From: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Date: Fri, 21 May 2010 18:14:45 +0000 (-0400)
Subject: Use a circular buffer.
X-Git-Url: http://git.cascardo.eti.br/?p=cascardo%2Fkernel%2Fsamples%2Fchar2%2F.git;a=commitdiff_plain;h=91e474b7d6c004d5c0d3c5c1ff94721ad64d442d

Use a circular buffer.
---

diff --git a/hellochar.c b/hellochar.c
index 1b0c9f2..de99828 100644
--- a/hellochar.c
+++ b/hellochar.c
@@ -22,32 +22,21 @@
 #include <linux/slab.h>
 #include <linux/mutex.h>
 #include <asm/uaccess.h>
+#include <linux/circ_buf.h>
 
 MODULE_LICENSE("GPL");
 
-#define MAXLEN 4000
+#define MAXLEN 4096
 static dev_t devnum;
 static struct cdev *dev;
 static const char default_greeting[] = "Hello, World!\n";
 
-struct hello_buffer {
-	size_t len;
-	char buffer[0];
-};
-static struct hello_buffer *hello;
+static struct circ_buf *hello;
+static struct circ_buf hello_instance;
 static DEFINE_MUTEX(hello_mtx);
 
 static int hello_open(struct inode *ino, struct file *fp)
 {
-	if (mutex_lock_interruptible(&hello_mtx))
-		return -ERESTARTSYS;
-	if (fp->f_flags & O_TRUNC) {
-		memset(hello->buffer, 0, MAXLEN);
-		hello->len = 0;
-	}
-	if (fp->f_flags & O_APPEND)
-		fp->f_pos = hello->len;
-	mutex_unlock(&hello_mtx);
 	return 0;
 }
 
@@ -55,15 +44,25 @@ static ssize_t hello_read(struct file *fp, char __user *buf, size_t sz,
 	loff_t *pos)
 {
 	int r;
+	size_t fsz;
+	size_t ssz;
+	size_t len;
 	if (mutex_lock_interruptible(&hello_mtx))
 		return -ERESTARTSYS;
-	if (sz + *pos > hello->len)
-		sz = hello->len - *pos;
-	r = copy_to_user(buf, hello->buffer + *pos, sz);
+	len = CIRC_CNT(hello->head, hello->tail, MAXLEN);
+	if (sz > len)
+		sz = len;
+	/* First, copy at most until the limit of the buffer */
+	fsz = min(sz, CIRC_CNT_TO_END(hello->head, hello->tail, MAXLEN));
+	r = copy_to_user(buf, hello->buf + hello->tail, fsz);
+	ssz = sz - fsz;
+	if (!r && !ssz)
+		r = copy_to_user(buf + fsz, hello->buf, ssz);
+	if (!r)
+		hello->tail = (hello->tail + sz) & ~MAXLEN;
 	mutex_unlock(&hello_mtx);
 	if (r)
 		return -EFAULT;
-	*pos += sz;
 	return sz;
 }
 
@@ -71,19 +70,24 @@ static ssize_t hello_write(struct file *fp, const char __user *buf, size_t sz,
 	loff_t *pos)
 {
 	int r;
+	size_t fsz;
+	size_t ssz;
+	size_t len;
 	if (mutex_lock_interruptible(&hello_mtx))
 		return -ERESTARTSYS;
-	if (sz + *pos > MAXLEN)
-		sz = MAXLEN - *pos;
-	r = copy_from_user(hello->buffer + *pos, buf, sz);
-	if (r) {
-		mutex_unlock(&hello_mtx);
-		return -EFAULT;
-	}
-	*pos += sz;
-	if (hello->len < *pos)
-		hello->len = *pos;
+	len = CIRC_SPACE(hello->head, hello->tail, MAXLEN);
+	if (sz > len)
+		sz = len;
+	fsz = min(sz, CIRC_SPACE_TO_END(hello->head, hello->tail, MAXLEN));
+	r = copy_from_user(hello->buf + hello->head, buf, fsz);
+	ssz = sz - fsz;
+	if (!r && !ssz)
+		r = copy_from_user(hello->buf, buf + fsz, ssz);
+	if (!r)
+		hello->head = (hello->head + sz) & ~MAXLEN;
 	mutex_unlock(&hello_mtx);
+	if (r)
+		return -EFAULT;
 	return sz;
 }
 
@@ -103,13 +107,14 @@ static const struct file_operations hello_fops = {
 static int __init ch_init(void)
 {
 	int r = 0;
-	hello = kzalloc(sizeof(*hello) + MAXLEN, GFP_KERNEL);
+	hello = &hello_instance;
+	hello->buf = kzalloc(sizeof(*hello) + MAXLEN, GFP_KERNEL);
 	if (!hello) {
 		r = -ENOMEM;
 		goto out;
 	}
-	memcpy(hello->buffer, default_greeting, sizeof(default_greeting));
-	hello->len = sizeof(default_greeting);
+	memcpy(hello->buf, default_greeting, sizeof(default_greeting));
+	hello->head = sizeof(default_greeting);
 	r = alloc_chrdev_region(&devnum, 0, 256, "hello");
 	if (r)
 		goto reg_out;
@@ -122,7 +127,6 @@ static int __init ch_init(void)
 	r = cdev_add(dev, devnum, 256);
 	if (r)
 		goto add_out;
-	printk(KERN_DEBUG "Allocate major %d\n", MAJOR(devnum));
 	return 0;
 add_out:
 	kfree(dev);