mirror of
https://github.com/torvalds/linux.git
synced 2024-11-17 17:41:44 +00:00
1da177e4c3
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
346 lines
14 KiB
Plaintext
346 lines
14 KiB
Plaintext
The Frame Buffer Device
|
||
-----------------------
|
||
|
||
Maintained by Geert Uytterhoeven <geert@linux-m68k.org>
|
||
Last revised: May 10, 2001
|
||
|
||
|
||
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
|
||
1 = /dev/fb1 Second frame buffer
|
||
...
|
||
31 = /dev/fb31 32nd 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 a frame buffer device. 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. Starting with XFree86 release 3.2, the X server is part of
|
||
XFree86 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/fb0 (or $FRAMEBUFFER, if set). You
|
||
still have to specify the color depth (using the Depth keyword) and virtual
|
||
resolution (using the Virtual keyword) though. This is the default for the
|
||
configuration file supplied with XFree86. It's the most simple
|
||
configuration, 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).
|
||
|
||
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 color models, 1 electron beam for monochrome monitors). The front of
|
||
the screen is covered by a pattern of colored phosphors (pixels). If a phosphor
|
||
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 `lines'). 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 | | |
|
||
| | <20> | | |
|
||
+----------###############################################----------+-------+
|
||
| # ^ # | |
|
||
| # | # | |
|
||
| # | # | |
|
||
| # | # | |
|
||
| left # | # right | hsync |
|
||
| margin # | xres # margin | len |
|
||
|<-------->#<---------------+--------------------------->#<-------->|<----->|
|
||
| # | # | |
|
||
| # | # | |
|
||
| # | # | |
|
||
| # |yres # | |
|
||
| # | # | |
|
||
| # | # | |
|
||
| # | # | |
|
||
| # | # | |
|
||
| # | # | |
|
||
| # | # | |
|
||
| # | # | |
|
||
| # | # | |
|
||
| # <20> # | |
|
||
+----------###############################################----------+-------+
|
||
| | ^ | | |
|
||
| | |lower_margin | | |
|
||
| | <20> | | |
|
||
+----------+---------------------------------------------+----------+-------+
|
||
| | ^ | | |
|
||
| | |vsync_len | | |
|
||
| | <20> | | |
|
||
+----------+---------------------------------------------+----------+-------+
|
||
|
||
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 Linux-fbdev website:
|
||
|
||
http://linux-fbdev.sourceforge.net/
|
||
|
||
and to the following documentation:
|
||
|
||
- The manual pages for fbset: fbset(8), fb.modes(5)
|
||
- The manual pages for XFree86: XF68_FBDev(1), XF86Config(4/5)
|
||
- The mighty kernel sources:
|
||
o linux/drivers/video/
|
||
o linux/include/linux/fb.h
|
||
o linux/include/video/
|
||
|
||
|
||
|
||
8. Mailing list
|
||
---------------
|
||
|
||
There are several frame buffer device related mailing lists at SourceForge:
|
||
- linux-fbdev-announce@lists.sourceforge.net, for announcements,
|
||
- linux-fbdev-user@lists.sourceforge.net, for generic user support,
|
||
- linux-fbdev-devel@lists.sourceforge.net, for project developers.
|
||
|
||
Point your web browser to http://sourceforge.net/projects/linux-fbdev/ for
|
||
subscription information and archive browsing.
|
||
|
||
|
||
9. Downloading
|
||
--------------
|
||
|
||
All necessary files can be found at
|
||
|
||
ftp://ftp.uni-erlangen.de/pub/Linux/LOCAL/680x0/
|
||
|
||
and on its mirrors.
|
||
|
||
The latest version of fbset can be found at
|
||
|
||
http://home.tvd.be/cr26864/Linux/fbdev/
|
||
|
||
|
||
10. 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.
|