[PATCH v3] i2c: i801: fix potential race in i801_block_transaction_byte_by_byte

[Heiner Kallweit <hkallweit1@gmail.com>]

Currently we set SMBHSTCNT_LAST_BYTE only after the host has started
receiving the last byte. If we get e.g. preempted before setting
SMBHSTCNT_LAST_BYTE, the host may be finished with receiving the byte
before SMBHSTCNT_LAST_BYTE is set.
Therefore change the code to set SMBHSTCNT_LAST_BYTE before writing
SMBHSTSTS_BYTE_DONE for the byte before the last byte. Now the code
is also consistent with what we do in i801_isr_byte_done().

Reported-by: Jean Delvare <jdelvare@suse.com>
Closes: https://lore.kernel.org/linux-i2c/20230828152747.09444625@endymion.delvare/
Cc: stable@vger.kernel.org
Acked-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
---
v3:
- Apparently potential issue has been there forever, therefore cc
  to stable w/o Fixes tag.
---
 drivers/i2c/busses/i2c-i801.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/drivers/i2c/busses/i2c-i801.c b/drivers/i2c/busses/i2c-i801.c
index 7a0ccc584..8acf09539 100644
--- a/drivers/i2c/busses/i2c-i801.c
+++ b/drivers/i2c/busses/i2c-i801.c
@@ -679,15 +679,11 @@ static int i801_block_transaction_byte_by_byte(struct i801_priv *priv,
 		return result ? priv->status : -ETIMEDOUT;
 	}
 
-	for (i = 1; i <= len; i++) {
-		if (i == len && read_write == I2C_SMBUS_READ)
-			smbcmd |= SMBHSTCNT_LAST_BYTE;
-		outb_p(smbcmd, SMBHSTCNT(priv));
-
-		if (i == 1)
-			outb_p(inb(SMBHSTCNT(priv)) | SMBHSTCNT_START,
-			       SMBHSTCNT(priv));
+	if (len == 1 && read_write == I2C_SMBUS_READ)
+		smbcmd |= SMBHSTCNT_LAST_BYTE;
+	outb_p(smbcmd | SMBHSTCNT_START, SMBHSTCNT(priv));
 
+	for (i = 1; i <= len; i++) {
 		status = i801_wait_byte_done(priv);
 		if (status)
 			return status;
@@ -710,9 +706,12 @@ static int i801_block_transaction_byte_by_byte(struct i801_priv *priv,
 			data->block[0] = len;
 		}
 
-		/* Retrieve/store value in SMBBLKDAT */
-		if (read_write == I2C_SMBUS_READ)
+		if (read_write == I2C_SMBUS_READ) {
 			data->block[i] = inb_p(SMBBLKDAT(priv));
+			if (i == len - 1)
+				outb_p(smbcmd | SMBHSTCNT_LAST_BYTE, SMBHSTCNT(priv));
+		}
+
 		if (read_write == I2C_SMBUS_WRITE && i+1 <= len)
 			outb_p(data->block[i+1], SMBBLKDAT(priv));
 
-- 
2.42.0