本文详细介绍了Linux系统的帧缓冲设备原理及应用，包括设备文件、硬件抽象层、编程接口、分辨率设置工具fbset以及X服务器配置等内容。
1 The Frame Buffer Device
2 -----------------------
3
4 Maintained by Geert Uytterhoeven <geert@linux-m68k.org>
5 Last revised: May 10, 2001
6
7
8 0. Introduction
9 ---------------
10
11 The frame buffer device provides an abstraction for the graphics hardware. It
12 represents the frame buffer of some video hardware and allows application
13 software to access the graphics hardware through a well-defined interface, so
14 the software doesn't need to know anything about the low-level (hardware
15 register) stuff.
16
17 The device is accessed through special device nodes, usually located in the
18 /dev directory, i.e. /dev/fb*.
19
20
21 1. User's View of /dev/fb*
22 --------------------------
23
24 From the user's point of view, the frame buffer device looks just like any
25 other device in /dev. It's a character device using major 29; the minor
26 specifies the frame buffer number.
27
28 By convention, the following device nodes are used (numbers indicate the device
29 minor numbers):
30
31 0 = /dev/fb0 First frame buffer
32 1 = /dev/fb1 Second frame buffer
33 ...
34 31 = /dev/fb31 32nd frame buffer
35
36 For backwards compatibility, you may want to create the following symbolic
37 links:
38
39 /dev/fb0current -> fb0
40 /dev/fb1current -> fb1
41
42 and so on...
43
44 The frame buffer devices are also `normal' memory devices, this means, you can
45 read and write their contents. You can, for example, make a screen snapshot by
46
47 cp /dev/fb0 myfile
48
49 There also can be more than one frame buffer at a time, e.g. if you have a
50 graphics card in addition to the built-in hardware. The corresponding frame
51 buffer devices (/dev/fb0 and /dev/fb1 etc.) work independently.
52
53 Application software that uses the frame buffer device (e.g. the X server) will
54 use /dev/fb0 by default (older software uses /dev/fb0current). You can specify
55 an alternative frame buffer device by setting the environment variable
56 $FRAMEBUFFER to the path name of a frame buffer device, e.g. (for sh/bash
57 users):
58
59 export FRAMEBUFFER=/dev/fb1
60
61 or (for csh users):
62
63 setenv FRAMEBUFFER /dev/fb1
64
65 After this the X server will use the second frame buffer.
66
67
68 2. Programmer's View of /dev/fb*
69 --------------------------------
70
71 As you already know, a frame buffer device is a memory device like /dev/mem and
72 it has the same features. You can read it, write it, seek to some location in
73 it and mmap() it (the main usage). The difference is just that the memory that
74 appears in the special file is not the whole memory, but the frame buffer of
75 some video hardware.
76
77 /dev/fb* also allows several ioctls on it, by which lots of information about
78 the hardware can be queried and set. The color map handling works via ioctls,
79 too. Look into <linux/fb.h> for more information on what ioctls exist and on
80 which data structures they work. Here's just a brief overview:
81
82 - You can request unchangeable information about the hardware, like name,
83 organization of the screen memory (planes, packed pixels, ...) and address
84 and length of the screen memory.
85
86 - You can request and change variable information about the hardware, like
87 visible and virtual geometry, depth, color map format, timing, and so on.
88 If you try to change that information, the driver maybe will round up some
89 values to meet the hardware's capabilities (or return EINVAL if that isn't
90 possible).
91
92 - You can get and set parts of the color map. Communication is done with 16
93 bits per color part (red, green, blue, transparency) to support all
94 existing hardware. The driver does all the computations needed to apply
95 it to the hardware (round it down to less bits, maybe throw away
96 transparency).
97
98 All this hardware abstraction makes the implementation of application programs
99 easier and more portable. E.g. the X server works completely on /dev/fb* and
100 thus doesn't need to know, for example, how the color registers of the concrete
101 hardware are organized. XF68_FBDev is a general X server for bitmapped,
102 unaccelerated video hardware. The only thing that has to be built into
103 application programs is the screen organization (bitplanes or chunky pixels
104 etc.), because it works on the frame buffer image data directly.
105
106 For the future it is planned that frame buffer drivers for graphics cards and
107 the like can be implemented as kernel modules that are loaded at runtime. Such
108 a driver just has to call register_framebuffer() and supply some functions.
109 Writing and distributing such drivers independently from the kernel will save
110 much trouble...
111
112
113 3. Frame Buffer Resolution Maintenance
114 --------------------------------------
115
116 Frame buffer resolutions are maintained using the utility `fbset'. It can
117 change the video mode properties of a frame buffer device. Its main usage is
118 to change the current video mode, e.g. during boot up in one of your /etc/rc.*
119 or /etc/init.d/* files.
120
121 Fbset uses a video mode database stored in a configuration file, so you can
122 easily add your own modes and refer to them with a simple identifier.
123
124
125 4. The X Server
126 ---------------
127
128 The X server (XF68_FBDev) is the most notable application program for the frame
129 buffer device. Starting with XFree86 release 3.2, the X server is part of
130 XFree86 and has 2 modes:
131
132 - If the `Display' subsection for the `fbdev' driver in the /etc/XF86Config
133 file contains a
134
135 Modes "default"
136
137 line, the X server will use the scheme discussed above, i.e. it will start
138 up in the resolution determined by /dev/fb0 (or $FRAMEBUFFER, if set). You
139 still have to specify the color depth (using the Depth keyword) and virtual
140 resolution (using the Virtual keyword) though. This is the default for the
141 configuration file supplied with XFree86. It's the most simple
142 configuration, but it has some limitations.
143
144 - Therefore it's also possible to specify resolutions in the /etc/XF86Config
145 file. This allows for on-the-fly resolution switching while retaining the
146 same virtual desktop size. The frame buffer device that's used is still
147 /dev/fb0current (or $FRAMEBUFFER), but the available resolutions are
148 defined by /etc/XF86Config now. The disadvantage is that you have to
149 specify the timings in a different format (but `fbset -x' may help).
150
151 To tune a video mode, you can use fbset or xvidtune. Note that xvidtune doesn't
152 work 100% with XF68_FBDev: the reported clock values are always incorrect.
153
154
155 5. Video Mode Timings
156 ---------------------
157
158 A monitor draws an image on the screen by using an electron beam (3 electron
159 beams for color models, 1 electron beam for monochrome monitors). The front of
160 the screen is covered by a pattern of colored phosphors (pixels). If a phosphor
161 is hit by an electron, it emits a photon and thus becomes visible.
162
163 The electron beam draws horizontal lines (scanlines) from left to right, and
164 from the top to the bottom of the screen. By modifying the intensity of the
165 electron beam, pixels with various colors and intensities can be shown.
166
167 After each scanline the electron beam has to move back to the left side of the
168 screen and to the next line: this is called the horizontal retrace. After the
169 whole screen (frame) was painted, the beam moves back to the upper left corner:
170 this is called the vertical retrace. During both the horizontal and vertical
171 retrace, the electron beam is turned off (blanked).
172
173 The speed at which the electron beam paints the pixels is determined by the
174 dotclock in the graphics board. For a dotclock of e.g. 28.37516 MHz (millions
175 of cycles per second), each pixel is 35242 ps (picoseconds) long:
176
177 1/(28.37516E6 Hz) = 35.242E-9 s
178
179 If the screen resolution is 640x480, it will take
180
181 640*35.242E-9 s = 22.555E-6 s
182
183 to paint the 640 (xres) pixels on one scanline. But the horizontal retrace
184 also takes time (e.g. 272 `pixels'), so a full scanline takes
185
186 (640+272)*35.242E-9 s = 32.141E-6 s
187
188 We'll say that the horizontal scanrate is about 31 kHz:
189
190 1/(32.141E-6 s) = 31.113E3 Hz
191
192 A full screen counts 480 (yres) lines, but we have to consider the vertical
193 retrace too (e.g. 49 `pixels'). So a full screen will take
194
195 (480+49)*32.141E-6 s = 17.002E-3 s
196
197 The vertical scanrate is about 59 Hz:
198
199 1/(17.002E-3 s) = 58.815 Hz
200
201 This means the screen data is refreshed about 59 times per second. To have a
202 stable picture without visible flicker, VESA recommends a vertical scanrate of
203 at least 72 Hz. But the perceived flicker is very human dependent: some people
204 can use 50 Hz without any trouble, while I'll notice if it's less than 80 Hz.
205
206 Since the monitor doesn't know when a new scanline starts, the graphics board
207 will supply a synchronization pulse (horizontal sync or hsync) for each
208 scanline. Similarly it supplies a synchronization pulse (vertical sync or
209 vsync) for each new frame. The position of the image on the screen is
210 influenced by the moments at which the synchronization pulses occur.
211
212 The following picture summarizes all timings. The horizontal retrace time is
213 the sum of the left margin, the right margin and the hsync length, while the
214 vertical retrace time is the sum of the upper margin, the lower margin and the
215 vsync length.
216
217 +----------+---------------------------------------------+----------+-------+
218 | | ^ | | |
219 | | |upper_margin | | |
220 | | ¥ | | |
221 +----------###############################################----------+-------+
222 | # ^ # | |
223 | # | # | |
224 | # | # | |
225 | # | # | |
226 | left # | # right | hsync |
227 | margin # | xres # margin | len |
228 |<-------->#<---------------+--------------------------->#<-------->|<----->|
229 | # | # | |
230 | # | # | |
231 | # | # | |
232 | # |yres # | |
233 | # | # | |
234 | # | # | |
235 | # | # | |
236 | # | # | |
237 | # | # | |
238 | # | # | |
239 | # | # | |
240 | # | # | |
241 | # ¥ # | |
242 +----------###############################################----------+-------+
243 | | ^ | | |
244 | | |lower_margin | | |
245 | | ¥ | | |
246 +----------+---------------------------------------------+----------+-------+
247 | | ^ | | |
248 | | |vsync_len | | |
249 | | ¥ | | |
250 +----------+---------------------------------------------+----------+-------+
251
252 The frame buffer device expects all horizontal timings in number of dotclocks
253 (in picoseconds, 1E-12 s), and vertical timings in number of scanlines.
254
255
256 6. Converting XFree86 timing values info frame buffer device timings
257 --------------------------------------------------------------------
258
259 An XFree86 mode line consists of the following fields:
260 "800x600" 50 800 856 976 1040 600 637 643 666
261 < name > DCF HR SH1 SH2 HFL VR SV1 SV2 VFL
262
263 The frame buffer device uses the following fields:
264
265 - pixclock: pixel clock in ps (pico seconds)
266 - left_margin: time from sync to picture
267 - right_margin: time from picture to sync
268 - upper_margin: time from sync to picture
269 - lower_margin: time from picture to sync
270 - hsync_len: length of horizontal sync
271 - vsync_len: length of vertical sync
272
273 1) Pixelclock:
274 xfree: in MHz
275 fb: in picoseconds (ps)
276
277 pixclock = 1000000 / DCF
278
279 2) horizontal timings:
280 left_margin = HFL - SH2
281 right_margin = SH1 - HR
282 hsync_len = SH2 - SH1
283
284 3) vertical timings:
285 upper_margin = VFL - SV2
286 lower_margin = SV1 - VR
287 vsync_len = SV2 - SV1
288
289 Good examples for VESA timings can be found in the XFree86 source tree,
290 under "xc/programs/Xserver/hw/xfree86/doc/modeDB.txt".
291
292
293 7. References
294 -------------
295
296 For more specific information about the frame buffer device and its
297 applications, please refer to the Linux-fbdev website:
298
299 http://linux-fbdev.sourceforge.net/
300
301 and to the following documentation:
302
303 - The manual pages for fbset: fbset(8), fb.modes(5)
304 - The manual pages for XFree86: XF68_FBDev(1), XF86Config(4/5)
305 - The mighty kernel sources:
306 o linux/drivers/video/
307 o linux/include/linux/fb.h
308 o linux/include/video/
309
310
311
312 8. Mailing list
313 ---------------
314
315 There are several frame buffer device related mailing lists at SourceForge:
316 - linux-fbdev-announce@lists.sourceforge.net, for announcements,
317 - linux-fbdev-user@lists.sourceforge.net, for generic user support,
318 - linux-fbdev-devel@lists.sourceforge.net, for project developers.
319
320 Point your web browser to http://sourceforge.net/projects/linux-fbdev/ for
321 subscription information and archive browsing.
322
323
324 9. Downloading
325 --------------
326
327 All necessary files can be found at
328
329 ftp://ftp.uni-erlangen.de/pub/Linux/LOCAL/680x0/
330
331 and on its mirrors.
332
333 The latest version of fbset can be found at
334
335 http://home.tvd.be/cr26864/Linux/fbdev/
336
337
338 10. Credits
339 ----------
340
341 This readme was written by Geert Uytterhoeven, partly based on the original
342 `X-framebuffer.README' by Roman Hodek and Martin Schaller. Section 6 was
343 provided by Frank Neumann.
344
345 The frame buffer device abstraction was designed by Martin Schaller.
附录（added by wave）：
1. 在csdn上找到一篇此文的翻译版本，这就省去了我翻译的时间了。链接如下：
http://blog.youkuaiyun.com/ganxingming/archive/2006/06/05/774869.aspx
2. 帧缓冲驱动网站：
http://linux-fbdev.sourceforge.net
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