Root/Documentation/fb/vesafb.txt

1
2What is vesafb?
3===============
4
5This is a generic driver for a graphic framebuffer on intel boxes.
6
7The idea is simple: Turn on graphics mode at boot time with the help
8of the BIOS, and use this as framebuffer device /dev/fb0, like the m68k
9(and other) ports do.
10
11This means we decide at boot time whenever we want to run in text or
12graphics mode. Switching mode later on (in protected mode) is
13impossible; BIOS calls work in real mode only. VESA BIOS Extensions
14Version 2.0 are required, because we need a linear frame buffer.
15
16Advantages:
17
18 * It provides a nice large console (128 cols + 48 lines with 1024x768)
19   without using tiny, unreadable fonts.
20 * You can run XF68_FBDev on top of /dev/fb0 (=> non-accelerated X11
21   support for every VBE 2.0 compliant graphics board).
22 * Most important: boot logo :-)
23
24Disadvantages:
25
26 * graphic mode is slower than text mode...
27
28
29How to use it?
30==============
31
32Switching modes is done using the vga=... boot parameter. Read
33Documentation/svga.txt for details.
34
35You should compile in both vgacon (for text mode) and vesafb (for
36graphics mode). Which of them takes over the console depends on
37whenever the specified mode is text or graphics.
38
39The graphic modes are NOT in the list which you get if you boot with
40vga=ask and hit return. The mode you wish to use is derived from the
41VESA mode number. Here are those VESA mode numbers:
42
43    | 640x480 800x600 1024x768 1280x1024
44----+-------------------------------------
45256 | 0x101 0x103 0x105 0x107
4632k | 0x110 0x113 0x116 0x119
4764k | 0x111 0x114 0x117 0x11A
4816M | 0x112 0x115 0x118 0x11B
49
50The video mode number of the Linux kernel is the VESA mode number plus
510x200.
52 
53 Linux_kernel_mode_number = VESA_mode_number + 0x200
54
55So the table for the Kernel mode numbers are:
56
57    | 640x480 800x600 1024x768 1280x1024
58----+-------------------------------------
59256 | 0x301 0x303 0x305 0x307
6032k | 0x310 0x313 0x316 0x319
6164k | 0x311 0x314 0x317 0x31A
6216M | 0x312 0x315 0x318 0x31B
63
64To enable one of those modes you have to specify "vga=ask" in the
65lilo.conf file and rerun LILO. Then you can type in the desired
66mode at the "vga=ask" prompt. For example if you like to use
671024x768x256 colors you have to say "305" at this prompt.
68
69If this does not work, this might be because your BIOS does not support
70linear framebuffers or because it does not support this mode at all.
71Even if your board does, it might be the BIOS which does not. VESA BIOS
72Extensions v2.0 are required, 1.2 is NOT sufficient. You will get a
73"bad mode number" message if something goes wrong.
74
751. Note: LILO cannot handle hex, for booting directly with
76         "vga=mode-number" you have to transform the numbers to decimal.
772. Note: Some newer versions of LILO appear to work with those hex values,
78         if you set the 0x in front of the numbers.
79
80X11
81===
82
83XF68_FBDev should work just fine, but it is non-accelerated. Running
84another (accelerated) X-Server like XF86_SVGA might or might not work.
85It depends on X-Server and graphics board.
86
87The X-Server must restore the video mode correctly, else you end up
88with a broken console (and vesafb cannot do anything about this).
89
90
91Refresh rates
92=============
93
94There is no way to change the vesafb video mode and/or timings after
95booting linux. If you are not happy with the 60 Hz refresh rate, you
96have these options:
97
98 * configure and load the DOS-Tools for the graphics board (if
99   available) and boot linux with loadlin.
100 * use a native driver (matroxfb/atyfb) instead if vesafb. If none
101   is available, write a new one!
102 * VBE 3.0 might work too. I have neither a gfx board with VBE 3.0
103   support nor the specs, so I have not checked this yet.
104
105
106Configuration
107=============
108
109The VESA BIOS provides protected mode interface for changing
110some parameters. vesafb can use it for palette changes and
111to pan the display. It is turned off by default because it
112seems not to work with some BIOS versions, but there are options
113to turn it on.
114
115You can pass options to vesafb using "video=vesafb:option" on
116the kernel command line. Multiple options should be separated
117by comma, like this: "video=vesafb:ypan,invers"
118
119Accepted options:
120
121invers no comment...
122
123ypan enable display panning using the VESA protected mode
124    interface. The visible screen is just a window of the
125    video memory, console scrolling is done by changing the
126    start of the window.
127    pro: * scrolling (fullscreen) is fast, because there is
128          no need to copy around data.
129        * You'll get scrollback (the Shift-PgUp thing),
130          the video memory can be used as scrollback buffer
131    kontra: * scrolling only parts of the screen causes some
132          ugly flicker effects (boot logo flickers for
133          example).
134
135ywrap Same as ypan, but assumes your gfx board can wrap-around
136    the video memory (i.e. starts reading from top if it
137    reaches the end of video memory). Faster than ypan.
138
139redraw scroll by redrawing the affected part of the screen, this
140    is the safe (and slow) default.
141
142
143vgapal Use the standard vga registers for palette changes.
144    This is the default.
145pmipal Use the protected mode interface for palette changes.
146
147mtrr:n setup memory type range registers for the vesafb framebuffer
148    where n:
149          0 - disabled (equivalent to nomtrr) (default)
150          1 - uncachable
151          2 - write-back
152          3 - write-combining
153          4 - write-through
154
155    If you see the following in dmesg, choose the type that matches the
156    old one. In this example, use "mtrr:2".
157...
158mtrr: type mismatch for e0000000,8000000 old: write-back new: write-combining
159...
160
161nomtrr disable mtrr
162
163vremap:n
164        remap 'n' MiB of video RAM. If 0 or not specified, remap memory
165    according to video mode. (2.5.66 patch/idea by Antonino Daplas
166    reversed to give override possibility (allocate more fb memory
167    than the kernel would) to 2.4 by tmb@iki.fi)
168
169vtotal:n
170        if the video BIOS of your card incorrectly determines the total
171        amount of video RAM, use this option to override the BIOS (in MiB).
172
173Have fun!
174
175  Gerd
176
177--
178Gerd Knorr <kraxel@goldbach.in-berlin.de>
179
180Minor (mostly typo) changes
181by Nico Schmoigl <schmoigl@rumms.uni-mannheim.de>
182

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