Hardware Framebuffer

From OpenTom

General

• Information can be gathered from the Hello world example
• Framebuffer is now working with linux 2.6

• Screen size and colour depth (bits per pixel) can be aquired using IOCTL calls

Open Framebuffer

The framebuffer is opened and mapped to memory. A second screen buffer is also allocated.

int fbfd = open(FB_DEVICE_NAME, O_RDWR);
if (!fbfd) {
    printf("Could not open framebuffer.\n");
    return -1;
}

// Get fixed screen information
if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo)) {
    printf("Could not read fixed screen info.\n");
    return -2;
}

// Get variable screen information
if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)) {
    printf("Could not read variable screen info.\n");
    return -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) {
    printf("Fout: gefaald bij het mappen van de framebuffer device in het geheugen.\n");
    return -4;
}
fbbackp = (char*)malloc(screensize);
printf("framebuffer ready.\n");

Use the Framebuffer

Manipulate the second buffer *fbbackp, then copy all the data to the first one.

// set pixel x,y to color
unsigned short *ptr  = (unsigned short*)(fbbackp+y*((vinfo.xres*vinfo.bits_per_pixel)/8));
ptr+=x;
*ptr = color;

Flush the changes to the real buffer

memcpy(fbp,fbbackp,screensize);

Or display the second buffer (faster) :

struct fb_var_screeninfo fb_var;
ioctl(fb,FBIOGET_VSCREENINFO,&fb_var))
fb_var.yoffset=fb_var.yres;
ioctl(fb,FBIOPAN_DISPLAY,&fb_var))

(That's how TT does it.)

To switch back to the first buffer :

fb_var.yoffset=0;
ioctl(fb,FBIOPAN_DISPLAY,&fb_var))

Read more about programming in LibSDL.