linux-kernel - [PATCH] i2c: stm32f7: Fix PEC handling in case of SMBUS transfers

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Message-ID: <20231002084211.1108940-1-alain.volmat@foss.st.com>
Date:   Mon, 2 Oct 2023 10:42:10 +0200
From:   Alain Volmat <alain.volmat@...s.st.com>
To:     Pierre-Yves MORDRET <pierre-yves.mordret@...s.st.com>,
        Alain Volmat <alain.volmat@...s.st.com>,
        Andi Shyti <andi.shyti@...nel.org>,
        "Maxime Coquelin" <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        M'boumba Cedric Madianga <cedric.madianga@...il.com>,
        Wolfram Sang <wsa@...nel.org>
CC:     Pierre-Yves MORDRET <pierre-yves.mordret@...com>,
        <linux-i2c@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: [PATCH] i2c: stm32f7: Fix PEC handling in case of SMBUS transfers

The PECBYTE bit allows to generate (in case of write) or
compute/compare the PEC byte (in case of read).  In case
of reading a value (performed by first sending a write
command, then followed by a read command) the PECBYTE should
only be set before starting the read command and not before
the first write command.

Fixes: 9e48155f6bfe ("i2c: i2c-stm32f7: Add initial SMBus protocols support")

Signed-off-by: Alain Volmat <alain.volmat@...s.st.com>
---
 drivers/i2c/busses/i2c-stm32f7.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/i2c/busses/i2c-stm32f7.c b/drivers/i2c/busses/i2c-stm32f7.c
index 579b30581725..0d3c9a041b56 100644
--- a/drivers/i2c/busses/i2c-stm32f7.c
+++ b/drivers/i2c/busses/i2c-stm32f7.c
@@ -1059,9 +1059,10 @@ static int stm32f7_i2c_smbus_xfer_msg(struct stm32f7_i2c_dev *i2c_dev,
 	/* Configure PEC */
 	if ((flags & I2C_CLIENT_PEC) && f7_msg->size != I2C_SMBUS_QUICK) {
 		cr1 |= STM32F7_I2C_CR1_PECEN;
-		cr2 |= STM32F7_I2C_CR2_PECBYTE;
-		if (!f7_msg->read_write)
+		if (!f7_msg->read_write) {
+			cr2 |= STM32F7_I2C_CR2_PECBYTE;
 			f7_msg->count++;
+		}
 	} else {
 		cr1 &= ~STM32F7_I2C_CR1_PECEN;
 		cr2 &= ~STM32F7_I2C_CR2_PECBYTE;
@@ -1149,8 +1150,10 @@ static void stm32f7_i2c_smbus_rep_start(struct stm32f7_i2c_dev *i2c_dev)
 	f7_msg->stop = true;
 
 	/* Add one byte for PEC if needed */
-	if (cr1 & STM32F7_I2C_CR1_PECEN)
+	if (cr1 & STM32F7_I2C_CR1_PECEN) {
+		cr2 |= STM32F7_I2C_CR2_PECBYTE;
 		f7_msg->count++;
+	}
 
 	/* Set number of bytes to be transferred */
 	cr2 &= ~(STM32F7_I2C_CR2_NBYTES_MASK);
-- 
2.25.1