[PATCH net-next v3] net: phy: sfp: detect presence via I2C when no MOD_DEF0 GPIO

[PATCH net-next v3] net: phy: sfp: detect presence via I2C when no MOD_DEF0 GPIO

[Greg Patrick]

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>
---
v3:
 - Reduce the presence re-probe interval to 500 ms and require 3 consecutive
   failed probes (R_PROBE_ABSENT) to declare a live module removed, cutting
   removal-detection latency from ~4 s to ~1.5 s while still riding out a
   transient I2C error on a live module. Polling is gated by the existing
   need_poll path (100 ms), so this is one single-byte SMBus read per cage
   every 500 ms. Note that, as a polled scheme with no presence edge, a
   same-cage swap to a different module within the removal window cannot be
   observed; this is inherent to boards lacking a readable presence line.
   (Maxime Chevallier)
 - Hardware-validated on the NicGiga (now running from flash): insertion into
   a cage that was empty at boot is detected and links within ~1 s, and clean
   removal of a live module is detected in ~1.2-1.5 s.
 - Note the Horaco ZX-SW82TS-L2P (RTL9302D, 2x SFP+) as a second, independent
   board with the same no-MOD_DEF0 wiring. The Tested-by reflects a test of
   the I2C-presence mechanism on that board; the v3 delta is the timing
   constants above.

v2: https://lore.kernel.org/netdev/20260604151614.3310544-1-gregspatrick@hotmail.com/
v1: https://lore.kernel.org/netdev/20260602235528.2795028-1-gregspatrick@hotmail.com/

 drivers/net/phy/sfp.c | 83 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 80 insertions(+), 3 deletions(-)

diff --git a/drivers/net/phy/sfp.c b/drivers/net/phy/sfp.c
index 376c705a9..5e952d095 100644
--- 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
@@ -863,6 +880,45 @@ static int sfp_read(struct sfp *sfp, bool a2, u8 addr, void *buf, size_t len)
 	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);
@@ -3168,9 +3224,30 @@ static int sfp_probe(struct platform_device *pdev)
 	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);
-- 
2.53.0

Re: [PATCH net-next v3] net: phy: sfp: detect presence via I2C when no MOD_DEF0 GPIO

[Maxime Chevallier]

Hi,

On 6/11/26 19:53, Greg Patrick wrote:
> 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>

And it doesn't regress any boards I've tested this on (although this was
very quick testing), so

Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>

I'm wondering if a followup could be to add another loud warning to
dmesg that the HW design is broken, leading to potentially quirky behaviour.

We have lots of those for SFP.

Maxime

Re: [PATCH net-next v3] net: phy: sfp: detect presence via I2C when no MOD_DEF0 GPIO

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Thu, 11 Jun 2026 17:53:41 +0000 you wrote:
> 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.
> 
> [...]

Here is the summary with links:
  - [net-next,v3] net: phy: sfp: detect presence via I2C when no MOD_DEF0 GPIO
    https://git.kernel.org/netdev/net-next/c/8ac44d24c3a1

You are awesome, thank you!
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