drivers: i2c: add an option to skip auto-sending stop on last message

The I2C transfer API has been recently changed to always automatically
set a STOP on the last message, which was well documented but
implemented only by few drivers.

Unfortunately, while documented, this is a change in the current
behavior and it turns out that some applications depended on it for some
complex operations.

Add a flag to temporarily restore the old behavior, buying time to fix
the application code depending on this.

Signed-off-by: Fabio Baltieri <[email protected]>

drivers/i2c/Kconfig

@@ -54,6 +54,14 @@ config I2C_CALLBACK
 	help
 	  API and implementations of i2c_transfer_cb.
 
+config I2C_LAST_MESSAGE_IMPLICIT_STOP
+	bool "Always send a STOP on the last message of an I2C transfer"
+	default y
+	help
+	  The last message of an I2C transaction implies a STOP whether or not
+	  it is explicitly set. This is the default behavior and the option
+	  will be removed in Zephyr 4.1.
+
 config HAS_I2C_RTIO
 	bool
 	help

include/zephyr/drivers/i2c.h

@@ -789,7 +789,9 @@ static inline int z_impl_i2c_transfer(const struct device *dev,
 		return 0;
 	}
 
-	msgs[num_msgs - 1].flags |= I2C_MSG_STOP;
+	if (IS_ENABLED(CONFIG_I2C_LAST_MESSAGE_IMPLICIT_STOP)) {
+		msgs[num_msgs - 1].flags |= I2C_MSG_STOP;
+	}
 
 	int res =  api->transfer(dev, msgs, num_msgs, addr);
 
@@ -845,7 +847,9 @@ static inline int i2c_transfer_cb(const struct device *dev,
 		return 0;
 	}
 
-	msgs[num_msgs - 1].flags |= I2C_MSG_STOP;
+	if (IS_ENABLED(CONFIG_I2C_LAST_MESSAGE_IMPLICIT_STOP)) {
+		msgs[num_msgs - 1].flags |= I2C_MSG_STOP;
+	}
 
 	return api->transfer_cb(dev, msgs, num_msgs, addr, cb, userdata);
 }