[PATCH iwl-net] e1000e: fix flow control stuck off after link partner reset

["Craig I. Hagan" <hagan@amazon.com>]

On an I219 (pch_spt+) copper with jumbo MTU enabled, a link partner
reset (e.g. a switch reboot) can leave the PHY's pause advertisement
bits dropped. Nothing re-asserts them on link-up, so every renegotiation
resolves flow control to none. rx_crc_errors climb and traffic stalls until
the interface is manually reset. Observed on I219-LM (8086:15b7) at
MTU 9000 behind a switch that was power-cycled.

In the watchdog, detect a completed autonegotiation that resolved
to no flow control despite pause being requested and the pause bits
missing from the advertisement, and schedule reset_task to rebuild
the advertisement (as ethtool -r does). Remediation is limited to three
attempts, 30s apart, until the next link down.

Remediate via the existing reset path rather than re-asserting the
advertisement on link-up; a root-cause rework can follow in net-next.

Fixes: 79849ebc0e06 ("e1000e: initial support for i219")
Cc: stable@vger.kernel.org
Link: https://github.com/systemd/systemd/issues/25925
Assisted-by: LLM
Signed-off-by: Craig I. Hagan <hagan@amazon.com>
---
 drivers/net/ethernet/intel/e1000e/e1000.h  |  11 ++
 drivers/net/ethernet/intel/e1000e/netdev.c | 132 +++++++++++++++++++++
 2 files changed, 143 insertions(+)

diff --git a/drivers/net/ethernet/intel/e1000e/e1000.h b/drivers/net/ethernet/intel/e1000e/e1000.h
index 63ebe0037..4353519d4 100644
--- a/drivers/net/ethernet/intel/e1000e/e1000.h
+++ b/drivers/net/ethernet/intel/e1000e/e1000.h
@@ -210,6 +210,17 @@ struct e1000_adapter {
 	/* track device up/down/testing state */
 	unsigned long state;
 
+	/* Flow-control advertisement-latch auto-recovery.
+	 * These MUST persist across the reinit that remediation triggers
+	 * (the reinit generates its own carrier-up), so they are NOT reset
+	 * on link-up. The holdoff and per-episode attempt budget prevent a
+	 * reset/flap storm against a partner that is still booting or that
+	 * genuinely never advertises pause.
+	 */
+	unsigned long fclatch_last_fix;	/* jiffies of last remediation, 0=none */
+	u8 fclatch_attempts;		/* attempts in the current link episode */
+	bool fclatch_gave_up;		/* terminal for episode; re-armed on link-down */
+
 	/* Interrupt Throttle Rate */
 	u32 itr;
 	u32 itr_setting;
diff --git a/drivers/net/ethernet/intel/e1000e/netdev.c b/drivers/net/ethernet/intel/e1000e/netdev.c
index 844f31ab3..28bd33335 100644
--- a/drivers/net/ethernet/intel/e1000e/netdev.c
+++ b/drivers/net/ethernet/intel/e1000e/netdev.c
@@ -38,6 +38,15 @@ static int debug = -1;
 module_param(debug, int, 0);
 MODULE_PARM_DESC(debug, "Debug level (0=none,...,16=all)");
 
+/* Flow-control advertisement-latch auto-recovery tunables. Fixed constants
+ * rather than user knobs: the values only need to be safe, not tunable.
+ *   MAX_ATTEMPTS : autoneg-restart attempts per link episode before giving up
+ *   HOLDOFF      : minimum spacing between attempts; longer than a switch's
+ *                  autoneg settle so we do not fight a still-booting partner
+ */
+#define E1000E_FCLATCH_MAX_ATTEMPTS	3	/* autoneg-restarts per link episode */
+#define E1000E_FCLATCH_HOLDOFF		(30 * HZ)	/* min spacing between attempts */
+
 static const struct e1000_info *e1000_info_tbl[] = {
 	[board_82571]		= &e1000_82571_info,
 	[board_82572]		= &e1000_82572_info,
@@ -5217,6 +5226,113 @@ static void e1000_watchdog(struct timer_list *t)
 	/* TODO: make this use queue_delayed_work() */
 }
 
+/**
+ * e1000e_check_fc_latch - re-assert a dropped pause advertisement
+ * @adapter: board private structure
+ *
+ * On the I219/PCH family at jumbo MTU, an upstream switch reboot can bring the
+ * link back with the PHY's pause-advertisement bits dropped and flow control
+ * resolved to none, even when a pause mode was requested. The driver does not
+ * re-assert the advertisement on a plain link-up (e1000_phy_setup_autoneg()
+ * runs only during a full setup pass), so every later renegotiation resolves
+ * to none and the link does not recover on its own.
+ *
+ * Detect that state from a fresh PHY read and schedule a full reconfigure (the
+ * reset_task path, as ethtool -r uses) to rebuild the advertisement. reset_task
+ * bounces the link and re-enters this check, so the attempt count is kept
+ * across the bounce rather than reset on link-up. Remediation stops after three
+ * tries per link episode; the give-up state is re-armed only on a real
+ * link-down (peer/cable change), so a partner that never advertises pause is
+ * reset three times and then left alone until the link is re-established --
+ * it cannot flap indefinitely.
+ */
+static void e1000e_check_fc_latch(struct e1000_adapter *adapter)
+{
+	struct e1000_hw *hw = &adapter->hw;
+	struct net_device *netdev = adapter->netdev;
+	u16 bmsr = 0, adv = 0, expected;
+	unsigned long now;
+	int rv;
+
+	/* Skip while down/resetting or with a reset already pending. */
+	if (!netif_carrier_ok(netdev) ||
+	    test_bit(__E1000_DOWN, &adapter->state) ||
+	    test_bit(__E1000_RESETTING, &adapter->state) ||
+	    (adapter->flags & FLAG_RESTART_NOW))
+		return;
+
+	/* Limited to I219/PCH copper at jumbo MTU, where the behavior is seen
+	 * and the RX-side impact matters. Act only when a pause mode was
+	 * requested but autoneg resolved none. Pause is only meaningful on a
+	 * full-duplex link, so a half-duplex result legitimately carries no
+	 * pause bits and must not be treated as the fault.
+	 */
+	if (hw->phy.media_type != e1000_media_type_copper ||
+	    hw->mac.type < e1000_pch_spt ||		/* pch_spt+ (I219 and later) */
+	    netdev->mtu <= ETH_DATA_LEN ||		/* jumbo only */
+	    adapter->link_duplex != FULL_DUPLEX ||
+	    !hw->mac.autoneg || !adapter->fc_autoneg ||
+	    hw->fc.current_mode != e1000_fc_none)
+		return;
+	if (hw->fc.requested_mode != e1000_fc_full &&
+	    hw->fc.requested_mode != e1000_fc_rx_pause &&
+	    hw->fc.requested_mode != e1000_fc_tx_pause)
+		return;
+
+	/* Fresh, self-contained reads; act only on a good, autoneg-complete read. */
+	rv = e1e_rphy(hw, MII_BMSR, &bmsr);
+	if (!rv)
+		rv = e1e_rphy(hw, MII_ADVERTISE, &adv);
+	if (rv || !(bmsr & BMSR_ANEGCOMPLETE))
+		return;
+
+	/* Pause bits we should be advertising for the requested mode (mirrors
+	 * e1000_phy_setup_autoneg): full/rx use CAP|ASYM, tx uses ASYM only.
+	 */
+	if (hw->fc.requested_mode == e1000_fc_tx_pause)
+		expected = ADVERTISE_PAUSE_ASYM;
+	else
+		expected = ADVERTISE_PAUSE_CAP | ADVERTISE_PAUSE_ASYM;
+
+	/* The advertisement should carry these bits; missing them while a pause
+	 * mode was requested is the failure this recovers from.
+	 */
+	if ((adv & expected) == expected)
+		return;
+
+	now = jiffies;
+
+	/* Give-up is terminal for this link episode; it is re-armed only on a
+	 * real link-down (see the watchdog link-down path), i.e. when the peer
+	 * or cabling actually changes -- not on a timer, which cannot tell a
+	 * recovered switch from the same never-pause partner 60s later.
+	 */
+	if (adapter->fclatch_gave_up)
+		return;
+	if (adapter->fclatch_last_fix &&
+	    time_before(now, adapter->fclatch_last_fix + E1000E_FCLATCH_HOLDOFF))
+		return;				/* within holdoff */
+
+	if (adapter->fclatch_attempts >= E1000E_FCLATCH_MAX_ATTEMPTS) {
+		adapter->fclatch_gave_up = true;
+		e_warn("flow-control advertisement latch persists after %u autoneg-restart attempts (adv=0x%04x, requested fc=%u); giving up until the link goes down and back up\n",
+		       adapter->fclatch_attempts, adv, hw->fc.requested_mode);
+		return;
+	}
+
+	adapter->fclatch_attempts++;
+	adapter->fclatch_last_fix = now;
+	e_warn("flow-control advertisement latch detected (adv=0x%04x missing pause for requested fc=%u); forcing autoneg restart (attempt %u/%d)\n",
+	       adv, hw->fc.requested_mode, adapter->fclatch_attempts,
+	       E1000E_FCLATCH_MAX_ATTEMPTS);
+	/* Request the reset via FLAG_RESTART_NOW; the watchdog schedules
+	 * reset_task on this flag just below the call site. The flag also
+	 * tells reset_task this is intentional, suppressing its "Reset
+	 * adapter unexpectedly" warning + register dump.
+	 */
+	adapter->flags |= FLAG_RESTART_NOW;
+}
+
 static void e1000_watchdog_task(struct work_struct *work)
 {
 	struct e1000_adapter *adapter = container_of(work,
@@ -5353,6 +5469,17 @@ static void e1000_watchdog_task(struct work_struct *work)
 			netdev_info(netdev, "NIC Link is Down\n");
 			netif_carrier_off(netdev);
 			netif_stop_queue(netdev);
+
+			/* A real link-down ends the current fc-latch episode:
+			 * re-arm remediation so the next link-up (e.g. the
+			 * switch finishing its reboot) gets a fresh set of
+			 * attempts. Our own reset_task bounce does not reach
+			 * here (it goes through __E1000_DOWN/RESETTING), so
+			 * this does not defeat the per-episode attempt cap.
+			 */
+			adapter->fclatch_attempts = 0;
+			adapter->fclatch_gave_up = false;
+			adapter->fclatch_last_fix = 0;
 			if (!test_bit(__E1000_DOWN, &adapter->state))
 				mod_timer(&adapter->phy_info_timer,
 					  round_jiffies(jiffies + 2 * HZ));
@@ -5384,6 +5511,11 @@ static void e1000_watchdog_task(struct work_struct *work)
 	adapter->gotc_old = adapter->stats.gotc;
 	spin_unlock(&adapter->stats64_lock);
 
+	/* Auto-recover from a latched pause advertisement (I219/PCH,
+	 * jumbo) after an upstream switch reboot; see e1000e_check_fc_latch().
+	 */
+	e1000e_check_fc_latch(adapter);
+
 	/* If the link is lost the controller stops DMA, but
 	 * if there is queued Tx work it cannot be done.  So
 	 * reset the controller to flush the Tx packet buffers.
-- 
2.47.3