[PATCH RFC net-next] net: phy: sfp: drop 1000Base-T support for FCLF8521P2BTL

[Michael Walle <mwalle@kernel.org>]

The FCLF8521P2BTL is marketed as a drop in replacement for fiber
modules using 1000Base-X autoneg towards the host as default. See the
referenced application note, esp. question #11. Drop the 1000baseT
capability, so 1000Base-X will eventually be used.

This is esp. important if the TX_DISABLE pin is not connected on a
board. Usually, pin is used as a reset line to the PHY on the copper
SFP. If a bootloader expects the default mode and doesn't do any
reconfiguration of the SFP module, a link might not be established.

Link: https://www.coherent.com/resources/application-note/networking/1000base-t-sfp-faq-an-2036.pdf
Signed-off-by: Michael Walle <mwalle@kernel.org>
---
I'm not sure, this is the correct place for the fix, nor if it goes in
the right direction. There is a comment in

/*
 * Clause 22 copper SFP modules normally operate in Cisco SGMII mode with
 * negotiation enabled, but some may be in 1000base-X - which is for the
 * PHY driver to determine.
 */

I haven't found any traces of that though. So any pointers, what flags
have to be modified is appreciated (phydev->supported?
phydev->possible_interfaces?).

I also compared the EEPROMs of both a FCLF-8520-3 and a FCLF-8521-3 (I
don't have a FCLF8520P2BTL) and they are the same except for the P/N and
the serial number. So they both advertise the same capabilites. Although
I'd suspect that the power-on strapping is different and could be read
by the PHY driver.
---
 drivers/net/phy/sfp.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/net/phy/sfp.c b/drivers/net/phy/sfp.c
index b9d84612ac6f..387753cb5c62 100644
--- a/drivers/net/phy/sfp.c
+++ b/drivers/net/phy/sfp.c
@@ -478,6 +478,16 @@ static void sfp_quirk_2500basex(const struct sfp_eeprom_id *id,
 	__set_bit(PHY_INTERFACE_MODE_2500BASEX, caps->interfaces);
 }
 
+static void sfp_quirk_no_sgmii(const struct sfp_eeprom_id *id,
+			       struct sfp_module_caps *caps)
+{
+	linkmode_clear_bit(ETHTOOL_LINK_MODE_1000baseT_Half_BIT,
+			   caps->link_modes);
+	linkmode_clear_bit(ETHTOOL_LINK_MODE_1000baseT_Full_BIT,
+			   caps->link_modes);
+	__clear_bit(PHY_INTERFACE_MODE_SGMII, caps->interfaces);
+}
+
 static void sfp_quirk_disable_autoneg(const struct sfp_eeprom_id *id,
 				      struct sfp_module_caps *caps)
 {
@@ -539,6 +549,10 @@ static const struct sfp_quirk sfp_quirks[] = {
 	// PHY.
 	SFP_QUIRK_F("FS", "SFP-10G-T", sfp_fixup_fs_10gt),
 
+	// These are sold as 1000base-X compatible. Thus, make sure we don't
+	// use SGMII.
+	SFP_QUIRK_S("FINISAR CORP.", "FCLF8521P2BTL", sfp_quirk_no_sgmii),
+
 	// Fiberstore SFP-2.5G-T and SFP-10GM-T uses Rollball protocol to talk
 	// to the PHY and needs 4 sec wait before probing the PHY.
 	SFP_QUIRK_F("FS", "SFP-2.5G-T", sfp_fixup_rollball_wait4s),
-- 
2.47.3