[PATCH net 1/2] net: dsa: microchip: Fix KSZ9477 set_ageing_time function

[<Tristram.Ha@microchip.com>]

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

The aging count is not a simple 11-bit value but comprises a 3-bit
multiplier and an 8-bit second count.  The code tries to use the
original multiplier which is 4 as the second count is still 300 seconds
by default.

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/ksz9477.c     | 47 +++++++++++++++++++------
 drivers/net/dsa/microchip/ksz9477_reg.h |  4 +--
 2 files changed, 37 insertions(+), 14 deletions(-)

diff --git a/drivers/net/dsa/microchip/ksz9477.c b/drivers/net/dsa/microchip/ksz9477.c
index d16817e0476f..29fe79ea74cd 100644
--- a/drivers/net/dsa/microchip/ksz9477.c
+++ b/drivers/net/dsa/microchip/ksz9477.c
@@ -2,7 +2,7 @@
 /*
  * Microchip KSZ9477 switch driver main logic
  *
- * Copyright (C) 2017-2019 Microchip Technology Inc.
+ * Copyright (C) 2017-2024 Microchip Technology Inc.
  */
 
 #include <linux/kernel.h>
@@ -983,26 +983,51 @@ void ksz9477_get_caps(struct ksz_device *dev, int port,
 int ksz9477_set_ageing_time(struct ksz_device *dev, unsigned int msecs)
 {
 	u32 secs = msecs / 1000;
-	u8 value;
-	u8 data;
+	u8 data, mult, value;
+	u32 max_val;
 	int ret;
 
-	value = FIELD_GET(SW_AGE_PERIOD_7_0_M, secs);
+#define MAX_TIMER_VAL	((1 << 8) - 1)
 
-	ret = ksz_write8(dev, REG_SW_LUE_CTRL_3, value);
-	if (ret < 0)
-		return ret;
+	/* The aging timer comprises a 3-bit multiplier and an 8-bit second
+	 * value.  Either of them cannot be zero.  The maximum timer is then
+	 * 7 * 255 = 1785 seconds.
+	 */
+	if (!secs)
+		secs = 1;
 
-	data = FIELD_GET(SW_AGE_PERIOD_10_8_M, secs);
+	/* Return error if too large. */
+	else if (secs > 7 * MAX_TIMER_VAL)
+		return -EINVAL;
 
 	ret = ksz_read8(dev, REG_SW_LUE_CTRL_0, &value);
 	if (ret < 0)
 		return ret;
 
-	value &= ~SW_AGE_CNT_M;
-	value |= FIELD_PREP(SW_AGE_CNT_M, data);
+	/* Check whether there is need to update the multiplier. */
+	mult = FIELD_GET(SW_AGE_CNT_M, value);
+	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) {
+		value &= ~SW_AGE_CNT_M;
+		value |= FIELD_PREP(SW_AGE_CNT_M, data);
+		ret = ksz_write8(dev, REG_SW_LUE_CTRL_0, value);
+		if (ret < 0)
+			return ret;
+	}
 
-	return ksz_write8(dev, REG_SW_LUE_CTRL_0, value);
+	value = DIV_ROUND_UP(secs, data);
+	return ksz_write8(dev, REG_SW_LUE_CTRL_3, value);
 }
 
 void ksz9477_port_queue_split(struct ksz_device *dev, int port)
diff --git a/drivers/net/dsa/microchip/ksz9477_reg.h b/drivers/net/dsa/microchip/ksz9477_reg.h
index 04235c22bf40..ff579920078e 100644
--- a/drivers/net/dsa/microchip/ksz9477_reg.h
+++ b/drivers/net/dsa/microchip/ksz9477_reg.h
@@ -2,7 +2,7 @@
 /*
  * Microchip KSZ9477 register definitions
  *
- * Copyright (C) 2017-2018 Microchip Technology Inc.
+ * Copyright (C) 2017-2024 Microchip Technology Inc.
  */
 
 #ifndef __KSZ9477_REGS_H
@@ -165,8 +165,6 @@
 #define SW_VLAN_ENABLE			BIT(7)
 #define SW_DROP_INVALID_VID		BIT(6)
 #define SW_AGE_CNT_M			GENMASK(5, 3)
-#define SW_AGE_CNT_S			3
-#define SW_AGE_PERIOD_10_8_M		GENMASK(10, 8)
 #define SW_RESV_MCAST_ENABLE		BIT(2)
 #define SW_HASH_OPTION_M		0x03
 #define SW_HASH_OPTION_CRC		1
-- 
2.34.1