mailbox: mailbox-test: don't rely on rx_buffer content to signal data ready

Currently we rely on the first byte of the Rx buffer to check if there's
any data available to be read. If the first byte of the received buffer
is zero (i.e. null character), then we fail to signal that data is
available even when it's available.

Instead introduce a boolean variable to track the data availability and
update it in the channel receive callback as ready and clear it when the
data is read.
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/mailbox-test.c

@@ -30,6 +30,7 @@
 #define MBOX_HEXDUMP_MAX_LEN	(MBOX_HEXDUMP_LINE_LEN *		\
 				 (MBOX_MAX_MSG_LEN / MBOX_BYTES_PER_LINE))
 
+static bool mbox_data_ready;
 static struct dentry *root_debugfs_dir;
 
 struct mbox_test_device {
@@ -152,16 +153,14 @@ static ssize_t mbox_test_message_write(struct file *filp,
 
 static bool mbox_test_message_data_ready(struct mbox_test_device *tdev)
 {
-	unsigned char data;
+	bool data_ready;
 	unsigned long flags;
 
 	spin_lock_irqsave(&tdev->lock, flags);
-	data = tdev->rx_buffer[0];
+	data_ready = mbox_data_ready;
 	spin_unlock_irqrestore(&tdev->lock, flags);
 
-	if (data != '\0')
-		return true;
-	return false;
+	return data_ready;
 }
 
 static ssize_t mbox_test_message_read(struct file *filp, char __user *userbuf,
@@ -223,6 +222,7 @@ static ssize_t mbox_test_message_read(struct file *filp, char __user *userbuf,
 	*(touser + l) = '\0';
 
 	memset(tdev->rx_buffer, 0, MBOX_MAX_MSG_LEN);
+	mbox_data_ready = false;
 
 	spin_unlock_irqrestore(&tdev->lock, flags);
 
@@ -292,6 +292,7 @@ static void mbox_test_receive_message(struct mbox_client *client, void *message)
 				     message, MBOX_MAX_MSG_LEN);
 		memcpy(tdev->rx_buffer, message, MBOX_MAX_MSG_LEN);
 	}
+	mbox_data_ready = true;
 	spin_unlock_irqrestore(&tdev->lock, flags);
 
 	wake_up_interruptible(&tdev->waitq);