i2c: designware: fix race between subsequent xfers

The designware block is not always properly disabled in the case of
transfer errors. Interrupts from aborted transfers might be handled
after the data structures for the following transfer are initialised but
before the hardware is set up. This can corrupt the data structures to
the point that the system is stuck in an infinite interrupt loop (where
FIFOs are never emptied because dev->msg_read_idx == dev->msgs_num).

This patch cleanly disables the designware-i2c hardware at the end of
every transfer, be it successful or not.
Signed-off-by: Christian Ruppert <christian.ruppert@abilis.com>
[wsa: extended the comment]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

drivers/i2c/busses/i2c-designware-core.c

@@ -583,11 +583,21 @@ i2c_dw_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[], int num)
 	ret = wait_for_completion_timeout(&dev->cmd_complete, HZ);
 	if (ret == 0) {
 		dev_err(dev->dev, "controller timed out\n");
+		/* i2c_dw_init implicitly disables the adapter */
 		i2c_dw_init(dev);
 		ret = -ETIMEDOUT;
 		goto done;
 	}
 
+	/*
+	 * We must disable the adapter before unlocking the &dev->lock mutex
+	 * below. Otherwise the hardware might continue generating interrupts
+	 * which in turn causes a race condition with the following transfer.
+	 * Needs some more investigation if the additional interrupts are
+	 * a hardware bug or this driver doesn't handle them correctly yet.
+	 */
+	__i2c_dw_enable(dev, false);
+
 	if (dev->msg_err) {
 		ret = dev->msg_err;
 		goto done;
@@ -595,8 +605,6 @@ i2c_dw_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[], int num)
 
 	/* no error */
 	if (likely(!dev->cmd_err)) {
-		/* Disable the adapter */
-		__i2c_dw_enable(dev, false);
 		ret = num;
 		goto done;
 	}