net: phy: dp83tg720: Add randomized polling intervals for link detection

Address the limitations of the DP83TG720 PHY, which cannot reliably
detect or report a stable link state. To handle this, the PHY must be
periodically reset when the link is down. However, synchronized reset
intervals between the PHY and its link partner can result in a deadlock,
preventing the link from re-establishing.

This change introduces a randomized polling interval when the link is
down to desynchronize resets between link partners.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250210082358.200751-3-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: olerem

drivers/net/phy/dp83tg720.c

@@ -4,12 +4,31 @@
  */
 #include <linux/bitfield.h>
 #include <linux/ethtool_netlink.h>
+#include <linux/jiffies.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/phy.h>
+#include <linux/random.h>
 
 #include "open_alliance_helpers.h"
 
+/*
+ * DP83TG720S_POLL_ACTIVE_LINK - Polling interval in milliseconds when the link
+ *				 is active.
+ * DP83TG720S_POLL_NO_LINK_MIN - Minimum polling interval in milliseconds when
+ *				 the link is down.
+ * DP83TG720S_POLL_NO_LINK_MAX - Maximum polling interval in milliseconds when
+ *				 the link is down.
+ *
+ * These values are not documented or officially recommended by the vendor but
+ * were determined through empirical testing. They achieve a good balance in
+ * minimizing the number of reset retries while ensuring reliable link recovery
+ * within a reasonable timeframe.
+ */
+#define DP83TG720S_POLL_ACTIVE_LINK		1000
+#define DP83TG720S_POLL_NO_LINK_MIN		100
+#define DP83TG720S_POLL_NO_LINK_MAX		1000
+
 #define DP83TG720S_PHY_ID			0x2000a284
 
 /* MDIO_MMD_VEND2 registers */
@@ -371,6 +390,13 @@ static int dp83tg720_read_status(struct phy_device *phydev)
 		if (ret)
 			return ret;
 
+		/* Sleep 600ms for PHY stabilization post-reset.
+		 * Empirically chosen value (not documented).
+		 * Helps reduce reset bounces with link partners having similar
+		 * issues.
+		 */
+		msleep(600);
+
 		/* After HW reset we need to restore master/slave configuration.
 		 * genphy_c45_pma_baset1_read_master_slave() call will be done
 		 * by the dp83tg720_config_aneg() function.
@@ -498,6 +524,57 @@ static int dp83tg720_probe(struct phy_device *phydev)
 	return 0;
 }
 
+/**
+ * dp83tg720_get_next_update_time - Determine the next update time for PHY
+ *                                  state
+ * @phydev: Pointer to the phy_device structure
+ *
+ * This function addresses a limitation of the DP83TG720 PHY, which cannot
+ * reliably detect or report a stable link state. To recover from such
+ * scenarios, the PHY must be periodically reset when the link is down. However,
+ * if the link partner also runs Linux with the same driver, synchronized reset
+ * intervals can lead to a deadlock where the link never establishes due to
+ * simultaneous resets on both sides.
+ *
+ * To avoid this, the function implements randomized polling intervals when the
+ * link is down. It ensures that reset intervals are desynchronized by
+ * introducing a random delay between a configured minimum and maximum range.
+ * When the link is up, a fixed polling interval is used to minimize overhead.
+ *
+ * This mechanism guarantees that the link will reestablish within 10 seconds
+ * in the worst-case scenario.
+ *
+ * Return: Time (in jiffies) until the next update event for the PHY state
+ * machine.
+ */
+static unsigned int dp83tg720_get_next_update_time(struct phy_device *phydev)
+{
+	unsigned int next_time_jiffies;
+
+	if (phydev->link) {
+		/* When the link is up, use a fixed 1000ms interval
+		 * (in jiffies)
+		 */
+		next_time_jiffies =
+			msecs_to_jiffies(DP83TG720S_POLL_ACTIVE_LINK);
+	} else {
+		unsigned int min_jiffies, max_jiffies, rand_jiffies;
+
+		/* When the link is down, randomize interval between min/max
+		 * (in jiffies)
+		 */
+		min_jiffies = msecs_to_jiffies(DP83TG720S_POLL_NO_LINK_MIN);
+		max_jiffies = msecs_to_jiffies(DP83TG720S_POLL_NO_LINK_MAX);
+
+		rand_jiffies = min_jiffies +
+			get_random_u32_below(max_jiffies - min_jiffies + 1);
+		next_time_jiffies = rand_jiffies;
+	}
+
+	/* Ensure the polling time is at least one jiffy */
+	return max(next_time_jiffies, 1U);
+}
+
 static struct phy_driver dp83tg720_driver[] = {
 {
 	PHY_ID_MATCH_MODEL(DP83TG720S_PHY_ID),
@@ -516,6 +593,7 @@ static struct phy_driver dp83tg720_driver[] = {
 	.get_link_stats	= dp83tg720_get_link_stats,
 	.get_phy_stats	= dp83tg720_get_phy_stats,
 	.update_stats	= dp83tg720_update_stats,
+	.get_next_update_time = dp83tg720_get_next_update_time,
 
 	.suspend	= genphy_suspend,
 	.resume		= genphy_resume,