linux-kernel - [PATCH 3/6] i2c: spacemit: disable SDA glitch fix to avoid restart delay

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Message-Id: <20250827-k1-i2c-atomic-v1-3-e59bea02d680@linux.spacemit.com>
Date: Wed, 27 Aug 2025 15:39:10 +0800
From: Troy Mitchell <troy.mitchell@...ux.spacemit.com>
To: Andi Shyti <andi.shyti@...nel.org>, Yixun Lan <dlan@...too.org>, 
 Alex Elder <elder@...cstar.com>, Troy Mitchell <troymitchell988@...il.com>
Cc: linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-riscv@...ts.infradead.org, spacemit@...ts.linux.dev, 
 Troy Mitchell <troy.mitchell@...ux.spacemit.com>
Subject: [PATCH 3/6] i2c: spacemit: disable SDA glitch fix to avoid restart
 delay

The K1 I2C controller has an SDA glitch fix that introduces a small
delay on restart signals. While this feature can suppress glitches
on SDA when SCL = 0, it also delays the restart signal, which may
cause unexpected behavior in some transfers.

The glitch itself does not affect normal I2C operation, because
the I2C specification allows SDA to change while SCL is low.

To ensure correct transmission for every message, we disable the
SDA glitch fix by setting the RCR.SDA_GLITCH_NOFIX bit during
initialization.

This guarantees that restarts are issued promptly without
unintended delays.

Fixes: 5ea558473fa31 ("i2c: spacemit: add support for SpacemiT K1 SoC")
Signed-off-by: Troy Mitchell <troy.mitchell@...ux.spacemit.com>
---
 drivers/i2c/busses/i2c-k1.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/i2c/busses/i2c-k1.c b/drivers/i2c/busses/i2c-k1.c
index d342752030d077953adf84a2886211de96e843c4..c1656b78f1681729ccc2ebca6e290259d14889d9 100644
--- a/drivers/i2c/busses/i2c-k1.c
+++ b/drivers/i2c/busses/i2c-k1.c
@@ -14,6 +14,7 @@
 #define SPACEMIT_ICR		 0x0		/* Control register */
 #define SPACEMIT_ISR		 0x4		/* Status register */
 #define SPACEMIT_IDBR		 0xc		/* Data buffer register */
+#define SPACEMIT_IRCR		 0x18		/* Reset cycle counter */
 #define SPACEMIT_IBMR		 0x1c		/* Bus monitor register */
 
 /* SPACEMIT_ICR register fields */
@@ -76,6 +77,8 @@
 					SPACEMIT_SR_GCAD | SPACEMIT_SR_IRF | SPACEMIT_SR_ITE | \
 					SPACEMIT_SR_ALD)
 
+#define SPACEMIT_RCR_SDA_GLITCH_NOFIX		BIT(7)		/* bypass the SDA glitch fix */
+
 /* SPACEMIT_IBMR register fields */
 #define SPACEMIT_BMR_SDA         BIT(0)		/* SDA line level */
 #define SPACEMIT_BMR_SCL         BIT(1)		/* SCL line level */
@@ -237,6 +240,14 @@ static void spacemit_i2c_init(struct spacemit_i2c_dev *i2c)
 	val |= SPACEMIT_CR_MSDE | SPACEMIT_CR_MSDIE;
 
 	writel(val, i2c->base + SPACEMIT_ICR);
+
+	/*
+	 * The glitch fix in the K1 I2C controller introduces a delay
+	 * on restart signals, so we disable the fix here.
+	 */
+	val = readl(i2c->base + SPACEMIT_IRCR);
+	val |= SPACEMIT_RCR_SDA_GLITCH_NOFIX;
+	writel(val, i2c->base + SPACEMIT_IRCR);
 }
 
 static inline void

-- 
2.50.1