“上回书”说到kernel源代码/drivers/video/fbmem.c中有一些函数和结构需要弄清楚，我们在这里一个一个地了解。
module_init(fbmem_init);
subsys_initcall(fbmem_init);
无论是模块中还是子系统中初始化函数都调用了fbmem_init(),说明它对于framebuffer驱动来说极其重要，应该是所有程序的源头。
fbmem_init()在同文件中实现：
/**
* fbmem_init - init frame buffer subsystem
*
* Initialize the frame buffer subsystem.
*
* NOTE: This function is _only_ to be called by drivers/char/mem.c.
*
*/
static int __init
fbmem_init(void)
{
proc_create("fb", 0, NULL, &fb_proc_fops);
if (register_chrdev(FB_MAJOR,"fb",&fb_fops))
printk("unable to get major %d for fb devs\n", FB_MAJOR);
fb_class = class_create(THIS_MODULE, "graphics");
if (IS_ERR(fb_class)) {
printk(KERN_WARNING "Unable to create fb class; errno = %ld\n", PTR_ERR(fb_class));
fb_class = NULL;
}
return 0;
}
根据其函数说明，可知fbmen_init()的作用是初始化frame buffer子系统。下面来分析其具体内容：
其中调用了3个函数，先说第一个：proc_create() 。
创建一个 proc 文件
struct proc_dir_entry *proc_create(const char *name, mode_t mode, struct proc_dir_entry *parent, const struct file_operations *proc_fops)
参数 ：
name - 要创建的 proc 文件名；
mode - 该文件权限值，例如 S_IRUGO，可传入0表示采用系统默认值；
parent - 指定该文件的上层 proc 目录项，如果为 NULL，表示创建在 /proc 根目录下；
proc_fops - 如果要创建一个 proc 文件，并且不用 proc_fs 默认提供的 file_operations 的话，可以使用 proc_create() 函数，通过最后一个参数来指定要创建的 proc 文件的 file_operations。
根据上述解释，proc_create("fb", 0, NULL, &fb_proc_fops);这句的作用是在"/proc"下建立一个名为"fb"的文件，不使用proc文件系统默认提供的file_operation,而是单独提供名为"fb_proc_fops"的file_operation结构。此结构体在同文件中定义：
static const struct file_operations fb_proc_fops = {
.owner = THIS_MODULE,
.open = proc_fb_open,
.read = seq_read,
.llseek = seq_lseek,
.release = seq_release,
};
其中proc_fb_open()在同文件中实现：
static int proc_fb_open(struct inode *inode, struct file *file)
{
return seq_open(file, &proc_fb_seq_ops);
}
proc_fb_seq_ops结构体也在同文件中实现，就在proc_fb_open()的上头：
static const struct seq_operations proc_fb_seq_ops = {
.start = fb_seq_start,
.next = fb_seq_next,
.stop = fb_seq_stop,
.show = fb_seq_show,
};
proc_fb_seq_ops结构体中定义的4个函数在同文件中实现：
static void *fb_seq_start(struct seq_file *m, loff_t *pos)
{
mutex_lock(®istration_lock);
return (*pos < FB_MAX) ? pos : NULL;
}
static void *fb_seq_next(struct seq_file *m, void *v, loff_t *pos)
{
(*pos)++;
return (*pos < FB_MAX) ? pos : NULL;
}
static void fb_seq_stop(struct seq_file *m, void *v)
{
mutex_unlock(®istration_lock);
}
static int fb_seq_show(struct seq_file *m, void *v)
{
int i = *(loff_t *)v;
struct fb_info *fi = registered_fb[i];
if (fi)
seq_printf(m, "%d %s\n", fi->node, fi->fix.id);
return 0;
}
FB_MAX的定义在kernel源码/include/linux/fb.h中:
#define FB_MAX 32 /* sufficient for now */
即系统中最多有32个framebuffer设备。
上述内容中,有关seq_xx的函数被跳过了,因为seq_xx()的实现都不在本文件中,而是在kernel源代码/fs/seq_file.c中：
/**
* seq_open - initialize sequential file
* @file: file we initialize
* @op: method table describing the sequence
*
* seq_open() sets @file, associating it with a sequence described
* by @op. @op->start() sets the iterator up and returns the first
* element of sequence. @op->stop() shuts it down. @op->next()
* returns the next element of sequence. @op->show() prints element
* into the buffer. In case of error ->start() and ->next() return
* ERR_PTR(error). In the end of sequence they return %NULL. ->show()
* returns 0 in case of success and negative number in case of error.
* Returning SEQ_SKIP means "discard this element and move on".
*/
int seq_open(struct file *file, const struct seq_operations *op)
{
struct seq_file *p = file->private_data;
if (!p) {
p = kmalloc(sizeof(*p), GFP_KERNEL);
if (!p)
return -ENOMEM;
file->private_data = p;
}
memset(p, 0, sizeof(*p));
mutex_init(&p->lock);
p->op = op;
/*
* Wrappers around seq_open(e.g. swaps_open) need to be
* aware of this. If they set f_version themselves, they
* should call seq_open first and then set f_version.
*/
file->f_version = 0;
/*
* seq_files support lseek() and pread(). They do not implement
* write() at all, but we clear FMODE_PWRITE here for historical
* reasons.
*
* If a client of seq_files a) implements file.write() and b) wishes to
* support pwrite() then that client will need to implement its own
* file.open() which calls seq_open() and then sets FMODE_PWRITE.
*/
file->f_mode &= ~FMODE_PWRITE;
return 0;
}
EXPORT_SYMBOL(seq_open);
/**
* seq_release - free the structures associated with sequential file.
* @file: file in question
* @inode: file->f_path.dentry->d_inode
*
* Frees the structures associated with sequential file; can be used
* as ->f_op->release() if you don't have private data to destroy.
*/
int seq_release(struct inode *inode, struct file *file)
{
struct seq_file *m = file->private_data;
kfree(m->buf);
kfree(m);
return 0;
}
EXPORT_SYMBOL(seq_release);
/**
* seq_read - ->read() method for sequential files.
* @file: the file to read from
* @buf: the buffer to read to
* @size: the maximum number of bytes to read
* @ppos: the current position in the file
*
* Ready-made ->f_op->read()
*/
ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
{
struct seq_file *m = file->private_data;
size_t copied = 0;
loff_t pos;
size_t n;
void *p;
int err = 0;
mutex_lock(&m->lock);
/* Don't assume *ppos is where we left it */
if (unlikely(*ppos != m->read_pos)) {
m->read_pos = *ppos;
while ((err = traverse(m, *ppos)) == -EAGAIN)
;
if (err) {
/* With prejudice... */
m->read_pos = 0;
m->version = 0;
m->index = 0;
m->count = 0;
goto Done;
}
}
/*
* seq_file->op->..m_start/m_stop/m_next may do special actions
* or optimisations based on the file->f_version, so we want to
* pass the file->f_version to those methods.
*
* seq_file->version is just copy of f_version, and seq_file
* methods can treat it simply as file version.
* It is copied in first and copied out after all operations.
* It is convenient to have it as part of structure to avoid the
* need of passing another argument to all the seq_file methods.
*/
m->version = file->f_version;
/* grab buffer if we didn't have one */
if (!m->buf) {
m->buf = kmalloc(m->size = PAGE_SIZE, GFP_KERNEL);
if (!m->buf)
goto Enomem;
}
/* if not empty - flush it first */
if (m->count) {
n = min(m->count, size);
err = copy_to_user(buf, m->buf + m->from, n);
if (err)
goto Efault;
m->count -= n;
m->from += n;
size -= n;
buf += n;
copied += n;
if (!m->count)
m->index++;
if (!size)
goto Done;
}
/* we need at least one record in buffer */
pos = m->index;
p = m->op->start(m, &pos);
while (1) {
err = PTR_ERR(p);
if (!p || IS_ERR(p))
break;
err = m->op->show(m, p);
if (err < 0)
break;
if (unlikely(err))
m->count = 0;
if (unlikely(!m->count)) {
p = m->op->next(m, p, &pos);
m->index = pos;
continue;
}
if (m->count < m->size)
goto Fill;
m->op->stop(m, p);
kfree(m->buf);
m->buf = kmalloc(m->size <<= 1, GFP_KERNEL);
if (!m->buf)
goto Enomem;
m->count = 0;
m->version = 0;
pos = m->index;
p = m->op->start(m, &pos);
}
m->op->stop(m, p);
m->count = 0;
goto Done;
Fill:
/* they want more? let's try to get some more */
while (m->count < size) {
size_t offs = m->count;
loff_t next = pos;
p = m->op->next(m, p, &next);
if (!p || IS_ERR(p)) {
err = PTR_ERR(p);
break;
}
err = m->op->show(m, p);
if (m->count == m->size || err) {
m->count = offs;
if (likely(err <= 0))
break;
}
pos = next;
}
m->op->stop(m, p);
n = min(m->count, size);
err = copy_to_user(buf, m->buf, n);
if (err)
goto Efault;
copied += n;
m->count -= n;
if (m->count)
m->from = n;
else
pos++;
m->index = pos;
Done:
if (!copied)
copied = err;
else {
*ppos += copied;
m->read_pos += copied;
}
file->f_version = m->version;
mutex_unlock(&m->lock);
return copied;
Enomem:
err = -ENOMEM;
goto Done;
Efault:
err = -EFAULT;
goto Done;
}
EXPORT_SYMBOL(seq_read);
/**
* seq_lseek - ->llseek() method for sequential files.
* @file: the file in question
* @offset: new position
* @origin: 0 for absolute, 1 for relative position
*
* Ready-made ->f_op->llseek()
*/
loff_t seq_lseek(struct file *file, loff_t offset, int origin)
{
struct seq_file *m = file->private_data;
loff_t retval = -EINVAL;
mutex_lock(&m->lock);
m->version = file->f_version;
switch (origin) {
case 1:
offset += file->f_pos;
case 0:
if (offset < 0)
break;
retval = offset;
if (offset != m->read_pos) {
while ((retval=traverse(m, offset)) == -EAGAIN)
;
if (retval) {
/* with extreme prejudice... */
file->f_pos = 0;
m->read_pos = 0;
m->version = 0;
m->index = 0;
m->count = 0;
} else {
m->read_pos = offset;
retval = file->f_pos = offset;
}
}
}
file->f_version = m->version;
mutex_unlock(&m->lock);
return retval;
}
EXPORT_SYMBOL(seq_lseek);
