在/kernel/drivers/video/fbmem.c 文件中：
/**
* register_framebuffer - registers a frame buffer device
* @fb_info: frame buffer info structure
* Registers a frame buffer device @fb_info.
*
* Returns negative errno on error, or zero for success.
*/
int register_framebuffer(struct fb_info *fb_info)
/**
* unregister_framebuffer - releases a frame buffer device
* @fb_info: frame buffer info structure
* Unregisters a frame buffer device @fb_info.
*
* Returns negative errno on error, or zero for success.
*/
int unregister_framebuffer(struct fb_info *fb_info)
static int fb_open(struct inode *inode, struct file *file)
static int fb_release(struct inode *inode, struct file *file)
static ssize_t fb_read(struct file *file, char *buf, size_t count, loff_t *ppos)
static ssize_t fb_write(struct file *file, const char *buf, size_t count, loff_t *ppos)
static int fb_ioctl(struct inode *inode, struct file *file, unsigned int cmd, unsigned long arg)
static int fb_mmap(struct file *file, struct vm_area_struct * vma)
在该文件中包含了所有驱动LCD 的函数。在fb_read 和fb_write 这两个函数中，都对framebuffer 进行了操作。
fb_read 函数中：
char *base_addr;
base_addr = info->disp->screen_base;
count -= copy_to_user(buf, base_addr p, count);
fb_write 函数中：
char *base_addr;
base_addr = info->disp->screen_base;
count -= copy_from_user(base_addr p, buf, count);
所读写的framebuffer 的基地址就是disp->screen_base，也就是fbi->screen_cpu 所指的framebuffer 的虚拟地址。
从而得到：
framebuffer（虚拟地址）
|---------------|
| ... |
-------|---------------| <-- fbi->map_cpu
| 16K |
分配了|---------------| <-- fbi->screen_cpu = display->screen_base
176K | |
共11个| | 160K = 10个PAGE
PAGE | 160K | 可以容下所需的150K 视频缓冲区大小
(16K) | |
| |
-------|---------------|-------
| ... |
|---------------|
其中display->screen_base 结构在/kernel/include/video/fbcon.h 文件中定义。
得出结论，在分配framebuffer 时一共返回两个指针，虽然是同一块内存空间，但一个返回的是实际的物理地址，另一个返回的是经过地址转换的虚拟地址。在设置LCD 控制器中framebuffer 起始地址寄存器时，用的是所分配内存的物理地址；而当要对framebuffer 进行读写操作时，用的是同一块内存的物理地址所转换后的虚拟地址。由此可以知道，内核在对每个I/O 地址进行读写操作时用的都是经过转换的虚拟地址。
------------------------------------------------------------------------
在/kernel/include/asm-arm/arch-s3c2410/memory.h 文件中：
/*
* Page offset: 3GB
*/
#define PAGE_OFFSET (0xc0000000UL)
#define PHYS_OFFSET (0x30000000UL)
/*
* We take advantage of the fact that physical and virtual address can be the
* saem. Thu NUMA code is handling the large holes that might exist between
* all memory banks.
*/
#define __virt_to_phys__is_a_macro
#define __phys_to_virt__is_a_macro
#define __virt_to_phys(x) ((x) - PAGE_OFFSET PHYS_OFFSET)
#define __phys_to_virt(x) ((x) - PHYS_OFFSET PAGE_OFFSET)
物理地址与虚拟地址的间隔为：PAGE_OFFSET - PHYS_OFFSET。这样一来，以上对物理地址和虚拟地址之间转换的宏定义就很好理解了。
虚拟地址转为物理地址宏：__virt_to_phys(x)
= (x) - (PAGE_OFFSET - PHYS_OFFSET) = (x) - PAGE_OFFSET PHYS_OFFSET
物理地址转为虚拟地址宏：__phys_to_virt(x)
= (x) (PAGE_OFFSET - PHYS_OFFSET) = (x) PAGE_OFFSET - PHYS_OFFSET
内核虚拟地址和实际物理地址仅仅是相差一个偏移量（PAGE_OFFSET），可以很方便的将其转化为物理内存地址，同时内核也提供了virt_to_phys() 函数将内核虚拟空间中的物理影射区地址转化为物理地址。
/*
* Virtual view <-> DMA view memory address translations
* virt_to_bus: Used to translate the virtual address to an
* address suitable to be passed to set_dma_addr
* bus_to_virt: Used to convert an address for DMA operations
* to an address that the kernel can use.
*/
#define __virt_to_bus__is_a_macro
#define __bus_to_virt__is_a_macro
#define __virt_to_bus(x) __virt_to_phys(x)
#define __bus_to_virt(x) __phys_to_virt(x)
这里注意：__virt_to_bus(x) 就等于__virt_to_phys(x)
------------------------------------------------------------------------
在/kernel/include/asm-arm/memory.h 文件中：
/*
* These are *only* valid on the kernel direct mapped RAM memory.
*/
static inline unsigned long virt_to_phys(volatile void *x)
{
return __virt_to_phys((unsigned long)(x));
}
/*
* Virtual <-> DMA view memory address translations
* Again, these are *only* valid on the kernel direct mapped RAM
* memory.
*/
#define virt_to_bus(x) (__virt_to_bus((unsigned long)(x)))
由上面的分析可知：virt_to_bus(x) 和virt_to_phys(volatile void *x) 这两个函数调用的都是__virt_to_phys(x) 即((x) - PAGE_OFFSET PHYS_OFFSET)。所以这两个调用都是将虚拟地址转换为了物理地址。
------------------------------------------------------------------------
在/kernel/arch/arm/mm/ioremap.c 文件中：
/*
* Remap an arbitrary physical address space into the kernel virtual
* address space. Needed when the kernel wants to access high addresses
* directly.
*
* NOTE! We need to allow non-page-aligned mappings too: we will obviously
* have to convert them into an offset in a page-aligned mapping, but the
* caller shouldn\'t need to know that small detail.
*
* \'flags\' are the extra L_PTE_ flags that you want to specify for this
* mapping. See include/asm-arm/proc-armv/pgtable.h for more information.
*/
void * __ioremap(unsigned long phys_addr, size_t size, unsigned long flags)
ioremap 函数的作用是将physical address以及bus address映射为kernel的 virtual adrress。
ioremap 的作用是把I/O 内存地址（物理地址）映射到虚拟地址空间，使用之前需要分配I/O 内存区域。但是，ioremap 函数的内部实现并不是简单的（(IO的物理地址)-0x10000000 0xE4000000）。所以，得到的虚拟地址可能是不同的。
因为linux 用的是页面映射机制，CPU 不能按物理地址来访问存储空间，而必须使用虚拟地址，所以必须反向的从物理地址出发找到一片虚存空间并建立起映射。
-- 作者：asan-xu
-- 发布时间：2007-10-11 8:50:17
--
//*******************************************************
//* 2007.6.22
//*******************************************************
在/kernel/drivers/video/fbmem.c 文件中：
static struct file_operations fb_fops = {
owner: THIS_MODULE,
read: fb_read,
write: fb_write,
ioctl: fb_ioctl,
mmap: fb_mmap,
open: fb_open,
release: fb_release,
#ifdef HAVE_ARCH_FB_UNMAPPED_AREA
get_unmapped_area: get_fb_unmapped_area,
#endif
};
这个结构中定义了对LCD 所分配的framebuffer 进行读写操作，以及一些内存映射之类的函数，这些源函数都在该文件中。
/**
* register_framebuffer - registers a frame buffer device
* @fb_info: frame buffer info structure
*
* Registers a frame buffer device @fb_info.
*
* Returns negative errno on error, or zero for success.
*
*/
int register_framebuffer(struct fb_info *fb_info)
这个是对LCD 的framebuffer 设备进行注册时调用到的注册函数，该函数在/kernel/drivers/video/s3c2410fb.c 文件的s3c2410fb_init 函数里的最后部分被调用。
fb_info->devfs_handle =
devfs_register (devfs_handle, name_buf, DEVFS_FL_DEFAULT,
FB_MAJOR, i, S_IFCHR | S_IRUGO | S_IWUGO,
&fb_fops, NULL);
在注册函数的最后部分，将前面的fb_fops 结构里的各个驱动函数的入口点传入到devfs_register()设备注册函数中。
-- 作者：asan-xu
-- 发布时间：2007-10-11 8:50:36
--
//*******************************************************
//* 2007.6.26
//*******************************************************
上面这个devfs_register 函数（原型在/kernel/fs/devfs/base.c 文件中）执行前，在fbmem_init 函数中调用了函数：
devfs_handle = devfs_mk_dir (NULL, "fb", NULL);
在/kernel/fs/devfs/base.c 文件中：
/**
* devfs_mk_dir - Create a directory in the devfs namespace.
* @dir: The handle to the parent devfs directory entry. If this is %NULL the
* new name is relative to the root of the devfs.
* @name: The name of the entry.
* @info: An arbitrary pointer which will be associated with the entry.
*
* Use of this function is optional. The devfs_register() function
* will automatically create intermediate directories as needed. This function
* is provided for efficiency reasons, as it provides a handle to a directory.
* Returns a handle which may later be used in a call to devfs_unregister().
* On failure %NULL is returned.
*/
devfs_handle_t devfs_mk_dir (devfs_handle_t dir, const char *name, void *info)
这个devfs_mk_dir 函数会在设备文件系统中创建一个名为fb 的目录，并返回一个带有devfs 设备文件系统目录结构的数据结构变量devfs_handle。然后把这个数据结构作为下一步调用devfs_register 函数时的参数，该参数在调用设备文件系统注册清除函数devfs_unregister 时也要作为参数传入。
/**
* devfs_register - Register a device entry.
* @dir: The handle to the parent devfs directory entry. If this is %NULL the
* new name is relative to the root of the devfs.
* @name: The name of the entry.
* @flags: A set of bitwise-ORed flags (DEVFS_FL_*).
* @major: The major number. Not needed for regular files.
* @minor: The minor number. Not needed for regular files.
* @mode: The default file mode.
* @ops: The &file_operations or &block_device_operations structure.
* This must not be externally deallocated.
* @info: An arbitrary pointer which will be written to the @private_data
* field of the &file structure passed to the device driver. You can set
* this to whatever you like, and change it once the file is opened (the next
* file opened will not see this change).
*
* Returns a handle which may later be used in a call to devfs_unregister().
* On failure %NULL is returned.
*/
devfs_handle_t devfs_register (devfs_handle_t dir, const char *name,
unsigned int flags,
unsigned int major, unsigned int minor,
umode_t mode, void *ops, void *info)
函数devfs_register 是设备文件系统的主册函数，会在刚才创建的目录下再创建一个名为name 的设备文件节点。返回的devfs_handle_t 数据结构变量会在调用设备文件系统注册清除函数devfs_unregister 时作为参数传入。
fb_info->devfs_handle =
devfs_register (devfs_handle, name_buf, DEVFS_FL_DEFAULT,
FB_MAJOR, i, S_IFCHR | S_IRUGO | S_IWUGO,
&fb_fops, NULL);
调用该函数后，会在刚才创建的fb 目录下再创建一个名为0 （name_buf）的设备文件节点，并赋予相应的权限，以及主次设备号和file_operations 结构的函数入口。
这样一来，Linux 设备文件的创建，删除和目录层次等都由各设备驱动程序管理，再也不用手工创建设备文件节点了，再也不需要mknod 时查找对应的主设备号了，也不用依靠复杂的脚本来管理设备文件了。
