[PATCH net 2/2] net: dsa: microchip: Fix LAN937X set_ageing_time function

[<Tristram.Ha@microchip.com>]

From: Tristram Ha <tristram.ha@microchip.com>

The aging count is not a simple 20-bit value but comprises a 3-bit
multiplier and a 20-bit second time.  The code tries to use the
original multiplier which is 4 as the second count is still 300 seconds
by default.

As the 20-bit number is now too large for practical use there is an option
to interpret it as microseconds instead of seconds.

Fixes: 2c119d9982b1 ("net: dsa: microchip: add the support for set_ageing_time")
Signed-off-by: Tristram Ha <tristram.ha@microchip.com>
---
 drivers/net/dsa/microchip/lan937x_main.c | 62 ++++++++++++++++++++++--
 drivers/net/dsa/microchip/lan937x_reg.h  |  9 ++--
 2 files changed, 65 insertions(+), 6 deletions(-)

diff --git a/drivers/net/dsa/microchip/lan937x_main.c b/drivers/net/dsa/microchip/lan937x_main.c
index b7652efd632e..b1ae3b9de3d1 100644
--- a/drivers/net/dsa/microchip/lan937x_main.c
+++ b/drivers/net/dsa/microchip/lan937x_main.c
@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 /* Microchip LAN937X switch driver main logic
- * Copyright (C) 2019-2022 Microchip Technology Inc.
+ * Copyright (C) 2019-2024 Microchip Technology Inc.
  */
 #include <linux/kernel.h>
 #include <linux/module.h>
@@ -461,10 +461,66 @@ int lan937x_change_mtu(struct ksz_device *dev, int port, int new_mtu)
 
 int lan937x_set_ageing_time(struct ksz_device *dev, unsigned int msecs)
 {
-	u32 secs = msecs / 1000;
-	u32 value;
+	u8 data, mult, value8;
+	bool in_msec = false;
+	u32 max_val, value;
+	u32 secs = msecs;
 	int ret;
 
+#define MAX_TIMER_VAL	((1 << 20) - 1)
+
+	/* The aging timer comprises a 3-bit multiplier and a 20-bit second
+	 * value.  Either of them cannot be zero.  The maximum timer is then
+	 * 7 * 1048575 = 7340025 seconds.  As this value is too large for
+	 * practical use it can be interpreted as microseconds, making the
+	 * maximum timer 7340 seconds with finer control.  This allows for
+	 * maximum 122 minutes compared to 29 minutes in KSZ9477 switch.
+	 */
+	if (msecs % 1000)
+		in_msec = true;
+	else
+		secs /= 1000;
+	if (!secs)
+		secs = 1;
+
+	/* Return error if too large. */
+	else if (secs > 7 * MAX_TIMER_VAL)
+		return -EINVAL;
+
+	/* Configure how to interpret the number value. */
+	ret = ksz_rmw8(dev, REG_SW_LUE_CTRL_2, SW_AGE_CNT_IN_MICROSEC,
+		       in_msec ? SW_AGE_CNT_IN_MICROSEC : 0);
+	if (ret < 0)
+		return ret;
+
+	ret = ksz_read8(dev, REG_SW_LUE_CTRL_0, &value8);
+	if (ret < 0)
+		return ret;
+
+	/* Check whether there is need to update the multiplier. */
+	mult = FIELD_GET(SW_AGE_CNT_M, value8);
+	max_val = MAX_TIMER_VAL;
+	if (mult > 0) {
+		/* Try to use the same multiplier already in the register as
+		 * the hardware default uses multiplier 4 and 75 seconds for
+		 * 300 seconds.
+		 */
+		max_val = DIV_ROUND_UP(secs, mult);
+		if (max_val > MAX_TIMER_VAL || max_val * mult != secs)
+			max_val = MAX_TIMER_VAL;
+	}
+
+	data = DIV_ROUND_UP(secs, max_val);
+	if (mult != data) {
+		value8 &= ~SW_AGE_CNT_M;
+		value8 |= FIELD_PREP(SW_AGE_CNT_M, data);
+		ret = ksz_write8(dev, REG_SW_LUE_CTRL_0, value8);
+		if (ret < 0)
+			return ret;
+	}
+
+	secs = DIV_ROUND_UP(secs, data);
+
 	value = FIELD_GET(SW_AGE_PERIOD_7_0_M, secs);
 
 	ret = ksz_write8(dev, REG_SW_AGE_PERIOD__1, value);
diff --git a/drivers/net/dsa/microchip/lan937x_reg.h b/drivers/net/dsa/microchip/lan937x_reg.h
index 4ec93e421da4..72042fd64e5b 100644
--- a/drivers/net/dsa/microchip/lan937x_reg.h
+++ b/drivers/net/dsa/microchip/lan937x_reg.h
@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 /* Microchip LAN937X switch register definitions
- * Copyright (C) 2019-2021 Microchip Technology Inc.
+ * Copyright (C) 2019-2024 Microchip Technology Inc.
  */
 #ifndef __LAN937X_REG_H
 #define __LAN937X_REG_H
@@ -56,8 +56,7 @@
 
 #define SW_VLAN_ENABLE			BIT(7)
 #define SW_DROP_INVALID_VID		BIT(6)
-#define SW_AGE_CNT_M			0x7
-#define SW_AGE_CNT_S			3
+#define SW_AGE_CNT_M			GENMASK(5, 3)
 #define SW_RESV_MCAST_ENABLE		BIT(2)
 
 #define REG_SW_LUE_CTRL_1		0x0311
@@ -70,6 +69,10 @@
 #define SW_FAST_AGING			BIT(1)
 #define SW_LINK_AUTO_AGING		BIT(0)
 
+#define REG_SW_LUE_CTRL_2		0x0312
+
+#define SW_AGE_CNT_IN_MICROSEC		BIT(7)
+
 #define REG_SW_AGE_PERIOD__1		0x0313
 #define SW_AGE_PERIOD_7_0_M		GENMASK(7, 0)
 
-- 
2.34.1