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# Enabling the Linux Framebuffer

This is only a short guide. See /usr/src/linux/README and /usr/src/linux/Documentation/fb/ for detailed information. There is also a detailed explanation at http://www.linuxdoc.org/HOWTO/Framebuffer-HOWTO.html.

1. Make sure that you have the Linux kernel source code in /usr/src/linux/.

2. Log in as root and cd /usr/src/linux

3. Configure the kernel:

   Run:

           make menuconfig

   Select "Code maturity level options" and set "Prompt for development and/or incomplete code/drivers".

   Then select "Console drivers" set "Support for frame buffer devices" to built-in (even if it says EXPERIMENTAL). Then configure the driver. Most modern graphics cards can use the "VESA VGA graphics console"; use that or a driver that specifically matches your video card. Finally enable "Advanced low level driver options" and make sure that 16 and 32 bpp packed pixel support are enabled.

   When you are finished, chose exit and save.

4. Compile the kernel

   First do:

           make dep

   then:

           make bzImage

   The new kernel should now be in arch/i386/boot/bzImage.

5. Copy the kernel to the boot directory:

           cp arch/i386/boot/bzImage /boot/linux.vesafb

6. Edit /etc/lilo.conf.

   Warning: Keep a backup of /etc/lilo.conf, and have a rescue disk available. If you make a mistake here, the machine may not boot.

   The file /etc/lilo.conf specifies how the system boots. The precise contents of the file varies from system to system, this is one example:

       # LILO configuration file
       boot = /dev/hda3
       delay = 30
       image = /boot/vmlinuz
         root = /dev/hda3
         label = Linux
         read-only # Non-UMSDOS filesystems should be mounted read-only for checking
       other=/dev/hda1
               label=nt
               table=/dev/hda

   Warning: Keep a backup of /etc/lilo.conf, and have a rescue disk available. If you make a mistake here, the machine may not boot.

   Make a new "image" section that is a copy of the first one, but with image = /boot/linux.vesafb and label = Linux-vesafb. Place it just above the first image section.

   Add a line before the image section saying 'vga = 791'. (Meaning 1024x768, 16 bpp.)

   With the above example, lilo.conf would now be:

       # LILO configuration file
       boot = /dev/hda3
       delay = 30
       vga = 791
       image = /boot/linux.vesafb
         root = /dev/hda3
         label = Linux-vesafb
         read-only # Non-UMSDOS filesystems should be mounted read-only for checking
       image = /boot/vmlinuz
         root = /dev/hda3
         label = Linux
         read-only # Non-UMSDOS filesystems should be mounted read-only for checking
       other=/dev/hda1
               label=nt
               table=/dev/hda

   Do not change any existing lines in the file; just add new ones.

7. To make the new changes take effect, run the lilo program:

           lilo

8. Reboot the system. You should now see a penguin logo while the system is booting. (Or more than one on a multi-processor machine.)

9. If it does not boot properly with the new kernel, you can boot with the old kernel by entering the label of the old image section at the LILO prompt. (with the example lilo.conf file, the old label is Linux.)

   If that does not work (probably because of an error in lilo.conf), boot the machine using your rescue disk, restore /etc/lilo.conf from backup and re-run lilo.

10. Testing: Here's a short program that opens the frame buffer and draws a gradient-filled red square.

        #include <unistd.h>
        #include <stdio.h>
        #include <fcntl.h>
        #include <linux/fb.h>
        #include <sys/mman.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) {
                printf("Error: cannot open framebuffer device.\n");
                exit(1);
            }
            printf("The framebuffer device was opened successfully.\n");

            // Get fixed screen information
            if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo)) {
                printf("Error reading fixed information.\n");
                exit(2);
            }

            // Get variable screen information
            if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)) {
                printf("Error reading variable information.\n");
                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) {
                printf("Error: failed to map framebuffer device to memory.\n");
                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;
        }