From 01e3a7b3d1c481bf8b6187d90a739327a3f94e87 Mon Sep 17 00:00:00 2001
From: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Date: Tue, 8 Dec 2009 12:46:10 -0200
Subject: [PATCH] Allow writing to a buffer for our devices.

---
 helloc.c | 46 +++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 41 insertions(+), 5 deletions(-)

diff --git a/helloc.c b/helloc.c
index 8c085f3..00ce97d 100644
--- a/helloc.c
+++ b/helloc.c
@@ -12,9 +12,11 @@ MODULE_AUTHOR("Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>");
 MODULE_DESCRIPTION("A hello world char device");
 MODULE_VERSION("1.0.0");
 
+#define BUFFER_SIZE 4096
+
 /* Message buffer we send to upstream */
-static char hello_message[] = "hello, world\n";
-static char goodbye_message[] = "goodbye, world\n";
+static char *hello_message;
+static char *goodbye_message;
 
 struct message {
 	char *text;
@@ -30,10 +32,10 @@ static int helloc_open(struct inode *ino, struct file *filp)
 		return -ENOMEM;
 	if (iminor(ino) == 0) {
 		msg->text = hello_message;
-		msg->len = sizeof(hello_message);
+		msg->len = BUFFER_SIZE;
 	} else {
 		msg->text = goodbye_message;
-		msg->len = sizeof(goodbye_message);
+		msg->len = BUFFER_SIZE;
 	}
 	filp->private_data = msg;
 	return 0;
@@ -67,12 +69,34 @@ static ssize_t helloc_read(struct file *filp, char __user *buf, size_t len,
 	return len;
 }
 
+static ssize_t helloc_write(struct file *filp, const char __user *buf,
+			    size_t len, loff_t *pos)
+{
+	int r;
+	struct message *msg = filp->private_data;
+	/* do not read pass through the size of the message */
+	if (*pos >= msg->len)
+	/* return end of file */
+		return 0;
+	/* if len is bigger than the rest of the message, clamp it */
+	if (len > msg->len - *pos)
+		len = msg->len - *pos;
+	/* copy message to user space and return error if it fails */
+	r = copy_from_user(msg->text + *pos, buf, len);
+	if (r)
+		return -EFAULT;
+	/* update the file position */
+	*pos += len;
+	return len;
+}
+
 /* we only implement read */
 static struct file_operations helloc_fops = {
 	.owner = THIS_MODULE,
 	.open = helloc_open,
 	.release = helloc_release,
 	.read = helloc_read,
+	.write = helloc_write,
 };
 
 /* the device number and the char device struct */
@@ -81,8 +105,14 @@ static struct cdev *cdev;
 
 static int __init helloc_init(void)
 {
-	int r;
+	int r = -ENOMEM;
 	/* allocate any major number with only one minor */
+	hello_message = kzalloc(BUFFER_SIZE, GFP_KERNEL);
+	if (!hello_message)
+		goto out_hello;
+	goodbye_message = kzalloc(BUFFER_SIZE, GFP_KERNEL);
+	if (!goodbye_message)
+		goto out_goodbye;
 	r = alloc_chrdev_region(&dev, 0, 2, "helloc");
 	if (r)
 		goto out_region;
@@ -108,6 +138,10 @@ out_alloc:
 	/* release the device number allocated */
 	unregister_chrdev_region(dev, 2);
 out_region:
+	kfree(goodbye_message);
+out_goodbye:
+	kfree(hello_message);
+out_hello:
 	return r;
 }
 
@@ -117,6 +151,8 @@ static void __exit helloc_exit(void)
 	cdev_del(cdev);
 	/* release the device number allocated */
 	unregister_chrdev_region(dev, 2);
+	kfree(goodbye_message);
+	kfree(hello_message);
 }
 
 module_init(helloc_init);
-- 
2.20.1