mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-12-01 19:36:48 +07:00
c69f677cc8
The fbdev mailing lists at SourceForge have been migrated to a single mailing list at kernel.org: linux-fbdev@vger.kernel.org. Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org> Acked-by: Jean Delvare <khali@linux-fr.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
344 lines
14 KiB
Plaintext
344 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 | | |
|
|
| | ↓ | | |
|
|
+----------###############################################----------+-------+
|
|
| # ↑ # | |
|
|
| # | # | |
|
|
| # | # | |
|
|
| # | # | |
|
|
| 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 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 is a frame buffer device related mailing list at kernel.org:
|
|
linux-fbdev@vger.kernel.org.
|
|
|
|
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.
|