Linux内核修炼之framebuffer分析
Linux源代码包中/document/fb/framebuffer.txt有例如以下介绍：
The frame buffer device provides an abstraction for the graphics hardware. It represents the frame buffer of some video hardware and allows application software to access the graphics hardware through a well-defined interface, so the software doesn't need to know anything about the low-level (hardware register) stuff.Frame buffer机制为图形显示卡提供了一个抽象层。
能够使得应用程序不用考虑底层硬件的实现细节而通过一些API接口就可以訪问到显示设备。 但Framebuffer本身不具备不论什么运算数据的能力,就仅仅好比是一个临时存放水的水池。水池里的水就是显示的东西。CPU将运算后的结果放到这个水池,水池再将结果流到显示器（通常通过DMA传输）. 所以应用程序通过读写这个水池。就可以相当于操作了显示卡。系统中能够在/dev/fb*看到framebuffer设备。
以下这幅图非常好的描写叙述了framebuffer执行机制：
framebuffer子系统的层次结构：
上图主要在以下文件里：
drivers/vedio/fbmem.c 该文件是framebuffer实现的核心。与硬件无关
drivers/vedio/xxxfb.c 该文件主要是framebuffer 设备驱动的实现，如s3c2410fb.c实现了framebuffer设备驱动
fbmem.c是实现framebuffer的核心。与硬件无关。
它使用了以下这些数据结构：
struct fb_info *fb_info 该数据结构描写叙述了一个framebuffer device相关一系列信息struct fb_ops *fb_ops 该数据结构描写叙述了一个framebuffer device的操作函数集合。相似file_operations，但仅仅供内核使用
static const struct file_operations fb_fops 该数据结构为文件操作函数集合。当应用程序打开设备时。用户能够read,write,ioctl等
struct fb_var_screeninfo var 该数据结构描写叙述了framebuffer device显示特性，是能够更改的
struct fb_fix_screeninfo fix 该数据结构用于保存framebuffer device显示特性，是固定不变的，不能够更改
详细数据结构：
framebuffer设备的注冊与注销：
register_framebuffer(struct fb_info *fb_info);
unregister_framebuffer(struct fb_info *fb_info);
以下看看fb_ioctl 都做了什么？
static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
unsigned long arg)
{
struct fb_ops *fb;
struct fb_var_screeninfo var;
struct fb_fix_screeninfo fix;
struct fb_con2fbmap con2fb;
struct fb_cmap cmap_from;
struct fb_cmap_user cmap;
struct fb_event event;
void __user *argp = (void __user *)arg;
long ret = 0;
switch (cmd) {
case FBIOGET_VSCREENINFO:
if (!lock_fb_info(info))
return -ENODEV;
var = info->var;
unlock_fb_info(info);
ret = copy_to_user(argp, &var, sizeof(var)) ? -EFAULT : 0;
break;
case FBIOPUT_VSCREENINFO:
if (copy_from_user(&var, argp, sizeof(var)))
return -EFAULT;
if (!lock_fb_info(info))
return -ENODEV;
console_lock();
info->flags |= FBINFO_MISC_USEREVENT;
ret = fb_set_var(info, &var);
info->flags &= ~FBINFO_MISC_USEREVENT;
console_unlock();
unlock_fb_info(info);
if (!ret && copy_to_user(argp, &var, sizeof(var)))
ret = -EFAULT;
break;
case FBIOGET_FSCREENINFO:
if (!lock_fb_info(info))
return -ENODEV;
fix = info->fix;
unlock_fb_info(info);
ret = copy_to_user(argp, &fix, sizeof(fix)) ? -EFAULT : 0;
break;
case FBIOPUTCMAP:
if (copy_from_user(&cmap, argp, sizeof(cmap)))
return -EFAULT;
ret = fb_set_user_cmap(&cmap, info);
break;
case FBIOGETCMAP:
if (copy_from_user(&cmap, argp, sizeof(cmap)))
return -EFAULT;
if (!lock_fb_info(info))
return -ENODEV;
cmap_from = info->cmap;
unlock_fb_info(info);
ret = fb_cmap_to_user(&cmap_from, &cmap);
break;
case FBIOPAN_DISPLAY:
if (copy_from_user(&var, argp, sizeof(var)))
return -EFAULT;
if (!lock_fb_info(info))
return -ENODEV;
console_lock();
ret = fb_pan_display(info, &var);
console_unlock();
unlock_fb_info(info);
if (ret == 0 && copy_to_user(argp, &var, sizeof(var)))
return -EFAULT;
break;
case FBIO_CURSOR:
ret = -EINVAL;
break;
case FBIOGET_CON2FBMAP:
if (copy_from_user(&con2fb, argp, sizeof(con2fb)))
return -EFAULT;
if (con2fb.console < 1 || con2fb.console > MAX_NR_CONSOLES)
return -EINVAL;
con2fb.framebuffer = -1;
event.data = &con2fb;
if (!lock_fb_info(info))
return -ENODEV;
event.info = info;
fb_notifier_call_chain(FB_EVENT_GET_CONSOLE_MAP, &event);
unlock_fb_info(info);
ret = copy_to_user(argp, &con2fb, sizeof(con2fb)) ? -EFAULT : 0;
break;
case FBIOPUT_CON2FBMAP:
if (copy_from_user(&con2fb, argp, sizeof(con2fb)))
return -EFAULT;
if (con2fb.console < 1 || con2fb.console > MAX_NR_CONSOLES)
return -EINVAL;
if (con2fb.framebuffer < 0 || con2fb.framebuffer >= FB_MAX)
return -EINVAL;
if (!registered_fb[con2fb.framebuffer])
request_module("fb%d", con2fb.framebuffer);
if (!registered_fb[con2fb.framebuffer]) {
ret = -EINVAL;
break;
}
event.data = &con2fb;
if (!lock_fb_info(info))
return -ENODEV;
event.info = info;
ret = fb_notifier_call_chain(FB_EVENT_SET_CONSOLE_MAP, &event);
unlock_fb_info(info);
break;
case FBIOBLANK:
if (!lock_fb_info(info))
return -ENODEV;
console_lock();
info->flags |= FBINFO_MISC_USEREVENT;
ret = fb_blank(info, arg);
info->flags &= ~FBINFO_MISC_USEREVENT;
console_unlock();
unlock_fb_info(info);
break;
default:
if (!lock_fb_info(info))
return -ENODEV;
fb = info->fbops;
if (fb->fb_ioctl)
ret = fb->fb_ioctl(info, cmd, arg);
else
ret = -ENOTTY;
unlock_fb_info(info);
}
return ret;
}
static long fb_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
struct fb_info *info = file_fb_info(file);
printk(\nfb_ioctl mem\n);
if (!info)
return -ENODEV;
return do_fb_ioctl(info, cmd, arg);
}依据文件操作的static const struct file_operations fb_fops，应用程序在打开一个framebuffer设备时。能够使用read,write,ioctl来直接操作设备。
应用例程：
