[PATCH net v1 1/1] net: dsa: microchip: lan937x: Fix RGMII delay tuning

[PATCH net v1 1/1] net: dsa: microchip: lan937x: Fix RGMII delay tuning

[Oleksij Rempel]

Correct RGMII delay application logic in lan937x_set_tune_adj().

The function was missing `data16 &= ~PORT_TUNE_ADJ` before setting the
new delay value. This caused the new value to be bitwise-OR'd with the
existing PORT_TUNE_ADJ field instead of replacing it.

For example, when setting the RGMII 2 TX delay on port 4, the
intended TUNE_ADJUST value of 0 (RGMII_2_TX_DELAY_2NS) was
incorrectly OR'd with the default 0x1B (from register value 0xDA3),
leaving the delay at the wrong setting.

This patch adds the missing mask to clear the field, ensuring the
correct delay value is written. Physical measurements on the RGMII TX
lines confirm the fix, showing the delay changing from ~1ns (before
change) to ~2ns.

While testing on i.MX 8MP showed this was within the platform's timing
tolerance, it did not match the intended hardware-characterized value.

Fixes: b19ac41faa3f ("net: dsa: microchip: apply rgmii tx and rx delay in phylink mac config")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
 drivers/net/dsa/microchip/lan937x_main.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/dsa/microchip/lan937x_main.c b/drivers/net/dsa/microchip/lan937x_main.c
index b1ae3b9de3d1..5a1496fff445 100644
--- a/drivers/net/dsa/microchip/lan937x_main.c
+++ b/drivers/net/dsa/microchip/lan937x_main.c
@@ -540,6 +540,7 @@ static void lan937x_set_tune_adj(struct ksz_device *dev, int port,
 	ksz_pread16(dev, port, reg, &data16);
 
 	/* Update tune Adjust */
+	data16 &= ~PORT_TUNE_ADJ;
 	data16 |= FIELD_PREP(PORT_TUNE_ADJ, val);
 	ksz_pwrite16(dev, port, reg, data16);
 
-- 
2.47.3

Re: [PATCH net v1 1/1] net: dsa: microchip: lan937x: Fix RGMII delay tuning

[kernel test robot]

Hi,

Thanks for your patch.

FYI: kernel test robot notices the stable kernel rule is not satisfied.

The check is based on https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html#option-1

Rule: add the tag "Cc: stable@vger.kernel.org" in the sign-off area to have the patch automatically included in the stable tree.
Subject: [PATCH net v1 1/1] net: dsa: microchip: lan937x: Fix RGMII delay tuning
Link: https://lore.kernel.org/stable/20251112101119.3594611-1-o.rempel%40pengutronix.de

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