Re: [PATCH net-next v2 3/3] net: phy: marvell10g: add SFP+ support

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

On Sat, Nov 16, 2019 at 05:06:35PM +0100, Andrew Lunn wrote:
> > +static int mv3310_sfp_insert(void *upstream, const struct sfp_eeprom_id *id)
> > +{
> > +	struct phy_device *phydev = upstream;
> > +	__ETHTOOL_DECLARE_LINK_MODE_MASK(support) = { 0, };
> > +	phy_interface_t iface;
> > +
> > +	sfp_parse_support(phydev->sfp_bus, id, support);
> > +	iface = sfp_select_interface(phydev->sfp_bus, id, support);
> > +
> > +	if (iface != PHY_INTERFACE_MODE_10GKR) {
> > +		dev_err(&phydev->mdio.dev, "incompatible SFP module inserted\n");
> > +		return -EINVAL;
> > +	}
> 
> Hi Russell
> 
> Is it possible to put an SFP module into an SFP+ cage?
> sfp_select_interface() would then say 1000Base-X or 2500Base-X. The
> SFP+ cage has a single SERDES pair, so electrically, would it be
> possible to do 1000Base-X? Should mv3310_sfp_insert() be reconfiguring
> the PHY so the SFP side swaps to 1000Base-X?

The answer is... it depends.

Some SFP+ cages have stronger pull-ups and run the I2C bus at 400kHz.
SFP modules limit the pullups to 4.7k minimum and a bus speed of
100kHz.

Some SFP modules will stand the faster bus speed and the stronger
pull-ups.  Others will not.  Others may end up with EEPROM corruption.

It is possible to reconfigure the 3310 to 1000base-X (I don't think
2500base-X is possible on the fiber port) but it requires a PHY reset.
I have code to do it, but I haven't tested it - as the pullups on the
board I have to test with are too strong to allow the EEPROM in SFP
modules to be read.

If I get around to replacing the resistor packs, then I can test it,
but I'm not going to contribute completely untested code!

So, this patch reflects what can be done with the SFP+ slots on
Macchiatobin boards today.

-- 
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