[PATCH v2 2/6] i2c: spacemit: remove stop function to avoid bus error

From: Troy Mitchell <troy.mitchell@linux.spacemit.com>
To: Andi Shyti <andi.shyti@kernel.org>, Yixun Lan <dlan@gentoo.org>,
	 Alex Elder <elder@riscstar.com>,
	Troy Mitchell <troymitchell988@gmail.com>
Cc: linux-i2c@vger.kernel.org, linux-kernel@vger.kernel.org,
	 linux-riscv@lists.infradead.org, spacemit@lists.linux.dev,
	 Troy Mitchell <troy.mitchell@linux.spacemit.com>
Subject: [PATCH v2 2/6] i2c: spacemit: remove stop function to avoid bus error
Date: Thu, 25 Sep 2025 10:02:26 +0800	[thread overview]
Message-ID: <20250925-k1-i2c-atomic-v2-2-46dc13311cda@linux.spacemit.com> (raw)
In-Reply-To: <20250925-k1-i2c-atomic-v2-0-46dc13311cda@linux.spacemit.com>

Previously, STOP handling was split into two separate steps:
  1) clear TB/STOP/START/ACK bits
  2) issue STOP by calling spacemit_i2c_stop()

This left a small window where the control register was updated
twice, which can confuse the controller. While this race has not
been observed with interrupt-driven transfers, it reliably causes
bus errors in PIO mode.

Inline the STOP sequence into the IRQ handler and ensure that
control register bits are updated atomically in a single writel().

Fixes: 5ea558473fa31 ("i2c: spacemit: add support for SpacemiT K1 SoC")
Signed-off-by: Troy Mitchell <troy.mitchell@linux.spacemit.com>
---
 drivers/i2c/busses/i2c-k1.c | 26 +++++++-------------------
 1 file changed, 7 insertions(+), 19 deletions(-)

diff --git a/drivers/i2c/busses/i2c-k1.c b/drivers/i2c/busses/i2c-k1.c
index ee08811f4087c8e709d25dd314854ed643cc5a47..84f132d0504dc992f2f961253b011b86afcc2bbc 100644
--- a/drivers/i2c/busses/i2c-k1.c
+++ b/drivers/i2c/busses/i2c-k1.c
@@ -267,19 +267,6 @@ static void spacemit_i2c_start(struct spacemit_i2c_dev *i2c)
 	writel(val, i2c->base + SPACEMIT_ICR);
 }
 
-static void spacemit_i2c_stop(struct spacemit_i2c_dev *i2c)
-{
-	u32 val;
-
-	val = readl(i2c->base + SPACEMIT_ICR);
-	val |= SPACEMIT_CR_STOP | SPACEMIT_CR_ALDIE | SPACEMIT_CR_TB;
-
-	if (i2c->read)
-		val |= SPACEMIT_CR_ACKNAK;
-
-	writel(val, i2c->base + SPACEMIT_ICR);
-}
-
 static int spacemit_i2c_xfer_msg(struct spacemit_i2c_dev *i2c)
 {
 	unsigned long time_left;
@@ -412,7 +399,6 @@ static irqreturn_t spacemit_i2c_irq_handler(int irq, void *devid)
 
 	val = readl(i2c->base + SPACEMIT_ICR);
 	val &= ~(SPACEMIT_CR_TB | SPACEMIT_CR_ACKNAK | SPACEMIT_CR_STOP | SPACEMIT_CR_START);
-	writel(val, i2c->base + SPACEMIT_ICR);
 
 	switch (i2c->state) {
 	case SPACEMIT_STATE_START:
@@ -429,14 +415,16 @@ static irqreturn_t spacemit_i2c_irq_handler(int irq, void *devid)
 	}
 
 	if (i2c->state != SPACEMIT_STATE_IDLE) {
+		val |= SPACEMIT_CR_TB | SPACEMIT_CR_ALDIE;
+
 		if (spacemit_i2c_is_last_msg(i2c)) {
 			/* trigger next byte with stop */
-			spacemit_i2c_stop(i2c);
-		} else {
-			/* trigger next byte */
-			val |= SPACEMIT_CR_ALDIE | SPACEMIT_CR_TB;
-			writel(val, i2c->base + SPACEMIT_ICR);
+			val |= SPACEMIT_CR_STOP;
+
+			if (i2c->read)
+				val |= SPACEMIT_CR_ACKNAK;
 		}
+		writel(val, i2c->base + SPACEMIT_ICR);
 	}
 
 err_out:

-- 
2.51.0


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