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- Lines: 328
- Date:
Wed Jun 24 14:44:00 1998
- Orig file:
v2.1.106/linux/Documentation/fb/framebuffer.txt
- Orig date:
Wed Dec 31 16:00:00 1969
diff -u --recursive --new-file v2.1.106/linux/Documentation/fb/framebuffer.txt linux/Documentation/fb/framebuffer.txt
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+
+ The Frame Buffer Device
+ -----------------------
+
+Maintained by Geert Uytterhoeven (Geert.Uytterhoeven@cs.kuleuven.ac.be)
+Last revised: June 10, 1998
+
+
+0. Introduction
+---------------
+
+The frame buffer device provides an abstraction for the graphics hardware. It
+represents the frame buffer of some video hardware and allows application
+software to access the graphics hardware through a well-defined interface, so
+the software doesn't need to know anything about the low-level (hardware
+register) stuff.
+
+The device is accessed through special device nodes, usually located in the
+/dev directory, i.e. /dev/fb*.
+
+
+1. User's View of /dev/fb*
+--------------------------
+
+From the user's point of view, the frame buffer device looks just like any
+other device in /dev. It's a character device using major 29; the minor
+specifies the frame buffer number.
+
+By convention, the following device nodes are used (numbers indicate the device
+minor numbers):
+
+ 0 = /dev/fb0 First frame buffer
+ 32 = /dev/fb1 Second frame buffer
+ ...
+ 224 = /dev/fb7 8th frame buffer
+
+For backwards compatibility, you may want to create the following symbolic
+links:
+
+ /dev/fb0current -> fb0
+ /dev/fb1current -> fb1
+
+and so on...
+
+The frame buffer devices are also `normal' memory devices, this means, you can
+read and write their contents. You can, for example, make a screen snapshot by
+
+ cp /dev/fb0 myfile
+
+There also can be more than one frame buffer at a time, e.g. if you have a
+graphics card in addition to the built-in hardware. The corresponding frame
+buffer devices (/dev/fb0 and /dev/fb1 etc.) work independently.
+
+Application software that uses the frame buffer device (e.g. the X server) will
+use /dev/fb0 by default (older software uses /dev/fb0current). You can specify
+an alternative frame buffer device by setting the environment variable
+$FRAMEBUFFER to the path name of a frame buffer device, e.g. (for sh/bash
+users):
+
+ export FRAMEBUFFER=/dev/fb1
+
+or (for csh users):
+
+ setenv FRAMEBUFFER /dev/fb1
+
+After this the X server will use the second frame buffer.
+
+
+2. Programmer's View of /dev/fb*
+--------------------------------
+
+As you already know, a frame buffer device is a memory device like /dev/mem and
+it has the same features. You can read it, write it, seek to some location in
+it and mmap() it (the main usage). The difference is just that the memory that
+appears in the special file is not the whole memory, but the frame buffer of
+some video hardware.
+
+/dev/fb* also allows several ioctls on it, by which lots of information about
+the hardware can be queried and set. The color map handling works via ioctls,
+too. Look into <linux/fb.h> for more information on what ioctls exist and on
+which data structures they work. Here's just a brief overview:
+
+ - You can request unchangeable information about the hardware, like name,
+ organization of the screen memory (planes, packed pixels, ...) and address
+ and length of the screen memory.
+
+ - You can request and change variable information about the hardware, like
+ visible and virtual geometry, depth, color map format, timing, and so on.
+ If you try to change that information, the driver maybe will round up some
+ values to meet the hardware's capabilities (or return EINVAL if that isn't
+ possible).
+
+ - You can get and set parts of the color map. Communication is done with 16
+ bits per color part (red, green, blue, transparency) to support all
+ existing hardware. The driver does all the computations needed to apply
+ it to the hardware (round it down to less bits, maybe throw away
+ transparency).
+
+All this hardware abstraction makes the implementation of application programs
+easier and more portable. E.g. the X server works completely on /dev/fb* and
+thus doesn't need to know, for example, how the color registers of the concrete
+hardware are organized. XF68_FBDev is a general X server for bitmapped,
+unaccelerated video hardware. The only thing that has to be built into
+application programs is the screen organization (bitplanes or chunky pixels
+etc.), because it works on the frame buffer image data directly.
+
+For the future it is planned that frame buffer drivers for graphics cards and
+the like can be implemented as kernel modules that are loaded at runtime. Such
+a driver just has to call register_framebuffer() and supply some functions.
+Writing and distributing such drivers independently from the kernel will save
+much trouble...
+
+
+3. Frame Buffer Resolution Maintenance
+--------------------------------------
+
+Frame buffer resolutions are maintained using the utility `fbset'. It can
+change the video mode properties of the current resolution. Its main usage is
+to change the current video mode, e.g. during boot up in one of your /etc/rc.*
+or /etc/init.d/* files.
+
+Fbset uses a video mode database stored in a configuration file, so you can
+easily add your own modes and refer to them with a simple identifier.
+
+
+4. The X Server
+---------------
+
+The X server (XF68_FBDev) is the most notable application program for the frame
+buffer device. The current X server is part of the XFree86/XFree68 release
+3.3.1 package and has 2 modes:
+
+ - If the `Display' subsection for the `fbdev' driver in the /etc/XF86Config
+ file contains a
+
+ Modes "default"
+
+ line, the X server will use the scheme discussed above, i.e. it will start
+ up in the resolution determined by /dev/fb0current (or $FRAMEBUFFER, if
+ set). This is the default for the configuration file supplied with XFree68
+ 3.2. It's the most simple configuration (and the only possible one if you
+ want to have a broadcast compatible display, e.g. PAL or NTSC), but it has
+ some limitations.
+
+ - Therefore it's also possible to specify resolutions in the /etc/XF86Config
+ file. This allows for on-the-fly resolution switching while retaining the
+ same virtual desktop size. The frame buffer device that's used is still
+ /dev/fb0current (or $FRAMEBUFFER), but the available resolutions are
+ defined by /etc/XF86Config now. The disadvantage is that you have to
+ specify the timings in a different format (but `fbset -x' may help) and
+ that you can't have a broadcast compatible display (e.g. no PAL or NTSC).
+
+To tune a video mode, you can use fbset or xvidtune. Note that xvidtune doesn't
+work 100% with XF68_FBDev: the reported clock values are always incorrect.
+
+
+5. Video Mode Timings
+---------------------
+
+A monitor draws an image on the screen by using an electron beam (3 electron
+beams for most color models, 1 electron beam for Trinitron color monitors and
+monochrone monitors). The front of the screen is covered by a pattern of
+colored phospors (pixels). If a phospor is hit by an electron, it emits a
+photon and thus becomes visible.
+
+The electron beam draws horizontal lines (scanlines) from left to right, and
+from the top to the bottom of the screen. By modifying the intensity of the
+electron beam, pixels with various colors and intensities can be shown.
+
+After each scanline the electron beam has to move back to the left side of the
+screen and to the next line: this is called the horizontal retrace. After the
+whole screen (frame) was painted, the beam moves back to the upper left corner:
+this is called the vertical retrace. During both the horizontal and vertical
+retrace, the electron beam is turned off (blanked).
+
+The speed at which the electron beam paints the pixels is determined by the
+dotclock in the graphics board. For a dotclock of e.g. 28.37516 MHz (millions
+of cycles per second), each pixel is 35242 ps (picoseconds) long:
+
+ 1/(28.37516E6 Hz) = 35.242E-9 s
+
+If the screen resolution is 640x480, it will take
+
+ 640*35.242E-9 s = 22.555E-6 s
+
+to paint the 640 (xres) pixels on one scanline. But the horizontal retrace
+also takes time (e.g. 272 `pixels'), so a full scanline takes
+
+ (640+272)*35.242E-9 s = 32.141E-6 s
+
+We'll say that the horizontal scanrate is about 31 kHz:
+
+ 1/(32.141E-6 s) = 31.113E3 Hz
+
+A full screen counts 480 (yres) lines, but we have to consider the vertical
+retrace too (e.g. 49 `pixels'). So a full screen will take
+
+ (480+49)*32.141E-6 s = 17.002E-3 s
+
+The vertical scanrate is about 59 Hz:
+
+ 1/(17.002E-3 s) = 58.815 Hz
+
+This means the screen data is refreshed about 59 times per second. To have a
+stable picture without visible flicker, VESA recommends a vertical scanrate of
+at least 72 Hz. But the perceived flicker is very human dependent: some people
+can use 50 Hz without any trouble, while I'll notice if it's less than 80 Hz.
+
+Since the monitor doesn't know when a new scanline starts, the graphics board
+will supply a synchronization pulse (horizontal sync or hsync) for each
+scanline. Similarly it supplies a synchronization pulse (vertical sync or
+vsync) for each new frame. The position of the image on the screen is
+influenced by the moments at which the synchronization pulses occur.
+
+The following picture summarizes all timings. The horizontal retrace time is
+the sum of the left margin, the right margin and the hsync length, while the
+vertical retrace time is the sum of the upper margin, the lower margin and the
+vsync length.
+
+ +----------+---------------------------------------------+----------+-------+
+ | | ^ | | |
+ | | |upper_margin | | |
+ | | ¥ | | |
+ +----------###############################################----------+-------+
+ | # ^ # | |
+ | # | # | |
+ | # | # | |
+ | # | # | |
+ | left # | # right | hsync |
+ | margin # | xres # margin | len |
+ |<-------->#<---------------+--------------------------->#<-------->|<----->|
+ | # | # | |
+ | # | # | |
+ | # | # | |
+ | # |yres # | |
+ | # | # | |
+ | # | # | |
+ | # | # | |
+ | # | # | |
+ | # | # | |
+ | # | # | |
+ | # | # | |
+ | # | # | |
+ | # ¥ # | |
+ +----------###############################################----------+-------+
+ | | ^ | | |
+ | | |lower_margin | | |
+ | | ¥ | | |
+ +----------+---------------------------------------------+----------+-------+
+ | | ^ | | |
+ | | |vsync_len | | |
+ | | ¥ | | |
+ +----------+---------------------------------------------+----------+-------+
+
+The frame buffer device expects all horizontal timings in number of dotclocks
+(in picoseconds, 1E-12 s), and vertical timings in number of scanlines.
+
+
+6. Converting XFree86 timing values info frame buffer device timings
+--------------------------------------------------------------------
+
+An XFree86 mode line consists of the following fields:
+ "800x600" 50 800 856 976 1040 600 637 643 666
+ < name > DCF HR SH1 SH2 HFL VR SV1 SV2 VFL
+
+The frame buffer device uses the following fields:
+
+ - pixclock: pixel clock in ps (pico seconds)
+ - left_margin: time from sync to picture
+ - right_margin: time from picture to sync
+ - upper_margin: time from sync to picture
+ - lower_margin: time from picture to sync
+ - hsync_len: length of horizontal sync
+ - vsync_len: length of vertical sync
+
+1) Pixelclock:
+ xfree: in MHz
+ fb: In Picoseconds (ps)
+
+ pixclock = 1000000 / DCF
+
+2) horizontal timings:
+ left_margin = HFL - SH2
+ right_margin = SH1 - HR
+ hsync_len = SH2 - SH1
+
+3) vertical timings:
+ upper_margin = VFL - SV2
+ lower_margin = SV1 - VR
+ vsync_len = SV2 - SV1
+
+Good examples for VESA timings can be found in the XFree86 source tree,
+under "xc/programs/Xserver/hw/xfree86/doc/modeDB.txt".
+
+
+7. References
+-------------
+
+For more specific information about the frame buffer device and its
+applications, please refer to the following documentation:
+
+ - The manual pages for fbset: fbset(8), fb.modes(5)
+ - The manual pages for XFree68: XF68_FBDev(1), XF86Config(4/5)
+ - The mighty kernel sources:
+ o linux/include/linux/fb.h
+ o linux/drivers/char/fbmem.c
+ o linux/drivers/video/*fb.c
+
+
+8. Downloading
+--------------
+
+All necessary files can be found at
+
+ ftp://ftp.uni-erlangen.de/pub/Linux/LOCAL/680x0/
+
+and on its mirrors.
+
+
+9. Credits
+----------
+
+This readme was written by Geert Uytterhoeven, partly based on the original
+`X-framebuffer.README' by Roman Hodek and Martin Schaller. Section 6 was
+provided by Frank Neumann.
+
+The frame buffer device abstraction was designed by Martin Schaller.
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