
what is vesafb?
===============

This is a generic driver for a graphic framebuffer on intel boxes.

Idea is simple:  Turn on graphics mode at boot time with the help of
the BIOS, and use this as framebuffer device /dev/fb0, like the m68k
(and other) ports do.

This means we decide at boot time whenever we want to run in text or
graphics mode.  Switching mode later on (in protected mode) is
impossible, BIOS calls work in real mode only.  VESA BIOS Extentions
Version 2.0 are required, becauce we need a linear frame buffer.

Advantages:

 * It provides a nice large console (128 cols + 48 lines with 1024x768)
   without using tiny, unreadable fonts.
 * You can run XF68_FBDev on top of /dev/fb0 (=> non-accelerated X11
   support for every VBE 2.0 compliant graphics board).
 * Most important: boot logo :-)

Disadvantages:

 * graphic mode is slower than text mode...


How to use it?
==============

Switching modes is done using the vga=... boot parameter.  Read
Documentation/svga.txt for details.  With vesafb both text and
graphics modes work.  Text modes are handled by vgafb, graphic modes
by the new vesafb.c.

The graphic modes are not in the list which you get if you boot with
vga=ask and hit return.  Here are some mode numbers:

    | 640x480  800x600  1024x768
----+---------------------------
256 |  0x101    0x103    0x105
32k |  0x110    0x113    0x116
64k |  0x111    0x114    0x117
16M |  0x112    0x115    0x118

Note 1: this are the VESA mode numbers.  The video mode select code
        expects 0x200 + VESA mode number.
Note 2: lilo can't handle hex, for booting with "vga=??" you have to
        transform the numbers to decimal.


Speed it up!
============

Check /usr/src/linux/Documentation/mtrr.txt, enabling write-combining
for the framebuffer memory gives a performance boost.

There are two ways to do console scrolling: redraw the screen
completely, or by copying around the video memory.  You can select one
of them using the kernel command line: video=vesa:redraw or
video=vesa:memmove.  redraw is the default, becauce this one works
faster on my box.


Have fun!

  Gerd

--
Gerd Knorr <kraxel@cs.tu-berlin.de>
