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The Frame Buffer Device
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-----------------------
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Maintained by Geert Uytterhoeven <geert@linux-m68k.org>
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Last revised: May 10, 2001
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0. Introduction
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---------------
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The frame buffer device provides an abstraction for the graphics hardware. It
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represents the frame buffer of some video hardware and allows application
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software to access the graphics hardware through a well-defined interface, so
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the software doesn't need to know anything about the low-level (hardware
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register) stuff.
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The device is accessed through special device nodes, usually located in the
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/dev directory, i.e. /dev/fb*.
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1. User's View of /dev/fb*
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--------------------------
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From the user's point of view, the frame buffer device looks just like any
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other device in /dev. It's a character device using major 29; the minor
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specifies the frame buffer number.
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By convention, the following device nodes are used (numbers indicate the device
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minor numbers):
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0 = /dev/fb0 First frame buffer
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1 = /dev/fb1 Second frame buffer
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...
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31 = /dev/fb31 32nd frame buffer
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For backwards compatibility, you may want to create the following symbolic
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links:
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/dev/fb0current -> fb0
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/dev/fb1current -> fb1
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and so on...
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The frame buffer devices are also `normal' memory devices, this means, you can
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read and write their contents. You can, for example, make a screen snapshot by
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cp /dev/fb0 myfile
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There also can be more than one frame buffer at a time, e.g. if you have a
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graphics card in addition to the built-in hardware. The corresponding frame
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buffer devices (/dev/fb0 and /dev/fb1 etc.) work independently.
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Application software that uses the frame buffer device (e.g. the X server) will
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use /dev/fb0 by default (older software uses /dev/fb0current). You can specify
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an alternative frame buffer device by setting the environment variable
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$FRAMEBUFFER to the path name of a frame buffer device, e.g. (for sh/bash
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users):
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export FRAMEBUFFER=/dev/fb1
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or (for csh users):
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setenv FRAMEBUFFER /dev/fb1
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After this the X server will use the second frame buffer.
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2. Programmer's View of /dev/fb*
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--------------------------------
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As you already know, a frame buffer device is a memory device like /dev/mem and
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it has the same features. You can read it, write it, seek to some location in
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it and mmap() it (the main usage). The difference is just that the memory that
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appears in the special file is not the whole memory, but the frame buffer of
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some video hardware.
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/dev/fb* also allows several ioctls on it, by which lots of information about
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the hardware can be queried and set. The color map handling works via ioctls,
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too. Look into <linux/fb.h> for more information on what ioctls exist and on
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which data structures they work. Here's just a brief overview:
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- You can request unchangeable information about the hardware, like name,
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organization of the screen memory (planes, packed pixels, ...) and address
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and length of the screen memory.
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- You can request and change variable information about the hardware, like
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visible and virtual geometry, depth, color map format, timing, and so on.
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If you try to change that information, the driver maybe will round up some
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values to meet the hardware's capabilities (or return EINVAL if that isn't
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possible).
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- You can get and set parts of the color map. Communication is done with 16
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bits per color part (red, green, blue, transparency) to support all
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existing hardware. The driver does all the computations needed to apply
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it to the hardware (round it down to less bits, maybe throw away
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transparency).
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All this hardware abstraction makes the implementation of application programs
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easier and more portable. E.g. the X server works completely on /dev/fb* and
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thus doesn't need to know, for example, how the color registers of the concrete
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hardware are organized. XF68_FBDev is a general X server for bitmapped,
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unaccelerated video hardware. The only thing that has to be built into
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application programs is the screen organization (bitplanes or chunky pixels
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etc.), because it works on the frame buffer image data directly.
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For the future it is planned that frame buffer drivers for graphics cards and
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the like can be implemented as kernel modules that are loaded at runtime. Such
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a driver just has to call register_framebuffer() and supply some functions.
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Writing and distributing such drivers independently from the kernel will save
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much trouble...
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3. Frame Buffer Resolution Maintenance
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--------------------------------------
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Frame buffer resolutions are maintained using the utility `fbset'. It can
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change the video mode properties of a frame buffer device. Its main usage is
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to change the current video mode, e.g. during boot up in one of your /etc/rc.*
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or /etc/init.d/* files.
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Fbset uses a video mode database stored in a configuration file, so you can
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easily add your own modes and refer to them with a simple identifier.
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4. The X Server
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---------------
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The X server (XF68_FBDev) is the most notable application program for the frame
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buffer device. Starting with XFree86 release 3.2, the X server is part of
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XFree86 and has 2 modes:
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- If the `Display' subsection for the `fbdev' driver in the /etc/XF86Config
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file contains a
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Modes "default"
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line, the X server will use the scheme discussed above, i.e. it will start
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up in the resolution determined by /dev/fb0 (or $FRAMEBUFFER, if set). You
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still have to specify the color depth (using the Depth keyword) and virtual
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resolution (using the Virtual keyword) though. This is the default for the
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configuration file supplied with XFree86. It's the most simple
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configuration, but it has some limitations.
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- Therefore it's also possible to specify resolutions in the /etc/XF86Config
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file. This allows for on-the-fly resolution switching while retaining the
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same virtual desktop size. The frame buffer device that's used is still
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/dev/fb0current (or $FRAMEBUFFER), but the available resolutions are
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defined by /etc/XF86Config now. The disadvantage is that you have to
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specify the timings in a different format (but `fbset -x' may help).
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To tune a video mode, you can use fbset or xvidtune. Note that xvidtune doesn't
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work 100% with XF68_FBDev: the reported clock values are always incorrect.
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5. Video Mode Timings
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---------------------
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A monitor draws an image on the screen by using an electron beam (3 electron
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beams for color models, 1 electron beam for monochrome monitors). The front of
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the screen is covered by a pattern of colored phosphors (pixels). If a phosphor
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is hit by an electron, it emits a photon and thus becomes visible.
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The electron beam draws horizontal lines (scanlines) from left to right, and
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from the top to the bottom of the screen. By modifying the intensity of the
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electron beam, pixels with various colors and intensities can be shown.
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After each scanline the electron beam has to move back to the left side of the
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screen and to the next line: this is called the horizontal retrace. After the
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whole screen (frame) was painted, the beam moves back to the upper left corner:
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this is called the vertical retrace. During both the horizontal and vertical
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retrace, the electron beam is turned off (blanked).
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The speed at which the electron beam paints the pixels is determined by the
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dotclock in the graphics board. For a dotclock of e.g. 28.37516 MHz (millions
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of cycles per second), each pixel is 35242 ps (picoseconds) long:
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1/(28.37516E6 Hz) = 35.242E-9 s
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If the screen resolution is 640x480, it will take
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640*35.242E-9 s = 22.555E-6 s
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to paint the 640 (xres) pixels on one scanline. But the horizontal retrace
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also takes time (e.g. 272 `pixels'), so a full scanline takes
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(640+272)*35.242E-9 s = 32.141E-6 s
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We'll say that the horizontal scanrate is about 31 kHz:
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1/(32.141E-6 s) = 31.113E3 Hz
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A full screen counts 480 (yres) lines, but we have to consider the vertical
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retrace too (e.g. 49 `lines'). So a full screen will take
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(480+49)*32.141E-6 s = 17.002E-3 s
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The vertical scanrate is about 59 Hz:
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1/(17.002E-3 s) = 58.815 Hz
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This means the screen data is refreshed about 59 times per second. To have a
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stable picture without visible flicker, VESA recommends a vertical scanrate of
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at least 72 Hz. But the perceived flicker is very human dependent: some people
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can use 50 Hz without any trouble, while I'll notice if it's less than 80 Hz.
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Since the monitor doesn't know when a new scanline starts, the graphics board
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will supply a synchronization pulse (horizontal sync or hsync) for each
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scanline. Similarly it supplies a synchronization pulse (vertical sync or
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vsync) for each new frame. The position of the image on the screen is
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influenced by the moments at which the synchronization pulses occur.
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The following picture summarizes all timings. The horizontal retrace time is
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the sum of the left margin, the right margin and the hsync length, while the
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vertical retrace time is the sum of the upper margin, the lower margin and the
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vsync length.
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+----------+---------------------------------------------+----------+-------+
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| | ^ | | |
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| | |upper_margin | | |
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| | <EFBFBD> | | |
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+----------###############################################----------+-------+
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| # ^ # | |
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| # | # | |
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| # | # | |
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| # | # | |
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| left # | # right | hsync |
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| margin # | xres # margin | len |
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|<-------->#<---------------+--------------------------->#<-------->|<----->|
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| # | # | |
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| # | # | |
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| # | # | |
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| # |yres # | |
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| # | # | |
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| # | # | |
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| # | # | |
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| # | # | |
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| # | # | |
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| # | # | |
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| # | # | |
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| # | # | |
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| # <EFBFBD> # | |
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+----------###############################################----------+-------+
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| | ^ | | |
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| | |lower_margin | | |
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| | <EFBFBD> | | |
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+----------+---------------------------------------------+----------+-------+
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| | ^ | | |
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| | |vsync_len | | |
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| | <EFBFBD> | | |
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+----------+---------------------------------------------+----------+-------+
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The frame buffer device expects all horizontal timings in number of dotclocks
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(in picoseconds, 1E-12 s), and vertical timings in number of scanlines.
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6. Converting XFree86 timing values info frame buffer device timings
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--------------------------------------------------------------------
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An XFree86 mode line consists of the following fields:
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"800x600" 50 800 856 976 1040 600 637 643 666
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< name > DCF HR SH1 SH2 HFL VR SV1 SV2 VFL
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The frame buffer device uses the following fields:
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- pixclock: pixel clock in ps (pico seconds)
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- left_margin: time from sync to picture
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- right_margin: time from picture to sync
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- upper_margin: time from sync to picture
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- lower_margin: time from picture to sync
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- hsync_len: length of horizontal sync
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- vsync_len: length of vertical sync
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1) Pixelclock:
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xfree: in MHz
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fb: in picoseconds (ps)
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pixclock = 1000000 / DCF
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2) horizontal timings:
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left_margin = HFL - SH2
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right_margin = SH1 - HR
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hsync_len = SH2 - SH1
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3) vertical timings:
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upper_margin = VFL - SV2
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lower_margin = SV1 - VR
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vsync_len = SV2 - SV1
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Good examples for VESA timings can be found in the XFree86 source tree,
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under "xc/programs/Xserver/hw/xfree86/doc/modeDB.txt".
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7. References
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-------------
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For more specific information about the frame buffer device and its
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applications, please refer to the Linux-fbdev website:
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http://linux-fbdev.sourceforge.net/
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and to the following documentation:
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- The manual pages for fbset: fbset(8), fb.modes(5)
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- The manual pages for XFree86: XF68_FBDev(1), XF86Config(4/5)
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- The mighty kernel sources:
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o linux/drivers/video/
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o linux/include/linux/fb.h
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o linux/include/video/
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8. Mailing list
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---------------
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There are several frame buffer device related mailing lists at SourceForge:
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- linux-fbdev-announce@lists.sourceforge.net, for announcements,
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- linux-fbdev-user@lists.sourceforge.net, for generic user support,
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- linux-fbdev-devel@lists.sourceforge.net, for project developers.
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Point your web browser to http://sourceforge.net/projects/linux-fbdev/ for
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subscription information and archive browsing.
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9. Downloading
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--------------
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All necessary files can be found at
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ftp://ftp.uni-erlangen.de/pub/Linux/LOCAL/680x0/
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and on its mirrors.
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The latest version of fbset can be found at
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http://home.tvd.be/cr26864/Linux/fbdev/
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10. Credits
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----------
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This readme was written by Geert Uytterhoeven, partly based on the original
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`X-framebuffer.README' by Roman Hodek and Martin Schaller. Section 6 was
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provided by Frank Neumann.
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The frame buffer device abstraction was designed by Martin Schaller.
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