193 lines
6.6 KiB
ReStructuredText
193 lines
6.6 KiB
ReStructuredText
===============
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What is vesafb?
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===============
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This is a generic driver for a graphic framebuffer on intel boxes.
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The idea is simple: Turn on graphics mode at boot time with the help
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of the BIOS, and use this as framebuffer device /dev/fb0, like the m68k
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(and other) ports do.
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This means we decide at boot time whenever we want to run in text or
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graphics mode. Switching mode later on (in protected mode) is
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impossible; BIOS calls work in real mode only. VESA BIOS Extensions
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Version 2.0 are required, because we need a linear frame buffer.
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Advantages:
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* It provides a nice large console (128 cols + 48 lines with 1024x768)
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without using tiny, unreadable fonts.
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* You can run XF68_FBDev on top of /dev/fb0 (=> non-accelerated X11
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support for every VBE 2.0 compliant graphics board).
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* Most important: boot logo :-)
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Disadvantages:
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* graphic mode is slower than text mode...
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How to use it?
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==============
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Switching modes is done using the vga=... boot parameter. Read
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Documentation/admin-guide/svga.rst for details.
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You should compile in both vgacon (for text mode) and vesafb (for
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graphics mode). Which of them takes over the console depends on
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whenever the specified mode is text or graphics.
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The graphic modes are NOT in the list which you get if you boot with
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vga=ask and hit return. The mode you wish to use is derived from the
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VESA mode number. Here are those VESA mode numbers:
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====== ======= ======= ======== =========
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colors 640x480 800x600 1024x768 1280x1024
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====== ======= ======= ======== =========
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256 0x101 0x103 0x105 0x107
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32k 0x110 0x113 0x116 0x119
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64k 0x111 0x114 0x117 0x11A
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16M 0x112 0x115 0x118 0x11B
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====== ======= ======= ======== =========
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The video mode number of the Linux kernel is the VESA mode number plus
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0x200:
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Linux_kernel_mode_number = VESA_mode_number + 0x200
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So the table for the Kernel mode numbers are:
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====== ======= ======= ======== =========
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colors 640x480 800x600 1024x768 1280x1024
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====== ======= ======= ======== =========
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256 0x301 0x303 0x305 0x307
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32k 0x310 0x313 0x316 0x319
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64k 0x311 0x314 0x317 0x31A
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16M 0x312 0x315 0x318 0x31B
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====== ======= ======= ======== =========
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To enable one of those modes you have to specify "vga=ask" in the
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lilo.conf file and rerun LILO. Then you can type in the desired
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mode at the "vga=ask" prompt. For example if you like to use
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1024x768x256 colors you have to say "305" at this prompt.
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If this does not work, this might be because your BIOS does not support
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linear framebuffers or because it does not support this mode at all.
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Even if your board does, it might be the BIOS which does not. VESA BIOS
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Extensions v2.0 are required, 1.2 is NOT sufficient. You will get a
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"bad mode number" message if something goes wrong.
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1. Note: LILO cannot handle hex, for booting directly with
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"vga=mode-number" you have to transform the numbers to decimal.
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2. Note: Some newer versions of LILO appear to work with those hex values,
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if you set the 0x in front of the numbers.
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X11
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===
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XF68_FBDev should work just fine, but it is non-accelerated. Running
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another (accelerated) X-Server like XF86_SVGA might or might not work.
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It depends on X-Server and graphics board.
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The X-Server must restore the video mode correctly, else you end up
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with a broken console (and vesafb cannot do anything about this).
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Refresh rates
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=============
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There is no way to change the vesafb video mode and/or timings after
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booting linux. If you are not happy with the 60 Hz refresh rate, you
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have these options:
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* configure and load the DOS-Tools for the graphics board (if
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available) and boot linux with loadlin.
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* use a native driver (matroxfb/atyfb) instead if vesafb. If none
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is available, write a new one!
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* VBE 3.0 might work too. I have neither a gfx board with VBE 3.0
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support nor the specs, so I have not checked this yet.
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Configuration
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=============
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The VESA BIOS provides protected mode interface for changing
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some parameters. vesafb can use it for palette changes and
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to pan the display. It is turned off by default because it
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seems not to work with some BIOS versions, but there are options
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to turn it on.
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You can pass options to vesafb using "video=vesafb:option" on
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the kernel command line. Multiple options should be separated
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by comma, like this: "video=vesafb:ypan,inverse"
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Accepted options:
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inverse use inverse color map
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========= ======================================================================
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ypan enable display panning using the VESA protected mode
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interface. The visible screen is just a window of the
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video memory, console scrolling is done by changing the
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start of the window.
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pro:
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* scrolling (fullscreen) is fast, because there is
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no need to copy around data.
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* You'll get scrollback (the Shift-PgUp thing),
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the video memory can be used as scrollback buffer
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kontra:
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* scrolling only parts of the screen causes some
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ugly flicker effects (boot logo flickers for
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example).
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ywrap Same as ypan, but assumes your gfx board can wrap-around
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the video memory (i.e. starts reading from top if it
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reaches the end of video memory). Faster than ypan.
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redraw Scroll by redrawing the affected part of the screen, this
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is the safe (and slow) default.
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vgapal Use the standard vga registers for palette changes.
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This is the default.
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pmipal Use the protected mode interface for palette changes.
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mtrr:n Setup memory type range registers for the vesafb framebuffer
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where n:
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- 0 - disabled (equivalent to nomtrr) (default)
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- 1 - uncachable
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- 2 - write-back
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- 3 - write-combining
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- 4 - write-through
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If you see the following in dmesg, choose the type that matches the
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old one. In this example, use "mtrr:2".
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...
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mtrr: type mismatch for e0000000,8000000 old: write-back new:
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write-combining
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...
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nomtrr disable mtrr
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vremap:n
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Remap 'n' MiB of video RAM. If 0 or not specified, remap memory
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according to video mode. (2.5.66 patch/idea by Antonino Daplas
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reversed to give override possibility (allocate more fb memory
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than the kernel would) to 2.4 by tmb@iki.fi)
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vtotal:n If the video BIOS of your card incorrectly determines the total
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amount of video RAM, use this option to override the BIOS (in MiB).
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========= ======================================================================
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Have fun!
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Gerd Knorr <kraxel@goldbach.in-berlin.de>
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Minor (mostly typo) changes
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by Nico Schmoigl <schmoigl@rumms.uni-mannheim.de>
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