Enabling the Linux Framebuffer

jackzheng2002

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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.


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

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

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" and 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.


Compile the kernel
First do:

make dep

then:
make bzImage

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


Copy the kernel to the boot directory:
cp arch/i386/boot/bzImage /boot/linux.vesafb


Edit /etc/lilo.conf.
Warning: Keep a backup of /etc/lilo.conf, and have a rescue disk available. If you make a mistake, 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. Here is an 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

Make a new "image" section that is a copy of the first one, but with

image = /boot/linux.vesafb

and
label = Linux-vesafb
endcode
Place it just above the first image section.

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

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


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


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

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.


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/ +
(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;
}




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