i2c: altera: Fix race between xfer_msg and isr thread

Use a mutex to protect access to idev->msg_len, idev->buf, etc. which
are modified by both altr_i2c_xfer_msg() and altr_i2c_isr().

This is the minimal fix for easy backporting. A cleanup to remove the
spinlock will be added later.

Signed-off-by: Atsushi Nemoto <atsushi.nemoto@sord.co.jp>
Acked-by: Thor Thayer <thor.thayer@linux.intel.com>
[wsa: updated commit message]
Signed-off-by: Wolfram Sang <wsa@kernel.org>

drivers/i2c/busses/i2c-altera.c

@@ -70,6 +70,7 @@
  * @isr_mask: cached copy of local ISR enables.
  * @isr_status: cached copy of local ISR status.
  * @lock: spinlock for IRQ synchronization.
+ * @isr_mutex: mutex for IRQ thread.
  */
 struct altr_i2c_dev {
 	void __iomem *base;
@@ -86,6 +87,7 @@ struct altr_i2c_dev {
 	u32 isr_mask;
 	u32 isr_status;
 	spinlock_t lock;	/* IRQ synchronization */
+	struct mutex isr_mutex;
 };
 
 static void
@@ -245,10 +247,11 @@ static irqreturn_t altr_i2c_isr(int irq, void *_dev)
 	struct altr_i2c_dev *idev = _dev;
 	u32 status = idev->isr_status;
 
+	mutex_lock(&idev->isr_mutex);
 	if (!idev->msg) {
 		dev_warn(idev->dev, "unexpected interrupt\n");
 		altr_i2c_int_clear(idev, ALTR_I2C_ALL_IRQ);
-		return IRQ_HANDLED;
+		goto out;
 	}
 	read = (idev->msg->flags & I2C_M_RD) != 0;
 
@@ -301,6 +304,8 @@ static irqreturn_t altr_i2c_isr(int irq, void *_dev)
 		complete(&idev->msg_complete);
 		dev_dbg(idev->dev, "Message Complete\n");
 	}
+out:
+	mutex_unlock(&idev->isr_mutex);
 
 	return IRQ_HANDLED;
 }
@@ -312,6 +317,7 @@ static int altr_i2c_xfer_msg(struct altr_i2c_dev *idev, struct i2c_msg *msg)
 	u32 value;
 	u8 addr = i2c_8bit_addr_from_msg(msg);
 
+	mutex_lock(&idev->isr_mutex);
 	idev->msg = msg;
 	idev->msg_len = msg->len;
 	idev->buf = msg->buf;
@@ -336,6 +342,7 @@ static int altr_i2c_xfer_msg(struct altr_i2c_dev *idev, struct i2c_msg *msg)
 		altr_i2c_int_enable(idev, imask, true);
 		altr_i2c_fill_tx_fifo(idev);
 	}
+	mutex_unlock(&idev->isr_mutex);
 
 	time_left = wait_for_completion_timeout(&idev->msg_complete,
 						ALTR_I2C_XFER_TIMEOUT);
@@ -409,6 +416,7 @@ static int altr_i2c_probe(struct platform_device *pdev)
 	idev->dev = &pdev->dev;
 	init_completion(&idev->msg_complete);
 	spin_lock_init(&idev->lock);
+	mutex_init(&idev->isr_mutex);
 
 	ret = device_property_read_u32(idev->dev, "fifo-size",
 				       &idev->fifo_size);