Root/Documentation/fb/framebuffer.txt

1            The Frame Buffer Device
2            -----------------------
3
4Maintained by Geert Uytterhoeven <geert@linux-m68k.org>
5Last revised: May 10, 2001
6
7
80. Introduction
9---------------
10
11The frame buffer device provides an abstraction for the graphics hardware. It
12represents the frame buffer of some video hardware and allows application
13software to access the graphics hardware through a well-defined interface, so
14the software doesn't need to know anything about the low-level (hardware
15register) stuff.
16
17The device is accessed through special device nodes, usually located in the
18/dev directory, i.e. /dev/fb*.
19
20
211. User's View of /dev/fb*
22--------------------------
23
24From the user's point of view, the frame buffer device looks just like any
25other device in /dev. It's a character device using major 29; the minor
26specifies the frame buffer number.
27
28By convention, the following device nodes are used (numbers indicate the device
29minor 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
36For backwards compatibility, you may want to create the following symbolic
37links:
38
39    /dev/fb0current -> fb0
40    /dev/fb1current -> fb1
41
42and so on...
43
44The frame buffer devices are also `normal' memory devices, this means, you can
45read and write their contents. You can, for example, make a screen snapshot by
46
47  cp /dev/fb0 myfile
48
49There also can be more than one frame buffer at a time, e.g. if you have a
50graphics card in addition to the built-in hardware. The corresponding frame
51buffer devices (/dev/fb0 and /dev/fb1 etc.) work independently.
52
53Application software that uses the frame buffer device (e.g. the X server) will
54use /dev/fb0 by default (older software uses /dev/fb0current). You can specify
55an 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
57users):
58
59    export FRAMEBUFFER=/dev/fb1
60
61or (for csh users):
62
63    setenv FRAMEBUFFER /dev/fb1
64
65After this the X server will use the second frame buffer.
66
67
682. Programmer's View of /dev/fb*
69--------------------------------
70
71As you already know, a frame buffer device is a memory device like /dev/mem and
72it has the same features. You can read it, write it, seek to some location in
73it and mmap() it (the main usage). The difference is just that the memory that
74appears in the special file is not the whole memory, but the frame buffer of
75some video hardware.
76
77/dev/fb* also allows several ioctls on it, by which lots of information about
78the hardware can be queried and set. The color map handling works via ioctls,
79too. Look into <linux/fb.h> for more information on what ioctls exist and on
80which 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
98All this hardware abstraction makes the implementation of application programs
99easier and more portable. E.g. the X server works completely on /dev/fb* and
100thus doesn't need to know, for example, how the color registers of the concrete
101hardware are organized. XF68_FBDev is a general X server for bitmapped,
102unaccelerated video hardware. The only thing that has to be built into
103application programs is the screen organization (bitplanes or chunky pixels
104etc.), because it works on the frame buffer image data directly.
105
106For the future it is planned that frame buffer drivers for graphics cards and
107the like can be implemented as kernel modules that are loaded at runtime. Such
108a driver just has to call register_framebuffer() and supply some functions.
109Writing and distributing such drivers independently from the kernel will save
110much trouble...
111
112
1133. Frame Buffer Resolution Maintenance
114--------------------------------------
115
116Frame buffer resolutions are maintained using the utility `fbset'. It can
117change the video mode properties of a frame buffer device. Its main usage is
118to change the current video mode, e.g. during boot up in one of your /etc/rc.*
119or /etc/init.d/* files.
120
121Fbset uses a video mode database stored in a configuration file, so you can
122easily add your own modes and refer to them with a simple identifier.
123
124
1254. The X Server
126---------------
127
128The X server (XF68_FBDev) is the most notable application program for the frame
129buffer device. Starting with XFree86 release 3.2, the X server is part of
130XFree86 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
151To tune a video mode, you can use fbset or xvidtune. Note that xvidtune doesn't
152work 100% with XF68_FBDev: the reported clock values are always incorrect.
153
154
1555. Video Mode Timings
156---------------------
157
158A monitor draws an image on the screen by using an electron beam (3 electron
159beams for color models, 1 electron beam for monochrome monitors). The front of
160the screen is covered by a pattern of colored phosphors (pixels). If a phosphor
161is hit by an electron, it emits a photon and thus becomes visible.
162
163The electron beam draws horizontal lines (scanlines) from left to right, and
164from the top to the bottom of the screen. By modifying the intensity of the
165electron beam, pixels with various colors and intensities can be shown.
166
167After each scanline the electron beam has to move back to the left side of the
168screen and to the next line: this is called the horizontal retrace. After the
169whole screen (frame) was painted, the beam moves back to the upper left corner:
170this is called the vertical retrace. During both the horizontal and vertical
171retrace, the electron beam is turned off (blanked).
172
173The speed at which the electron beam paints the pixels is determined by the
174dotclock in the graphics board. For a dotclock of e.g. 28.37516 MHz (millions
175of cycles per second), each pixel is 35242 ps (picoseconds) long:
176
177    1/(28.37516E6 Hz) = 35.242E-9 s
178
179If the screen resolution is 640x480, it will take
180
181    640*35.242E-9 s = 22.555E-6 s
182
183to paint the 640 (xres) pixels on one scanline. But the horizontal retrace
184also takes time (e.g. 272 `pixels'), so a full scanline takes
185
186    (640+272)*35.242E-9 s = 32.141E-6 s
187
188We'll say that the horizontal scanrate is about 31 kHz:
189
190    1/(32.141E-6 s) = 31.113E3 Hz
191
192A full screen counts 480 (yres) lines, but we have to consider the vertical
193retrace too (e.g. 49 `lines'). So a full screen will take
194
195    (480+49)*32.141E-6 s = 17.002E-3 s
196
197The vertical scanrate is about 59 Hz:
198
199    1/(17.002E-3 s) = 58.815 Hz
200
201This means the screen data is refreshed about 59 times per second. To have a
202stable picture without visible flicker, VESA recommends a vertical scanrate of
203at least 72 Hz. But the perceived flicker is very human dependent: some people
204can use 50 Hz without any trouble, while I'll notice if it's less than 80 Hz.
205
206Since the monitor doesn't know when a new scanline starts, the graphics board
207will supply a synchronization pulse (horizontal sync or hsync) for each
208scanline. Similarly it supplies a synchronization pulse (vertical sync or
209vsync) for each new frame. The position of the image on the screen is
210influenced by the moments at which the synchronization pulses occur.
211
212The following picture summarizes all timings. The horizontal retrace time is
213the sum of the left margin, the right margin and the hsync length, while the
214vertical retrace time is the sum of the upper margin, the lower margin and the
215vsync 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
252The 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
2566. Converting XFree86 timing values info frame buffer device timings
257--------------------------------------------------------------------
258
259An 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
263The 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
2731) Pixelclock:
274   xfree: in MHz
275   fb: in picoseconds (ps)
276
277   pixclock = 1000000 / DCF
278
2792) horizontal timings:
280   left_margin = HFL - SH2
281   right_margin = SH1 - HR
282   hsync_len = SH2 - SH1
283
2843) vertical timings:
285   upper_margin = VFL - SV2
286   lower_margin = SV1 - VR
287   vsync_len = SV2 - SV1
288
289Good examples for VESA timings can be found in the XFree86 source tree,
290under "xc/programs/Xserver/hw/xfree86/doc/modeDB.txt".
291
292
2937. References
294-------------
295
296For more specific information about the frame buffer device and its
297applications, please refer to the Linux-fbdev website:
298
299    http://linux-fbdev.sourceforge.net/
300
301and 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
3128. Mailing list
313---------------
314
315There is a frame buffer device related mailing list at kernel.org:
316linux-fbdev@vger.kernel.org.
317
318Point your web browser to http://sourceforge.net/projects/linux-fbdev/ for
319subscription information and archive browsing.
320
321
3229. Downloading
323--------------
324
325All necessary files can be found at
326
327    ftp://ftp.uni-erlangen.de/pub/Linux/LOCAL/680x0/
328
329and on its mirrors.
330
331The latest version of fbset can be found at
332
333    http://www.linux-fbdev.org/
334
335  
33610. Credits
337----------
338                
339This readme was written by Geert Uytterhoeven, partly based on the original
340`X-framebuffer.README' by Roman Hodek and Martin Schaller. Section 6 was
341provided by Frank Neumann.
342
343The frame buffer device abstraction was designed by Martin Schaller.
344

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