1======================= 2The Framebuffer Console 3======================= 4 5The framebuffer console (fbcon), as its name implies, is a text 6console running on top of the framebuffer device. It has the functionality of 7any standard text console driver, such as the VGA console, with the added 8features that can be attributed to the graphical nature of the framebuffer. 9 10In the x86 architecture, the framebuffer console is optional, and 11some even treat it as a toy. For other architectures, it is the only available 12display device, text or graphical. 13 14What are the features of fbcon? The framebuffer console supports 15high resolutions, varying font types, display rotation, primitive multihead, 16etc. Theoretically, multi-colored fonts, blending, aliasing, and any feature 17made available by the underlying graphics card are also possible. 18 19A. Configuration 20================ 21 22The framebuffer console can be enabled by using your favorite kernel 23configuration tool. It is under Device Drivers->Graphics Support-> 24Console display driver support->Framebuffer Console Support. 25Select 'y' to compile support statically or 'm' for module support. The 26module will be fbcon. 27 28In order for fbcon to activate, at least one framebuffer driver is 29required, so choose from any of the numerous drivers available. For x86 30systems, they almost universally have VGA cards, so vga16fb and vesafb will 31always be available. However, using a chipset-specific driver will give you 32more speed and features, such as the ability to change the video mode 33dynamically. 34 35To display the penguin logo, choose any logo available in Graphics 36support->Bootup logo. 37 38Also, you will need to select at least one compiled-in font, but if 39you don't do anything, the kernel configuration tool will select one for you, 40usually an 8x16 font. 41 42.. admonition:: GOTCHA 43 44 A common bug report is enabling the framebuffer without enabling the 45 framebuffer console. Depending on the driver, you may get a blanked or 46 garbled display, but the system still boots to completion. If you are 47 fortunate to have a driver that does not alter the graphics chip, then you 48 will still get a VGA console. 49 50B. Loading 51========== 52 53Possible scenarios: 54 551. Driver and fbcon are compiled statically 56 57 Usually, fbcon will automatically take over your console. The notable 58 exception is vesafb. It needs to be explicitly activated with the 59 vga= boot option parameter. 60 612. Driver is compiled statically, fbcon is compiled as a module 62 63 Depending on the driver, you either get a standard console, or a 64 garbled display, as mentioned above. To get a framebuffer console, 65 do a 'modprobe fbcon'. 66 673. Driver is compiled as a module, fbcon is compiled statically 68 69 You get your standard console. Once the driver is loaded with 70 'modprobe xxxfb', fbcon automatically takes over the console with 71 the possible exception of using the fbcon=map:n option. See below. 72 734. Driver and fbcon are compiled as a module. 74 75 You can load them in any order. Once both are loaded, fbcon will take 76 over the console. 77 78C. Boot options 79=============== 80 81 The framebuffer console has several, largely unknown, boot options 82 that can change its behavior. 83 841. fbcon=font:<name> 85 86 Select the initial font to use. The value 'name' can be any of the 87 compiled-in fonts: 10x18, 6x10, 6x8, 7x14, Acorn8x8, MINI4x6, 88 PEARL8x8, ProFont6x11, SUN12x22, SUN8x16, TER16x32, VGA8x16, VGA8x8. 89 90 Note, not all drivers can handle font with widths not divisible by 8, 91 such as vga16fb. 92 93 942. fbcon=map:<0123> 95 96 This is an interesting option. It tells which driver gets mapped to 97 which console. The value '0123' is a sequence that gets repeated until 98 the total length is 64 which is the number of consoles available. In 99 the above example, it is expanded to 012301230123... and the mapping 100 will be:: 101 102 tty | 1 2 3 4 5 6 7 8 9 ... 103 fb | 0 1 2 3 0 1 2 3 0 ... 104 105 ('cat /proc/fb' should tell you what the fb numbers are) 106 107 One side effect that may be useful is using a map value that exceeds 108 the number of loaded fb drivers. For example, if only one driver is 109 available, fb0, adding fbcon=map:1 tells fbcon not to take over the 110 console. 111 112 Later on, when you want to map the console the to the framebuffer 113 device, you can use the con2fbmap utility. 114 1153. fbcon=vc:<n1>-<n2> 116 117 This option tells fbcon to take over only a range of consoles as 118 specified by the values 'n1' and 'n2'. The rest of the consoles 119 outside the given range will still be controlled by the standard 120 console driver. 121 122 .. note:: 123 For x86 machines, the standard console is the VGA console which 124 is typically located on the same video card. Thus, the consoles that 125 are controlled by the VGA console will be garbled. 126 1274. fbcon=rotate:<n> 128 129 This option changes the orientation angle of the console display. The 130 value 'n' accepts the following: 131 132 - 0 - normal orientation (0 degree) 133 - 1 - clockwise orientation (90 degrees) 134 - 2 - upside down orientation (180 degrees) 135 - 3 - counterclockwise orientation (270 degrees) 136 137 The angle can be changed anytime afterwards by 'echoing' the same 138 numbers to any one of the 2 attributes found in 139 /sys/class/graphics/fbcon: 140 141 - rotate - rotate the display of the active console 142 - rotate_all - rotate the display of all consoles 143 144 Console rotation will only become available if Framebuffer Console 145 Rotation support is compiled in your kernel. 146 147 .. note:: 148 This is purely console rotation. Any other applications that 149 use the framebuffer will remain at their 'normal' orientation. 150 Actually, the underlying fb driver is totally ignorant of console 151 rotation. 152 1535. fbcon=margin:<color> 154 155 This option specifies the color of the margins. The margins are the 156 leftover area at the right and the bottom of the screen that are not 157 used by text. By default, this area will be black. The 'color' value 158 is an integer number that depends on the framebuffer driver being used. 159 1606. fbcon=nodefer 161 162 If the kernel is compiled with deferred fbcon takeover support, normally 163 the framebuffer contents, left in place by the firmware/bootloader, will 164 be preserved until there actually is some text is output to the console. 165 This option causes fbcon to bind immediately to the fbdev device. 166 1677. fbcon=logo-pos:<location> 168 169 The only possible 'location' is 'center' (without quotes), and when 170 given, the bootup logo is moved from the default top-left corner 171 location to the center of the framebuffer. If more than one logo is 172 displayed due to multiple CPUs, the collected line of logos is moved 173 as a whole. 174 1758. fbcon=logo-count:<n> 176 177 The value 'n' overrides the number of bootup logos. 0 disables the 178 logo, and -1 gives the default which is the number of online CPUs. 179 180D. Attaching, Detaching and Unloading 181===================================== 182 183Before going on to how to attach, detach and unload the framebuffer console, an 184illustration of the dependencies may help. 185 186The console layer, as with most subsystems, needs a driver that interfaces with 187the hardware. Thus, in a VGA console:: 188 189 console ---> VGA driver ---> hardware. 190 191Assuming the VGA driver can be unloaded, one must first unbind the VGA driver 192from the console layer before unloading the driver. The VGA driver cannot be 193unloaded if it is still bound to the console layer. (See 194Documentation/driver-api/console.rst for more information). 195 196This is more complicated in the case of the framebuffer console (fbcon), 197because fbcon is an intermediate layer between the console and the drivers:: 198 199 console ---> fbcon ---> fbdev drivers ---> hardware 200 201The fbdev drivers cannot be unloaded if bound to fbcon, and fbcon cannot 202be unloaded if it's bound to the console layer. 203 204So to unload the fbdev drivers, one must first unbind fbcon from the console, 205then unbind the fbdev drivers from fbcon. Fortunately, unbinding fbcon from 206the console layer will automatically unbind framebuffer drivers from 207fbcon. Thus, there is no need to explicitly unbind the fbdev drivers from 208fbcon. 209 210So, how do we unbind fbcon from the console? Part of the answer is in 211Documentation/driver-api/console.rst. To summarize: 212 213Echo a value to the bind file that represents the framebuffer console 214driver. So assuming vtcon1 represents fbcon, then:: 215 216 echo 1 > /sys/class/vtconsole/vtcon1/bind - attach framebuffer console to 217 console layer 218 echo 0 > /sys/class/vtconsole/vtcon1/bind - detach framebuffer console from 219 console layer 220 221If fbcon is detached from the console layer, your boot console driver (which is 222usually VGA text mode) will take over. A few drivers (rivafb and i810fb) will 223restore VGA text mode for you. With the rest, before detaching fbcon, you 224must take a few additional steps to make sure that your VGA text mode is 225restored properly. The following is one of the several methods that you can do: 226 2271. Download or install vbetool. This utility is included with most 228 distributions nowadays, and is usually part of the suspend/resume tool. 229 2302. In your kernel configuration, ensure that CONFIG_FRAMEBUFFER_CONSOLE is set 231 to 'y' or 'm'. Enable one or more of your favorite framebuffer drivers. 232 2333. Boot into text mode and as root run:: 234 235 vbetool vbestate save > <vga state file> 236 237 The above command saves the register contents of your graphics 238 hardware to <vga state file>. You need to do this step only once as 239 the state file can be reused. 240 2414. If fbcon is compiled as a module, load fbcon by doing:: 242 243 modprobe fbcon 244 2455. Now to detach fbcon:: 246 247 vbetool vbestate restore < <vga state file> && \ 248 echo 0 > /sys/class/vtconsole/vtcon1/bind 249 2506. That's it, you're back to VGA mode. And if you compiled fbcon as a module, 251 you can unload it by 'rmmod fbcon'. 252 2537. To reattach fbcon:: 254 255 echo 1 > /sys/class/vtconsole/vtcon1/bind 256 2578. Once fbcon is unbound, all drivers registered to the system will also 258 become unbound. This means that fbcon and individual framebuffer drivers 259 can be unloaded or reloaded at will. Reloading the drivers or fbcon will 260 automatically bind the console, fbcon and the drivers together. Unloading 261 all the drivers without unloading fbcon will make it impossible for the 262 console to bind fbcon. 263 264Notes for vesafb users: 265======================= 266 267Unfortunately, if your bootline includes a vga=xxx parameter that sets the 268hardware in graphics mode, such as when loading vesafb, vgacon will not load. 269Instead, vgacon will replace the default boot console with dummycon, and you 270won't get any display after detaching fbcon. Your machine is still alive, so 271you can reattach vesafb. However, to reattach vesafb, you need to do one of 272the following: 273 274Variation 1: 275 276 a. Before detaching fbcon, do:: 277 278 vbetool vbemode save > <vesa state file> # do once for each vesafb mode, 279 # the file can be reused 280 281 b. Detach fbcon as in step 5. 282 283 c. Attach fbcon:: 284 285 vbetool vbestate restore < <vesa state file> && \ 286 echo 1 > /sys/class/vtconsole/vtcon1/bind 287 288Variation 2: 289 290 a. Before detaching fbcon, do:: 291 292 echo <ID> > /sys/class/tty/console/bind 293 294 vbetool vbemode get 295 296 b. Take note of the mode number 297 298 b. Detach fbcon as in step 5. 299 300 c. Attach fbcon:: 301 302 vbetool vbemode set <mode number> && \ 303 echo 1 > /sys/class/vtconsole/vtcon1/bind 304 305Samples: 306======== 307 308Here are 2 sample bash scripts that you can use to bind or unbind the 309framebuffer console driver if you are on an X86 box:: 310 311 #!/bin/bash 312 # Unbind fbcon 313 314 # Change this to where your actual vgastate file is located 315 # Or Use VGASTATE=$1 to indicate the state file at runtime 316 VGASTATE=/tmp/vgastate 317 318 # path to vbetool 319 VBETOOL=/usr/local/bin 320 321 322 for (( i = 0; i < 16; i++)) 323 do 324 if test -x /sys/class/vtconsole/vtcon$i; then 325 if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \ 326 = 1 ]; then 327 if test -x $VBETOOL/vbetool; then 328 echo Unbinding vtcon$i 329 $VBETOOL/vbetool vbestate restore < $VGASTATE 330 echo 0 > /sys/class/vtconsole/vtcon$i/bind 331 fi 332 fi 333 fi 334 done 335 336--------------------------------------------------------------------------- 337 338:: 339 340 #!/bin/bash 341 # Bind fbcon 342 343 for (( i = 0; i < 16; i++)) 344 do 345 if test -x /sys/class/vtconsole/vtcon$i; then 346 if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \ 347 = 1 ]; then 348 echo Unbinding vtcon$i 349 echo 1 > /sys/class/vtconsole/vtcon$i/bind 350 fi 351 fi 352 done 353 354Antonino Daplas <adaplas@pol.net> 355