What is vesafb?

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

The idea is simple: Turn on graphics mode at boot time with the helpof 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 orgraphics mode. Switching mode later on (in protected mode) isimpossible; BIOS calls work in real mode only. VESA BIOS ExtensionsVersion 2.0 are required, because 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 X11support 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. ReadDocumentation/admin-guide/svga.rst for details.

You should compile in both vgacon (for text mode) and vesafb (forgraphics mode). Which of them takes over the console depends onwhenever the specified mode is text or graphics.

The graphic modes are NOT in the list which you get if you boot withvga=ask and hit return. The mode you wish to use is derived from theVESA mode number. Here are those VESA mode numbers:

colors640x480800x6001024x7681280x1024
2560x1010x1030x1050x107
32k0x1100x1130x1160x119
64k0x1110x1140x1170x11A
16M0x1120x1150x1180x11B

The video mode number of the Linux kernel is the VESA mode number plus0x200:

Linux_kernel_mode_number = VESA_mode_number + 0x200

So the table for the Kernel mode numbers are:

colors640x480800x6001024x7681280x1024
2560x3010x3030x3050x307
32k0x3100x3130x3160x319
64k0x3110x3140x3170x31A
16M0x3120x3150x3180x31B

To enable one of those modes you have to specify “vga=ask” in thelilo.conf file and rerun LILO. Then you can type in the desiredmode at the “vga=ask” prompt. For example if you like to use1024x768x256 colors you have to say “305” at this prompt.

If this does not work, this might be because your BIOS does not supportlinear framebuffers or because it does not support this mode at all.Even if your board does, it might be the BIOS which does not. VESA BIOSExtensions v2.0 are required, 1.2 is NOT sufficient. You will get a“bad mode number” message if something goes wrong.

  1. Note: LILO cannot handle hex, for booting directly with“vga=mode-number” you have to transform the numbers to decimal.
  2. Note: Some newer versions of LILO appear to work with those hex values,if you set the 0x in front of the numbers.

X11

XF68_FBDev should work just fine, but it is non-accelerated. Runninganother (accelerated) X-Server like XF86_SVGA might or might not work.It depends on X-Server and graphics board.

The X-Server must restore the video mode correctly, else you end upwith a broken console (and vesafb cannot do anything about this).

Refresh rates

There is no way to change the vesafb video mode and/or timings afterbooting linux. If you are not happy with the 60 Hz refresh rate, youhave these options:

  • configure and load the DOS-Tools for the graphics board (ifavailable) and boot linux with loadlin.
  • use a native driver (matroxfb/atyfb) instead if vesafb. If noneis available, write a new one!
  • VBE 3.0 might work too. I have neither a gfx board with VBE 3.0support nor the specs, so I have not checked this yet.

Configuration

The VESA BIOS provides protected mode interface for changingsome parameters. vesafb can use it for palette changes andto pan the display. It is turned off by default because itseems not to work with some BIOS versions, but there are optionsto turn it on.

You can pass options to vesafb using “video=vesafb:option” onthe kernel command line. Multiple options should be separatedby comma, like this: “video=vesafb:ypan,inverse”

Accepted options:

inverse use inverse color map

ypan

enable display panning using the VESA protected modeinterface. The visible screen is just a window of thevideo memory, console scrolling is done by changing thestart of the window.

pro:

  • scrolling (fullscreen) is fast, because there isno need to copy around data.
  • You’ll get scrollback (the Shift-PgUp thing),the video memory can be used as scrollback buffer

kontra:

  • scrolling only parts of the screen causes someugly flicker effects (boot logo flickers forexample).
ywrapSame as ypan, but assumes your gfx board can wrap-aroundthe video memory (i.e. starts reading from top if itreaches the end of video memory). Faster than ypan.
redrawScroll by redrawing the affected part of the screen, thisis the safe (and slow) default.
vgapalUse the standard vga registers for palette changes.This is the default.
pmipalUse the protected mode interface for palette changes.
mtrr:n

Setup memory type range registers for the vesafb framebufferwhere n:

  • 0 - disabled (equivalent to nomtrr) (default)
  • 1 - uncachable
  • 2 - write-back
  • 3 - write-combining
  • 4 - write-through

If you see the following in dmesg, choose the type that matches theold one. In this example, use “mtrr:2”.

… 
mtrr:type mismatch for e0000000,8000000 old: write-back new:write-combining
… 
nomtrrdisable mtrr
vremap:nRemap ‘n’ MiB of video RAM. If 0 or not specified, remap memoryaccording to video mode. (2.5.66 patch/idea by Antonino Daplasreversed to give override possibility (allocate more fb memorythan the kernel would) to 2.4 bytmb@iki.fi)
vtotal:nIf the video BIOS of your card incorrectly determines the totalamount of video RAM, use this option to override the BIOS (in MiB).

Have fun!

Gerd Knorr <kraxel@goldbach.in-berlin.de>

Minor (mostly typo) changesby Nico Schmoigl <schmoigl@rumms.uni-mannheim.de>