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

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Message-ID: <6ff4a225-07c0-40f6-9509-c4fa79966266@lunn.ch>
Date: Tue, 25 Feb 2025 14:41:04 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Maxime Chevallier <maxime.chevallier@...tlin.com>
Cc: davem@...emloft.net, Jakub Kicinski <kuba@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Russell King <linux@...linux.org.uk>,
	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 12:20:39PM +0100, Maxime Chevallier wrote:
> The SFP module's eeprom and internals are accessible through an i2c bus.
> However, all the i2c transfers that are performed are SMBus-style
> transfers for read and write operations.
> 
> It is possible that the SFP might be connected to an SMBus-only
> controller, such as the one found in some PHY devices in the VSC85xx
> family.
> 
> Introduce a set of sfp read/write ops that are going to be used if the
> i2c bus is only capable of doing smbus byte accesses.
> 
> As Single-byte SMBus transaction go against SFF-8472 and breaks the
> atomicity for diagnostics data access, hwmon is disabled in the case
> of SMBus access.
> 
> Moreover, as this may cause other instabilities, print a warning at
> probe time to indicate that the setup may be unreliable because of the
> hardware design.
> 
> Tested-by: Sean Anderson <sean.anderson@...ux.dev>
> Signed-off-by: Maxime Chevallier <maxime.chevallier@...tlin.com>
> ---
> 
> V2: - Added Sean's tested-by
>     - Added a warning indicating that operations won't be reliable, from
>       Russell and Andrew's reviews
>     - Also added a flag saying we're under a single-byte-access bus, to
>       both print the warning and disable hwmon.
> 
>  drivers/net/phy/sfp.c | 79 +++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 73 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/phy/sfp.c b/drivers/net/phy/sfp.c
> index 9369f5297769..6e9d3d95eb95 100644
> --- a/drivers/net/phy/sfp.c
> +++ b/drivers/net/phy/sfp.c
> @@ -282,6 +282,7 @@ struct sfp {
>  	unsigned int rs_state_mask;
>  
>  	bool have_a2;
> +	bool single_byte_access;
>  
>  	const struct sfp_quirk *quirk;
>  
> @@ -690,14 +691,70 @@ static int sfp_i2c_write(struct sfp *sfp, bool a2, u8 dev_addr, void *buf,
>  	return ret == ARRAY_SIZE(msgs) ? len : 0;
>  }
>  
> -static int sfp_i2c_configure(struct sfp *sfp, struct i2c_adapter *i2c)
> +static int sfp_smbus_read(struct sfp *sfp, bool a2, u8 dev_addr, void *buf,

Maybe call this sfp_smbus_byte_read(), leaving space for
sfp_smbus_word_read() in the future.

> +			  size_t len)
>  {
> -	if (!i2c_check_functionality(i2c, I2C_FUNC_I2C))
> -		return -EINVAL;
> +	u8 bus_addr = a2 ? 0x51 : 0x50;
> +	union i2c_smbus_data smbus_data;
> +	u8 *data = buf;
> +	int ret;
> +
> +	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?


    Andrew

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