[PATCH net-next v5] net: phy: sfp: probe for RollBall I2C-to-MDIO bridge in mdio-i2c

[Petr Wozniak <petr.wozniak@gmail.com>]

The "OEM"/"SFP-10G-T" quirk entry in sfp_fixup_rollball_cc()
unconditionally forces MDIO_I2C_ROLLBALL for all modules matching that
vendor/part-number combination.  This works for modules that genuinely
implement a RollBall I2C-to-MDIO bridge, but silently breaks modules
that share the same EEPROM strings without having such a bridge.

The Realtek RTL8261BE-CG is one such module: a pure copper 10G SFP+
media converter with no I2C-to-MDIO bridge.  Its EEPROM reports
vendor="OEM", part="SFP-10G-T-I", and -- critically -- Vendor OUI
00:00:00, making OUI-based differentiation impossible.  With
MDIO_I2C_ROLLBALL forced, the module silently ACKs the unlock password
write, the MDIO bus is created, but no PHY responds; the SFP state
machine cycles through the RollBall PHY-probe retry window before
reporting no PHY.

Move the probe into i2c_mii_init_rollball() in mdio-i2c.c, where the
RollBall protocol constants are already defined.  After sending the
unlock password, issue a CMD_READ and wait ~70 ms for CMD_DONE.  A
genuine RollBall bridge asserts CMD_DONE within that window; modules
without a bridge never do, so i2c_mii_init_rollball() returns -ENODEV.
mdio_i2c_alloc() propagates -ENODEV to the caller to signal that no
bridge is present and PHY probing should be skipped.
sfp_sm_add_mdio_bus() catches -ENODEV and transitions
sfp->mdio_protocol to MDIO_I2C_NONE so the rest of the state machine
skips PHY probing for this module.

Add "OEM"/"SFP-10G-T-I" to the quirk table so RTL8261BE modules enter
the probe path; genuine RollBall modules continue to work as before.

Signed-off-by: Petr Wozniak <petr.wozniak@gmail.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
---

Changes since v4 (feedback from Maxime Chevallier):
  - Fix commit message: replace "stalls" with accurate description of
    the RollBall PHY-probe retry window
  - Fix variable declaration order in i2c_mii_probe_rollball() to
    follow reverse-xmas tree (descending line length)
  - Remove spurious alignment space on "SFP-10G-T" quirk entry
  - Document that -ENODEV from mdio_i2c_alloc() means no bridge present,
    PHY probing should be skipped

Changes since v3 (feedback from Jakub Kicinski):
  - Drop spurious Tested-by: tag -- author and tester are the same person
  - Use PATCH net-next subject prefix
  - Move -ENODEV handling from sfp_i2c_mdiobus_create() into
    sfp_sm_add_mdio_bus() so bus-creation code does not mutate
    sfp->mdio_protocol; the state machine is the correct place for
    protocol-state transitions
  - Split combined variable declaration for clarity

Changes since v2:
  - Compile-tested and hardware-tested on BPI-R4 (MT7988A, 6.12.87)
  - RTL8261BE (OEM/SFP-10G-T-I): probes MDIO_I2C_NONE, link Up 10Gbps
  - Genuine RollBall (OEM/SFP-10G-T): bridge detected, link Up 10Gbps

 drivers/net/mdio/mdio-i2c.c | 60 +++++++++++++++++++++++++++++++-----
 drivers/net/phy/sfp.c       | 16 ++++++++--
 2 files changed, 65 insertions(+), 11 deletions(-)

--- a/drivers/net/mdio/mdio-i2c.c
+++ b/drivers/net/mdio/mdio-i2c.c
@@ -419,6 +419,46 @@
 	return 0;
 }

+static int i2c_mii_probe_rollball(struct i2c_adapter *i2c)
+{
+	u8 data_buf[] = { ROLLBALL_DATA_ADDR, 0x01, 0x00, 0x00 };
+	u8 cmd_buf[]  = { ROLLBALL_CMD_ADDR, ROLLBALL_CMD_READ };
+	u8 cmd_addr   = ROLLBALL_CMD_ADDR;
+	struct i2c_msg msgs[2];
+	u8 result;
+	int ret;
+
+	msgs[0].addr  = ROLLBALL_PHY_I2C_ADDR;
+	msgs[0].flags = 0;
+	msgs[0].len   = sizeof(data_buf);
+	msgs[0].buf   = data_buf;
+	msgs[1].addr  = ROLLBALL_PHY_I2C_ADDR;
+	msgs[1].flags = 0;
+	msgs[1].len   = sizeof(cmd_buf);
+	msgs[1].buf   = cmd_buf;
+
+	ret = i2c_transfer_rollball(i2c, msgs, ARRAY_SIZE(msgs));
+	if (ret)
+		return ret;
+
+	msleep(70);
+
+	msgs[0].addr  = ROLLBALL_PHY_I2C_ADDR;
+	msgs[0].flags = 0;
+	msgs[0].len   = 1;
+	msgs[0].buf   = &cmd_addr;
+	msgs[1].addr  = ROLLBALL_PHY_I2C_ADDR;
+	msgs[1].flags = I2C_M_RD;
+	msgs[1].len   = 1;
+	msgs[1].buf   = &result;
+
+	ret = i2c_transfer_rollball(i2c, msgs, ARRAY_SIZE(msgs));
+	if (ret)
+		return ret;
+
+	/* -ENODEV: CMD_DONE not asserted — no RollBall bridge present */
+	return result == ROLLBALL_CMD_DONE ? 0 : -ENODEV;
+}
+
 static int i2c_mii_init_rollball(struct i2c_adapter *i2c)
 {
 	struct i2c_msg msg;
@@ -439,10 +479,10 @@
 	ret = i2c_transfer(i2c, &msg, 1);
 	if (ret < 0)
 		return ret;
-	else if (ret != 1)
+	if (ret != 1)
 		return -EIO;
-	else
-		return 0;
+
+	return i2c_mii_probe_rollball(i2c);
 }

 static bool mdio_i2c_check_functionality(struct i2c_adapter *i2c,
@@ -487,9 +527,13 @@
 	case MDIO_I2C_ROLLBALL:
 		ret = i2c_mii_init_rollball(i2c);
 		if (ret < 0) {
-			dev_err(parent,
-				"Cannot initialize RollBall MDIO I2C protocol: %d\n",
-				ret);
+			if (ret != -ENODEV)
+				dev_err(parent,
+					"Cannot initialize RollBall MDIO I2C protocol: %d\n",
+					ret);
+			/* -ENODEV propagates to caller: no bridge present,
+			 * PHY probing should be skipped for this module. */
 			mdiobus_free(mii);
 			return ERR_PTR(ret);
 		}
--- a/drivers/net/phy/sfp.c
+++ b/drivers/net/phy/sfp.c
@@ -579,7 +579,8 @@
 	// OEM SFP-GE-T is a 1000Base-T module with broken TX_FAULT indicator
 	SFP_QUIRK_F("OEM", "SFP-GE-T", sfp_fixup_ignore_tx_fault),

-	SFP_QUIRK_F("OEM", "SFP-10G-T", sfp_fixup_rollball_cc),
+	SFP_QUIRK_F("OEM", "SFP-10G-T-I", sfp_fixup_rollball),
+	SFP_QUIRK_F("OEM", "SFP-10G-T", sfp_fixup_rollball_cc),
 	SFP_QUIRK_S("OEM", "SFP-2.5G-T", sfp_quirk_oem_2_5g),
 	SFP_QUIRK_S("OEM", "SFP-2.5G-BX10-D", sfp_quirk_2500basex),
 	SFP_QUIRK_S("OEM", "SFP-2.5G-BX10-U", sfp_quirk_2500basex),
@@ -2022,10 +2023,18 @@

 static int sfp_sm_add_mdio_bus(struct sfp *sfp)
 {
-	if (sfp->mdio_protocol != MDIO_I2C_NONE)
-		return sfp_i2c_mdiobus_create(sfp);
+	int ret;

-	return 0;
+	if (sfp->mdio_protocol == MDIO_I2C_NONE)
+		return 0;
+
+	ret = sfp_i2c_mdiobus_create(sfp);
+	if (ret == -ENODEV) {
+		/* Probe confirmed no bridge present; skip PHY discovery. */
+		sfp->mdio_protocol = MDIO_I2C_NONE;
+		return 0;
+	}
+	return ret;
 }

 /* Probe a SFP for a PHY device if the module supports copper - the PHY
--
2.51.0

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