i2c: i801: fix potential race in i801_block_transaction_byte_by_byte

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 <[email protected]>
Closes: https://lore.kernel.org/linux-i2c/[email protected]/
Cc: [email protected]
Acked-by: Andi Shyti <[email protected]>
Signed-off-by: Heiner Kallweit <[email protected]>
Reviewed-by: Jean Delvare <[email protected]>
Signed-off-by: Wolfram Sang <[email protected]>

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));