Linux内核修炼之framebuffer分析

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内核版本：linux-2.6.30.4

Linux源码包中/document/fb/framebuffer.txt有如下介绍：

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。可以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接口即可访问到显示设备。  但Framebuffer本身不具备任何运算数据的能力,就只好比是一个暂时存放水的水池。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，即可相当于操作了显示卡。系统中可以在/dev/fb*看到framebuffer设备。下面这幅图很好的描述了framebuffer运行机制：

framebuffer子系统的层次结构：

上图主要在下面文件中：

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    该数据结构描述了一个framebuffer device相关一系列信息
struct fb_ops   *fb_ops      该数据结构描述了一个framebuffer device的操作函数集合，类似file_operations，但只供内核使用
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，用户可以read,write,ioctl等
struct fb_var_screeninfo var      该数据结构描述了framebuffer device显示特性，是可以更改的
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，不可以更改

具体数据结构：

struct fb_var_screeninfo {
	__u32 xres;			/* visible resolution		*/
	__u32 yres;
	__u32 xres_virtual;		/* virtual resolution		*/
	__u32 yres_virtual;
	__u32 xoffset;			/* offset from virtual to visible */
	__u32 yoffset;			/* resolution			*/

	__u32 bits_per_pixel;		/* guess what			*/
	__u32 grayscale;		/* != 0 Graylevels instead of colors */

	struct fb_bitfield red;		/* bitfield in fb mem if true color, */
	struct fb_bitfield green;	/* else only length is significant */
	struct fb_bitfield blue;
	struct fb_bitfield transp;	/* transparency			*/	

	__u32 nonstd;			/* != 0 Non standard pixel format */

	__u32 activate;			/* see FB_ACTIVATE_*		*/

	__u32 height;			/* height of picture in mm    */
	__u32 width;			/* width of picture in mm     */

	__u32 accel_flags;		/* (OBSOLETE) see fb_info.flags */

	/* Timing: All values in pixclocks, except pixclock (of course) */
	__u32 pixclock;			/* pixel clock in ps (pico seconds) */
	__u32 left_margin;		/* time from sync to picture	*/
	__u32 right_margin;		/* time from picture to sync	*/
	__u32 upper_margin;		/* time from sync to picture	*/
	__u32 lower_margin;
	__u32 hsync_len;		/* length of horizontal sync	*/
	__u32 vsync_len;		/* length of vertical sync	*/
	__u32 sync;			/* see FB_SYNC_*		*/
	__u32 vmode;			/* see FB_VMODE_*		*/
	__u32 rotate;			/* angle we rotate counter clockwise */
	__u32 reserved[5];		/* Reserved for future compatibility */
};

struct fb_fix_screeninfo {
	char id[16];			/* identification string eg "TT Builtin" */
	unsigned long smem_start;	/* Start of frame buffer mem */
					/* (physical address) */
	__u32 smem_len;			/* Length of frame buffer mem */
	__u32 type;			/* see FB_TYPE_*		*/
	__u32 type_aux;			/* Interleave for interleaved Planes */
	__u32 visual;			/* see FB_VISUAL_*		*/ 
	__u16 xpanstep;			/* zero if no hardware panning  */
	__u16 ypanstep;			/* zero if no hardware panning  */
	__u16 ywrapstep;		/* zero if no hardware ywrap    */
	__u32 line_length;		/* length of a line in bytes    */
	unsigned long mmio_start;	/* Start of Memory Mapped I/O   */
					/* (physical address) */
	__u32 mmio_len;			/* Length of Memory Mapped I/O  */
	__u32 accel;			/* Indicate to driver which	*/
					/*  specific chip/card we have	*/
	__u16 reserved[3];		/* Reserved for future compatibility */
};

struct fb_info {
	int node;
	int flags;
	struct mutex lock;		/* Lock for open/release/ioctl funcs */
	struct fb_var_screeninfo var;	/* Current var */
	struct fb_fix_screeninfo fix;	/* Current fix */
	struct fb_monspecs monspecs;	/* Current Monitor specs */
	struct work_struct queue;	/* Framebuffer event queue */
	struct fb_pixmap pixmap;	/* Image hardware mapper */
	struct fb_pixmap sprite;	/* Cursor hardware mapper */
	struct fb_cmap cmap;		/* Current cmap */
	struct list_head modelist;      /* mode list */
	struct fb_videomode *mode;	/* current mode */
        ...
	struct fb_ops *fbops;
	struct device *device;		/* This is the parent */
	struct device *dev;		/* This is this fb device */
	int class_flag;                    /* private sysfs flags */
#ifdef CONFIG_FB_TILEBLITTING
	struct fb_tile_ops *tileops;    /* Tile Blitting */
#endif
	char __iomem *screen_base;	/* Virtual address */
	unsigned long screen_size;	/* Amount of ioremapped VRAM or 0 */ 
	void *pseudo_palette;		/* Fake palette of 16 colors */ 
        ...
};

struct fb_ops {
	/* open/release and usage marking */
	struct module *owner;
	int (*fb_open)(struct fb_info *info, int user);
	int (*fb_release)(struct fb_info *info, int user);

	/* For framebuffers with strange non linear layouts or that do not
	 * work with normal memory mapped access
	 */
	ssize_t (*fb_read)(struct fb_info *info, char __user *buf,
			   size_t count, loff_t *ppos);
	ssize_t (*fb_write)(struct fb_info *info, const char __user *buf,
			    size_t count, loff_t *ppos);

	/* checks var and eventually tweaks it to something supported,
	 * DO NOT MODIFY PAR */
	int (*fb_check_var)(struct fb_var_screeninfo *var, struct fb_info *info);

	/* set the video mode according to info->var */
	int (*fb_set_par)(struct fb_info *info);

	/* set color register */
	int (*fb_setcolreg)(unsigned regno, unsigned red, unsigned green,
			    unsigned blue, unsigned transp, struct fb_info *info);

	/* set color registers in batch */
	int (*fb_setcmap)(struct fb_cmap *cmap, struct fb_info *info);

	/* blank display */
	int (*fb_blank)(int blank, struct fb_info *info);

	/* pan display */
	int (*fb_pan_display)(struct fb_var_screeninfo *var, struct fb_info *info);

	/* Draws a rectangle */
	void (*fb_fillrect) (struct fb_info *info, const struct fb_fillrect *rect);
	/* Copy data from area to another */
	void (*fb_copyarea) (struct fb_info *info, const struct fb_copyarea *region);
	/* Draws a image to the display */
	void (*fb_imageblit) (struct fb_info *info, const struct fb_image *image);

	/* Draws cursor */
	int (*fb_cursor) (struct fb_info *info, struct fb_cursor *cursor);

	/* Rotates the display */
	void (*fb_rotate)(struct fb_info *info, int angle);

	/* wait for blit idle, optional */
	int (*fb_sync)(struct fb_info *info);

	/* perform fb specific ioctl (optional) */
	int (*fb_ioctl)(struct fb_info *info, unsigned int cmd,
			unsigned long arg);

	/* Handle 32bit compat ioctl (optional) */
	int (*fb_compat_ioctl)(struct fb_info *info, unsigned cmd,
			unsigned long arg);

	/* perform fb specific mmap */
	int (*fb_mmap)(struct fb_info *info, struct vm_area_struct *vma);

	/* save current hardware state */
	void (*fb_save_state)(struct fb_info *info);

	/* restore saved state */
	void (*fb_restore_state)(struct fb_info *info);

	/* get capability given var */
	void (*fb_get_caps)(struct fb_info *info, struct fb_blit_caps *caps,
			    struct fb_var_screeninfo *var);
};

static const struct file_operations fb_fops = {
	.owner =	THIS_MODULE,
	.read =		fb_read,
	.write =	fb_write,
	.check_flags = my_check,
	.unlocked_ioctl = fb_ioctl,
#ifdef CONFIG_COMPAT
	.compat_ioctl = fb_compat_ioctl,
#endif
	.mmap =		fb_mmap,
	.open =		fb_open,
	.release =	fb_release,
#ifdef HAVE_ARCH_FB_UNMAPPED_AREA
	.get_unmapped_area = get_fb_unmapped_area,
#endif
#ifdef CONFIG_FB_DEFERRED_IO
	.fsync =	fb_deferred_io_fsync,
#endif
};

framebuffer设备的注册与注销：

register_framebuffer(struct fb_info *fb_info);

unregister_framebuffer(struct fb_info *fb_info);

根据文件操作的static const struct file_operations fb_fops，应用程序在打开一个framebuffer设备时，可以使用read,write,ioctl来直接操作设备。

应用例程：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <fcntl.h>        
#include <linux/fb.h>
#include <sys/mman.h>

struct fb_var_screeninfo vinfo;
struct fb_fix_screeninfo finfo;

static void fb_var_printf(struct fb_var_screeninfo tmp)
{


	printf("fb_var_screeninfo:n");
	printf("xres =%d, yres =%d, bits_per_pixel = %dn",tmp.xres,tmp.yres,tmp.bits_per_pixel);
	printf("height=%d,width = %dn",tmp.height,tmp.width);
	printf("xres_virtual =%d, yres_virtual =%d, xoffset=%d,yoffset=%dn",tmp.xres_virtual,tmp.yres_virtual,tmp.xoffset,tmp.yoffset);
	return ;
}
int main(void)
{
	int fbfd;
	int fbsize;
	unsigned char *fbbuf;
   	char buf[100];
	int i,res,adc_data;
	for (i=0; i<100; i++) buf[i] = 0xaa;
	if ((fbfd = open("/dev/fb0", O_RDWR)) < 0) {
            printf("open fb0 failedn");
	     return 1;
	}
	printf("fbfd = %dn", fbfd);
	if ((res =ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo))) { //获取设备显示特性信息
		printf("bad vscreeninfo ioctl.error = %dn",res);
	}

	fb_var_printf(vinfo);
	fbsize = vinfo.xres * vinfo.yres * (vinfo.bits_per_pixel/8); //计算显卡（LCD控制器）显存大小，也就是一整屏共占多少个字节
	printf("fbisze: %dn",fbsize);
	if ((fbbuf = mmap(0, fbsize, PROT_READ | PROT_WRITE, MAP_SHARED, fbfd, 0)) == (void*) -1)  //映射显卡设备的内存到用户控件，使得用户直接访问设备内存（显存）
	{
		printf("map video error.n");
	}
	
	for (i = 0; i< fbsize; i++) {  //填充farmebuffer缓冲区
		*(fbbuf+i) = 0xaa;  //颜色信息
	}
	munmap(fbbuf, fbsize);       
	close(fbfd);
	return 0;
}

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