Linux驱动修炼之道-framebuffer（下）

努力成为linux kernel hacker的人李万鹏原创作品，为梦而战。转载请标明出处

http://blog.csdn.net/woshixingaaa/archive/2011/05/29/6452690.aspx

用户写屏程序:

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

 int main()
 {
     int fbfd = 0;
     struct fb_var_screeninfo vinfo;
     struct fb_fix_screeninfo finfo;
     long int screensize = 0;
     char *fbp = 0;
     int x = 0, y = 0;
     long int location = 0;
 
     // Open the file for reading and writing
     fbfd = open("/dev/fb0", O_RDWR);
     if (fbfd == -1) {
         perror("Error: cannot open framebuffer device");
         exit(1);
     }
     printf("The framebuffer device was opened successfully.\n");
     // Get fixed screen information
     if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo) == -1) {
         perror("Error reading fixed information");
         exit(2);
     }
     // Get variable screen information

     if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo) == -1) {
        perror("Error reading variable information");
         exit(3);
     }

     printf("%dx%d, %dbpp\n", vinfo.xres, vinfo.yres, vinfo.bits_per_pixel);
     // Figure out the size of the screen in bytes
     screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;

     // Map the device to memory

     fbp = (char *)mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED,
                        fbfd, 0);

     if ((int)fbp == -1) {
         perror("Error: failed to map framebuffer device to memory");
         exit(4);
     }
     printf("The framebuffer device was mapped to memory successfully.\n");

     x = 100; y = 100;       // Where we are going to put the pixel

     // Figure out where in memory to put the pixel
     for (y = 100; y < 300; y++)
         for (x = 100; x < 300; x++) {
             location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +
                        (y+vinfo.yoffset) * finfo.line_length;

             if (vinfo.bits_per_pixel == 32) {
                 *(fbp + location) = 100;        // Some blue
                 *(fbp + location + 1) = 15+(x-100)/2;     // A little green
                 *(fbp + location + 2) = 200-(y-100)/5;    // A lot of red
                 *(fbp + location + 3) = 0;      // No transparency
             } else  { //assume 16bpp
                 int b = 10;
                 int g = (x-100)/6;     // A little green
                 int r = 31-(y-100)/16;    // A lot of red
                 unsigned short int t = r<<11 | g << 5 | b;
                 *((unsigned short int*)(fbp + location)) = t;
             }
         }
     munmap(fbp, screensize);
     close(fbfd);
     return 0;
 }

这个是网上流行的用户测试程序，总结一下用户程序写屏步骤:
1)打开设备节点
2)获得fb_info的固定参数与可变参数结构体
3)计算帧缓冲区大小
4)调用mmap将缓冲区映射到进程的地址空间
5)写屏
这里注意fb_var_screeninfo中的可见分辨率xres,yres是屏的实际大小，即320,240。而虚拟分辨率xres_virtual,yres_virtual是虚拟窗口的大小。但是在s3c2440中被设为相等的。看源码:
s3c24xxfb_probe函数中：

fbinfo->var.xres = display->xres;
fbinfo->var.yres = display->yres;
fbinfo->var.bits_per_pixel = display->bpp;

s3c2410fb_check_var函数中：

var->xres_virtual = display->xres;
var->yres_virtual = display->yres;

所以xres=xres_virtual,yres=yres_virtual。
而实际的xres_virtual=xres + 2 * x_offset,yres_virtual = yres + 2 * y_offset。可以看s3c2440的datasheet，有一个图，

看那个for循环,如果是在PC机上当然就是32BPP，R,G,B分别一个字节，另外一个字节空着或者干其他的事情，比如这里就用来表示透明度。如果在开发板上运行就是16BPP，在开发板上运行要改一下x,y的范围。在开发板上运行清屏命令:

dd if=/dev/zero of=/dev/fb0 bs=320 count=240 

然后运行效果如下：