netdev - [PATCH net-next 2/4] net: dsa: ks8995: Add proper RESET delay

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Message-Id: <20250813-ks8995-to-dsa-v1-2-75c359ede3a5@linaro.org>
Date: Wed, 13 Aug 2025 23:43:04 +0200
From: Linus Walleij <linus.walleij@...aro.org>
To: Andrew Lunn <andrew@...n.ch>, Vladimir Oltean <olteanv@...il.com>, 
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
 Heiner Kallweit <hkallweit1@...il.com>, 
 Russell King <linux@...linux.org.uk>
Cc: netdev@...r.kernel.org, Linus Walleij <linus.walleij@...aro.org>
Subject: [PATCH net-next 2/4] net: dsa: ks8995: Add proper RESET delay

According to the datasheet we need to wait 100us before accessing
any registers in the KS8995 after a reset de-assertion.

Add this delay, if and only if we obtained a GPIO descriptor,
otherwise it is just a pointless delay.

Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
---
 drivers/net/dsa/ks8995.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/net/dsa/ks8995.c b/drivers/net/dsa/ks8995.c
index d135b061d810b7ba0c0731d43d176f3ba46b3f52..bdee8c62315f336e380313558c66127ff0b701d3 100644
--- a/drivers/net/dsa/ks8995.c
+++ b/drivers/net/dsa/ks8995.c
@@ -438,9 +438,15 @@ static int ks8995_probe(struct spi_device *spi)
 	if (err)
 		return err;

-	/* de-assert switch reset */
-	/* FIXME: this likely requires a delay */
-	gpiod_set_value_cansleep(ks->reset_gpio, 0);
+	if (ks->reset_gpio) {
+		/*
+		 * If a reset line was obtained, wait for 100us after
+		 * de-asserting RESET before accessing any registers, see
+		 * the KS8995MA datasheet, page 44.
+		 */
+		gpiod_set_value_cansleep(ks->reset_gpio, 0);
+		udelay(100);
+	}

 	spi_set_drvdata(spi, ks);

-- 
2.50.1