1 ======================= 2 The Frame Buffer Device 3 ======================= 4 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 `lines`). 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:: 260 261 "800x600" 50 800 856 976 1040 600 637 643 666 262 < name > DCF HR SH1 SH2 HFL VR SV1 SV2 VFL 263 264 The frame buffer device uses the following fields: 265 266 - pixclock: pixel clock in ps (pico seconds) 267 - left_margin: time from sync to picture 268 - right_margin: time from picture to sync 269 - upper_margin: time from sync to picture 270 - lower_margin: time from picture to sync 271 - hsync_len: length of horizontal sync 272 - vsync_len: length of vertical sync 273 274 1) Pixelclock: 275 276 xfree: in MHz 277 278 fb: in picoseconds (ps) 279 280 pixclock = 1000000 / DCF 281 282 2) horizontal timings: 283 284 left_margin = HFL - SH2 285 286 right_margin = SH1 - HR 287 288 hsync_len = SH2 - SH1 289 290 3) vertical timings: 291 292 upper_margin = VFL - SV2 293 294 lower_margin = SV1 - VR 295 296 vsync_len = SV2 - SV1 297 298 Good examples for VESA timings can be found in the XFree86 source tree, 299 under "xc/programs/Xserver/hw/xfree86/doc/modeDB.txt". 300 301 302 7. References 303 ------------- 304 305 For more specific information about the frame buffer device and its 306 applications, please refer to the Linux-fbdev website: 307 308 http://linux-fbdev.sourceforge.net/ 309 310 and to the following documentation: 311 312 - The manual pages for fbset: fbset(8), fb.modes(5) 313 - The manual pages for XFree86: XF68_FBDev(1), XF86Config(4/5) 314 - The mighty kernel sources: 315 316 - linux/drivers/video/ 317 - linux/include/linux/fb.h 318 - linux/include/video/ 319 320 321 322 8. Mailing list 323 --------------- 324 325 There is a frame buffer device related mailing list at kernel.org: 326 linux-fbdev@vger.kernel.org. 327 328 Point your web browser to http://sourceforge.net/projects/linux-fbdev/ for 329 subscription information and archive browsing. 330 331 332 9. Downloading 333 -------------- 334 335 All necessary files can be found at 336 337 ftp://ftp.uni-erlangen.de/pub/Linux/LOCAL/680x0/ 338 339 and on its mirrors. 340 341 The latest version of fbset can be found at 342 343 http://www.linux-fbdev.org/ 344 345 346 10. Credits 347 ----------- 348 349 This readme was written by Geert Uytterhoeven, partly based on the original 350 `X-framebuffer.README` by Roman Hodek and Martin Schaller. Section 6 was 351 provided by Frank Neumann. 352 353 The frame buffer device abstraction was designed by Martin Schaller.
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.