell's suggestion of making
this generic is the way to go.

Maxime