Linux內核修煉之framebuffer分析

Linux源代碼包中/document/fb/framebuffer.txt有例如如下介紹：

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機制爲圖形顯示卡提供了一個抽象層。

可以使得應用程序不用考慮底層硬件的實現細節而經過一些API接口就能夠訪問到顯示設備。  但Framebuffer自己不具有不論什麼運算數據的能力,就僅僅比如是一個臨時存放水的水池。水池裏的水就是顯示的東西。CPU將運算後的結果放到這個水池,水池再將結果流到顯示器（一般經過DMA傳輸）. 因此應用程序經過讀寫這個水池。就能夠至關於操做了顯示卡。系統中可以在/dev/fb*看到framebuffer設備。

如下這幅圖很是好的描寫敘述了framebuffer執行機制：

framebuffer子系統的層次結構：

 

上圖主要在如下文件裏：

drivers/vedio/fbmem.c   該文件是framebuffer實現的核心。與硬件無關

drivers/vedio/xxxfb.c     該文件主要是framebuffer 設備驅動的實現，如s3c2410fb.c實現了framebuffer設備驅動

fbmem.c是實現framebuffer的核心。與硬件無關。

它使用瞭如下這些數據結構：

struct fb_info   *fb_info    該數據結構描寫敘述了一個framebuffer device相關一系列信息
struct fb_ops   *fb_ops      該數據結構描寫敘述了一個framebuffer device的操做函數集合。類似file_operations，但僅僅供內核使用
static const struct file_operations fb_fops   該數據結構爲文件操做函數集合。當應用程序打開設備時。用戶可以read,write,ioctl等
struct fb_var_screeninfo var      該數據結構描寫敘述了framebuffer device顯示特性，是可以更改的
struct fb_fix_screeninfo fix        該數據結構用於保存framebuffer device顯示特性，是固定不變的，不可以更改

詳細數據結構：

 

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1. 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 */  
   };  

    

 

 

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1. 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 */  
   };  

    

 

[cpp] view plain copy

1. 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 */   
           ...  
   }; 

    

[cpp] view plain copy

1. 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);  
   };  

    

 

 

[cpp] view plain copy

1. 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設備的註冊與註銷：

register_framebuffer(struct fb_info *fb_info);

unregister_framebuffer(struct fb_info *fb_info);

如下看看fb_ioctl 都作了什麼？

 

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);
}

 

 

依據文件操做的static const struct file_operations fb_fops，應用程序在打開一個framebuffer設備時。可以使用read,write,ioctl來直接操做設備。

應用例程：

 

[cpp] view plain copy

1. #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 = %d\n",tmp.xres,tmp.yres,tmp.bits_per_pixel);  
       printf("height=%d,width = %d\n",tmp.height,tmp.width);  
       printf("xres_virtual =%d, yres_virtual =%d, xoffset=%d,yoffset=%d\n",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 failed\n");  
            return 1;  
       }  
       printf("fbfd = %d\n", fbfd);  
       if ((res =ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo))) { //獲取設備顯示特性信息  
           printf("bad vscreeninfo ioctl.error = %d\n",res);  
       }  
     
       fb_var_printf(vinfo);  
       fbsize = vinfo.xres * vinfo.yres * (vinfo.bits_per_pixel/8); //計算顯卡（LCD控制器）顯存大小。也就是一整屏共佔多少個字節  
       printf("fbisze: %d\n",fbsize);  
       if ((fbbuf = mmap(0, fbsize, PROT_READ | PROT_WRITE, MAP_SHARED, fbfd, 0)) == (void*) -1)  //映射顯卡設備的內存到用戶控件，使得用戶直接訪問設備內存（顯存）  
       {  
           printf("map video error.\n");  
       }  
         
       for (i = 0; i< fbsize; i++) {  //填充farmebuffer緩衝區  
           *(fbbuf+i) = 0xaa;  //顏色信息  
       }  
       munmap(fbbuf, fbsize);         
       close(fbfd);  
       return 0;  
   }