From 8ac44d24c3a148c4177bd3ad790c377279f4674f Mon Sep 17 00:00:00 2001
From: Greg Patrick <gregspatrick@hotmail.com>
Date: Thu, 11 Jun 2026 17:53:41 +0000
Subject: [PATCH] net: phy: sfp: detect presence via I2C when no MOD_DEF0 GPIO

An SFP cage (compatible "sff,sfp") whose MOD_DEF0 signal is not wired to a
GPIO currently falls back to sff_gpio_get_state(), which unconditionally
reports the module as present. An empty cage therefore fails its probe and
is parked in SFP_MOD_ERROR forever; because SFP_F_PRESENT never deasserts
there is no REMOVE event to recover the state machine, so a module inserted
after boot is never detected, and empty cages spam -EIO at boot.

This affects boards that route none of the cage presence signal to a
software-readable input. On the NicGiga S100-0800S-M (RTL9303, 8x SFP+) the
cage I2C bus is the switch's SMBus master; TX_DISABLE is driven via a
PCA9534 I/O expander, but no MOD_ABS/MOD_DEF0 line reaches a readable GPIO
(the RTL9303 gpio0 lines read stuck-low, the single PCA9534 is fully
consumed by TX_DISABLE, and there is no RTL8231). The Horaco ZX-SW82TS-L2P
(RTL9302D, 2x SFP+) is independently affected in the same way.

For such an SFP cage, derive presence from a throttled single-byte I2C read
of the module EEPROM instead: a successful read asserts SFP_F_PRESENT,
R_PROBE_ABSENT consecutive failures clear it (to ride out a transient error
on a live module). The existing poll then emits SFP_E_INSERT / SFP_E_REMOVE
normally, giving working hot-plug and silencing the boot-time -EIO spam on
empty cages. Presence is re-probed every T_PROBE_PRESENT, so insertion is
detected within that interval and removal within
T_PROBE_PRESENT * R_PROBE_ABSENT.

A soldered-down module (compatible "sff,sff") has no presence signal and is
genuinely always present, so it continues to use sff_gpio_get_state(); the
new path is gated on the cage type advertising SFP_F_PRESENT.

Signed-off-by: Greg Patrick <gregspatrick@hotmail.com>
Tested-by: Manuel Stocker <mensi@mensi.ch>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20260611175341.2223184-1-gregspatrick@hotmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
 drivers/net/phy/sfp.c | 83 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 80 insertions(+), 3 deletions(-)

--- a/drivers/net/phy/sfp.c
+++ b/drivers/net/phy/sfp.c
@@ -206,6 +206,16 @@ static const enum gpiod_flags gpio_flags
 #define T_PROBE_RETRY_SLOW	msecs_to_jiffies(5000)
 #define R_PROBE_RETRY_SLOW	12
 
+/* Polling interval and consecutive-failure threshold for the I2C presence
+ * probe used on boards without a MOD_DEF0 GPIO (see sfp_i2c_get_state()).
+ * A single successful read asserts presence immediately; R_PROBE_ABSENT
+ * consecutive failures are required to declare a live module removed, to ride
+ * out a transient I2C error. Insertion is thus detected within
+ * T_PROBE_PRESENT and removal within T_PROBE_PRESENT * R_PROBE_ABSENT.
+ */
+#define T_PROBE_PRESENT		msecs_to_jiffies(500)
+#define R_PROBE_ABSENT		3
+
 /* SFP modules appear to always have their PHY configured for bus address
  * 0x56 (which with mdio-i2c, translates to a PHY address of 22).
  * RollBall SFPs access phy via SFP Enhanced Digital Diagnostic Interface
@@ -249,6 +259,13 @@ struct sfp {
 
 	bool need_poll;
 
+	/* I2C-probed presence, for boards without a MOD_DEF0 GPIO.
+	 * Access rules: st_mutex held (updated from the poll/state machine).
+	 */
+	bool i2c_present;
+	u8 i2c_present_nak;
+	unsigned long i2c_present_next;
+
 	/* Access rules:
 	 * state_hw_drive: st_mutex held
 	 * state_hw_mask: st_mutex held
@@ -861,6 +878,45 @@ static int sfp_read(struct sfp *sfp, boo
 	return sfp->read(sfp, a2, addr, buf, len);
 }
 
+/* Probe whether a module is physically present by attempting a single-byte
+ * I2C read of the EEPROM identifier (an empty cage NAKs). Used as the presence
+ * source on boards that do not wire MOD_DEF0 to a GPIO.
+ */
+static bool sfp_module_present_i2c(struct sfp *sfp)
+{
+	u8 id;
+
+	return sfp_read(sfp, false, SFP_PHYS_ID, &id, sizeof(id)) == sizeof(id);
+}
+
+/* get_state variant for boards without a MOD_DEF0 GPIO. Instead of assuming
+ * the module is always present, derive SFP_F_PRESENT from a throttled I2C
+ * probe so that hot-insertion and removal are detected. A single ACK asserts
+ * presence; R_PROBE_ABSENT consecutive failures clear it, to ride out a
+ * transient I2C error on a live module.
+ */
+static unsigned int sfp_i2c_get_state(struct sfp *sfp)
+{
+	unsigned int state = sfp_gpio_get_state(sfp);
+
+	if (time_after_eq(jiffies, sfp->i2c_present_next)) {
+		if (sfp_module_present_i2c(sfp)) {
+			sfp->i2c_present = true;
+			sfp->i2c_present_nak = 0;
+		} else if (sfp->i2c_present &&
+			   ++sfp->i2c_present_nak >= R_PROBE_ABSENT) {
+			sfp->i2c_present = false;
+			sfp->i2c_present_nak = 0;
+		}
+		sfp->i2c_present_next = jiffies + T_PROBE_PRESENT;
+	}
+
+	if (sfp->i2c_present)
+		state |= SFP_F_PRESENT;
+
+	return state;
+}
+
 static int sfp_write(struct sfp *sfp, bool a2, u8 addr, void *buf, size_t len)
 {
 	return sfp->write(sfp, a2, addr, buf, len);
@@ -3159,9 +3215,30 @@ static int sfp_probe(struct platform_dev
 	sfp->get_state = sfp_gpio_get_state;
 	sfp->set_state = sfp_gpio_set_state;
 
-	/* Modules that have no detect signal are always present */
-	if (!(sfp->gpio[GPIO_MODDEF0]))
-		sfp->get_state = sff_gpio_get_state;
+	/* An SFP cage with no MOD_DEF0 GPIO has no hardware presence signal.
+	 * Assuming the module is always present traps an empty cage in
+	 * MOD_ERROR and never detects hot-insertion, so derive presence from a
+	 * throttled I2C probe and poll for changes instead. sfp_i2c_configure()
+	 * has already set i2c_max_block_size; seed i2c_block_size so the
+	 * presence read does not issue a zero-length transfer before the first
+	 * EEPROM read. Seed i2c_present_next to jiffies so the first probe
+	 * happens immediately (a zero value would be in the past relative to
+	 * the negative INITIAL_JIFFIES at boot and delay detection).
+	 *
+	 * A soldered-down module (sff,sff) has no presence signal and is
+	 * genuinely always present, so it keeps the always-present behaviour;
+	 * the I2C probe is gated on the cage type advertising SFP_F_PRESENT.
+	 */
+	if (!sfp->gpio[GPIO_MODDEF0]) {
+		if (sff->gpios & SFP_F_PRESENT) {
+			sfp->get_state = sfp_i2c_get_state;
+			sfp->i2c_block_size = sfp->i2c_max_block_size;
+			sfp->i2c_present_next = jiffies;
+			sfp->need_poll = true;
+		} else {
+			sfp->get_state = sff_gpio_get_state;
+		}
+	}
 
 	device_property_read_u32(&pdev->dev, "maximum-power-milliwatt",
 				 &sfp->max_power_mW);
