Use a circular buffer.

diff --git a/hellochar.c b/hellochar.c
index 1b0c9f2..de99828 100644 (file)
--- 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);