mailbox: mailbox-test: add support for fasync/poll

Currently the read operation on the message debug file returns error if
there's no data ready to be read. It expects the userspace to retry if
it fails. Since the mailbox response could be asynchronous, it would be
good to add support to block the read until the data is available.

We can also implement poll file operations so that the userspace can
wait to become ready to perform any I/O.

This patch implements the poll and fasync file operation callback for
the test mailbox device.

Cc: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/mailbox-test.c

@@ -11,12 +11,14 @@
 
 #include <linux/debugfs.h>
 #include <linux/err.h>
+#include <linux/fs.h>
 #include <linux/io.h>
 #include <linux/kernel.h>
 #include <linux/mailbox_client.h>
 #include <linux/module.h>
 #include <linux/of.h>
 #include <linux/platform_device.h>
+#include <linux/poll.h>
 #include <linux/slab.h>
 #include <linux/uaccess.h>
 
@@ -39,6 +41,8 @@ struct mbox_test_device {
 	char			*signal;
 	char			*message;
 	spinlock_t		lock;
+	wait_queue_head_t	waitq;
+	struct fasync_struct	*async_queue;
 };
 
 static ssize_t mbox_test_signal_write(struct file *filp,
@@ -81,6 +85,13 @@ static const struct file_operations mbox_test_signal_ops = {
 	.llseek	= generic_file_llseek,
 };
 
+static int mbox_test_message_fasync(int fd, struct file *filp, int on)
+{
+	struct mbox_test_device *tdev = filp->private_data;
+
+	return fasync_helper(fd, filp, on, &tdev->async_queue);
+}
+
 static ssize_t mbox_test_message_write(struct file *filp,
 				       const char __user *userbuf,
 				       size_t count, loff_t *ppos)
@@ -138,6 +149,20 @@ out:
 	return ret < 0 ? ret : count;
 }
 
+static bool mbox_test_message_data_ready(struct mbox_test_device *tdev)
+{
+	unsigned char data;
+	unsigned long flags;
+
+	spin_lock_irqsave(&tdev->lock, flags);
+	data = tdev->rx_buffer[0];
+	spin_unlock_irqrestore(&tdev->lock, flags);
+
+	if (data != '\0')
+		return true;
+	return false;
+}
+
 static ssize_t mbox_test_message_read(struct file *filp, char __user *userbuf,
 				      size_t count, loff_t *ppos)
 {
@@ -147,6 +172,8 @@ static ssize_t mbox_test_message_read(struct file *filp, char __user *userbuf,
 	int l = 0;
 	int ret;
 
+	DECLARE_WAITQUEUE(wait, current);
+
 	touser = kzalloc(MBOX_HEXDUMP_MAX_LEN + 1, GFP_KERNEL);
 	if (!touser)
 		return -ENOMEM;
@@ -155,15 +182,29 @@ static ssize_t mbox_test_message_read(struct file *filp, char __user *userbuf,
 		ret = snprintf(touser, 20, "<NO RX CAPABILITY>\n");
 		ret = simple_read_from_buffer(userbuf, count, ppos,
 					      touser, ret);
-		goto out;
+		goto kfree_err;
 	}
 
-	if (tdev->rx_buffer[0] == '\0') {
-		ret = snprintf(touser, 9, "<EMPTY>\n");
-		ret = simple_read_from_buffer(userbuf, count, ppos,
-					      touser, ret);
-		goto out;
-	}
+	add_wait_queue(&tdev->waitq, &wait);
+
+	do {
+		__set_current_state(TASK_INTERRUPTIBLE);
+
+		if (mbox_test_message_data_ready(tdev))
+			break;
+
+		if (filp->f_flags & O_NONBLOCK) {
+			ret = -EAGAIN;
+			goto waitq_err;
+		}
+
+		if (signal_pending(current)) {
+			ret = -ERESTARTSYS;
+			goto waitq_err;
+		}
+		schedule();
+
+	} while (1);
 
 	spin_lock_irqsave(&tdev->lock, flags);
 
@@ -185,14 +226,31 @@ static ssize_t mbox_test_message_read(struct file *filp, char __user *userbuf,
 	spin_unlock_irqrestore(&tdev->lock, flags);
 
 	ret = simple_read_from_buffer(userbuf, count, ppos, touser, MBOX_HEXDUMP_MAX_LEN);
-out:
+waitq_err:
+	__set_current_state(TASK_RUNNING);
+	remove_wait_queue(&tdev->waitq, &wait);
+kfree_err:
 	kfree(touser);
 	return ret;
 }
 
+static unsigned int
+mbox_test_message_poll(struct file *filp, struct poll_table_struct *wait)
+{
+	struct mbox_test_device *tdev = filp->private_data;
+
+	poll_wait(filp, &tdev->waitq, wait);
+
+	if (mbox_test_message_data_ready(tdev))
+		return POLLIN | POLLRDNORM;
+	return 0;
+}
+
 static const struct file_operations mbox_test_message_ops = {
 	.write	= mbox_test_message_write,
 	.read	= mbox_test_message_read,
+	.fasync	= mbox_test_message_fasync,
+	.poll	= mbox_test_message_poll,
 	.open	= simple_open,
 	.llseek	= generic_file_llseek,
 };
@@ -234,6 +292,10 @@ static void mbox_test_receive_message(struct mbox_client *client, void *message)
 		memcpy(tdev->rx_buffer, message, MBOX_MAX_MSG_LEN);
 	}
 	spin_unlock_irqrestore(&tdev->lock, flags);
+
+	wake_up_interruptible(&tdev->waitq);
+
+	kill_fasync(&tdev->async_queue, SIGIO, POLL_IN);
 }
 
 static void mbox_test_prepare_message(struct mbox_client *client, void *message)
@@ -334,6 +396,7 @@ static int mbox_test_probe(struct platform_device *pdev)
 	if (ret)
 		return ret;
 
+	init_waitqueue_head(&tdev->waitq);
 	dev_info(&pdev->dev, "Successfully registered\n");
 
 	return 0;