linux/Documentation/fb/fbcon.txt
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   1The Framebuffer Console
   2=======================
   3
   4        The framebuffer console (fbcon), as its name implies, is a text
   5console running on top of the framebuffer device. It has the functionality of
   6any standard text console driver, such as the VGA console, with the added
   7features that can be attributed to the graphical nature of the framebuffer.
   8
   9         In the x86 architecture, the framebuffer console is optional, and
  10some even treat it as a toy. For other architectures, it is the only available
  11display device, text or graphical.
  12
  13         What are the features of fbcon?  The framebuffer console supports
  14high resolutions, varying font types, display rotation, primitive multihead,
  15etc. Theoretically, multi-colored fonts, blending, aliasing, and any feature
  16made available by the underlying graphics card are also possible.
  17
  18A. Configuration
  19
  20        The framebuffer console can be enabled by using your favorite kernel
  21configuration tool.  It is under Device Drivers->Graphics Support->Support for
  22framebuffer devices->Framebuffer Console Support. Select 'y' to compile
  23support statically, or 'm' for module support.  The module will be fbcon.
  24
  25        In order for fbcon to activate, at least one framebuffer driver is
  26required, so choose from any of the numerous drivers available. For x86
  27systems, they almost universally have VGA cards, so vga16fb and vesafb will
  28always be available. However, using a chipset-specific driver will give you
  29more speed and features, such as the ability to change the video mode
  30dynamically.
  31
  32        To display the penguin logo, choose any logo available in Logo
  33Configuration->Boot up logo.
  34
  35        Also, you will need to select at least one compiled-in fonts, but if
  36you don't do anything, the kernel configuration tool will select one for you,
  37usually an 8x16 font.
  38
  39GOTCHA: A common bug report is enabling the framebuffer without enabling the
  40framebuffer console.  Depending on the driver, you may get a blanked or
  41garbled display, but the system still boots to completion.  If you are
  42fortunate to have a driver that does not alter the graphics chip, then you
  43will still get a VGA console.
  44
  45B. Loading
  46
  47Possible scenarios:
  48
  491. Driver and fbcon are compiled statically
  50
  51         Usually, fbcon will automatically take over your console. The notable
  52         exception is vesafb.  It needs to be explicitly activated with the
  53         vga= boot option parameter.
  54
  552. Driver is compiled statically, fbcon is compiled as a module
  56
  57         Depending on the driver, you either get a standard console, or a
  58         garbled display, as mentioned above.  To get a framebuffer console,
  59         do a 'modprobe fbcon'.
  60
  613. Driver is compiled as a module, fbcon is compiled statically
  62
  63         You get your standard console.  Once the driver is loaded with
  64         'modprobe xxxfb', fbcon automatically takes over the console with
  65         the possible exception of using the fbcon=map:n option. See below.
  66
  674. Driver and fbcon are compiled as a module.
  68
  69         You can load them in any order. Once both are loaded, fbcon will take
  70         over the console.
  71
  72C. Boot options
  73
  74         The framebuffer console has several, largely unknown, boot options
  75         that can change its behavior.
  76
  771. fbcon=font:<name>
  78
  79        Select the initial font to use. The value 'name' can be any of the
  80        compiled-in fonts: VGA8x16, 7x14, 10x18, VGA8x8, MINI4x6, RomanLarge,
  81        SUN8x16, SUN12x22, ProFont6x11, Acorn8x8, PEARL8x8.
  82
  83        Note, not all drivers can handle font with widths not divisible by 8,
  84        such as vga16fb.
  85
  862. fbcon=scrollback:<value>[k]
  87
  88        The scrollback buffer is memory that is used to preserve display
  89        contents that has already scrolled past your view.  This is accessed
  90        by using the Shift-PageUp key combination.  The value 'value' is any
  91        integer. It defaults to 32KB.  The 'k' suffix is optional, and will
  92        multiply the 'value' by 1024.
  93
  943. 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
 1154. 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: For x86 machines, the standard console is the VGA console which
 123        is typically located on the same video card.  Thus, the consoles that
 124        are controlled by the VGA console will be garbled.
 125
 1264. fbcon=rotate:<n>
 127
 128        This option changes the orientation angle of the console display. The
 129        value 'n' accepts the following:
 130
 131              0 - normal orientation (0 degree)
 132              1 - clockwise orientation (90 degrees)
 133              2 - upside down orientation (180 degrees)
 134              3 - counterclockwise orientation (270 degrees)
 135
 136        The angle can be changed anytime afterwards by 'echoing' the same
 137        numbers to any one of the 2 attributes found in
 138         /sys/class/graphics/fbcon
 139
 140                rotate     - rotate the display of the active console
 141                rotate_all - rotate the display of all consoles
 142
 143        Console rotation will only become available if Console Rotation
 144        Support is compiled in your kernel.
 145
 146        NOTE: This is purely console rotation.  Any other applications that
 147        use the framebuffer will remain at their 'normal'orientation.
 148        Actually, the underlying fb driver is totally ignorant of console
 149        rotation.
 150
 151C. Attaching, Detaching and Unloading
 152
 153Before going on on how to attach, detach and unload the framebuffer console, an
 154illustration of the dependencies may help.
 155
 156The console layer, as with most subsystems, needs a driver that interfaces with
 157the hardware. Thus, in a VGA console:
 158
 159console ---> VGA driver ---> hardware.
 160
 161Assuming the VGA driver can be unloaded, one must first unbind the VGA driver
 162from the console layer before unloading the driver.  The VGA driver cannot be
 163unloaded if it is still bound to the console layer. (See
 164Documentation/console/console.txt for more information).
 165
 166This is more complicated in the case of the framebuffer console (fbcon),
 167because fbcon is an intermediate layer between the console and the drivers:
 168
 169console ---> fbcon ---> fbdev drivers ---> hardware
 170
 171The fbdev drivers cannot be unloaded if it's bound to fbcon, and fbcon cannot
 172be unloaded if it's bound to the console layer.
 173
 174So to unload the fbdev drivers, one must first unbind fbcon from the console,
 175then unbind the fbdev drivers from fbcon.  Fortunately, unbinding fbcon from
 176the console layer will automatically unbind framebuffer drivers from
 177fbcon. Thus, there is no need to explicitly unbind the fbdev drivers from
 178fbcon.
 179
 180So, how do we unbind fbcon from the console? Part of the answer is in
 181Documentation/console/console.txt. To summarize:
 182
 183Echo a value to the bind file that represents the framebuffer console
 184driver. So assuming vtcon1 represents fbcon, then:
 185
 186echo 1 > sys/class/vtconsole/vtcon1/bind - attach framebuffer console to
 187                                           console layer
 188echo 0 > sys/class/vtconsole/vtcon1/bind - detach framebuffer console from
 189                                           console layer
 190
 191If fbcon is detached from the console layer, your boot console driver (which is
 192usually VGA text mode) will take over.  A few drivers (rivafb and i810fb) will
 193restore VGA text mode for you.  With the rest, before detaching fbcon, you
 194must take a few additional steps to make sure that your VGA text mode is
 195restored properly. The following is one of the several methods that you can do:
 196
 1971. Download or install vbetool.  This utility is included with most
 198   distributions nowadays, and is usually part of the suspend/resume tool.
 199
 2002. In your kernel configuration, ensure that CONFIG_FRAMEBUFFER_CONSOLE is set
 201   to 'y' or 'm'. Enable one or more of your favorite framebuffer drivers.
 202
 2033. Boot into text mode and as root run:
 204
 205        vbetool vbestate save > <vga state file>
 206
 207        The above command saves the register contents of your graphics
 208        hardware to <vga state file>.  You need to do this step only once as
 209        the state file can be reused.
 210
 2114. If fbcon is compiled as a module, load fbcon by doing:
 212
 213       modprobe fbcon
 214
 2155. Now to detach fbcon:
 216
 217       vbetool vbestate restore < <vga state file> && \
 218       echo 0 > /sys/class/vtconsole/vtcon1/bind
 219
 2206. That's it, you're back to VGA mode. And if you compiled fbcon as a module,
 221   you can unload it by 'rmmod fbcon'
 222
 2237. To reattach fbcon:
 224
 225       echo 1 > /sys/class/vtconsole/vtcon1/bind
 226
 2278. Once fbcon is unbound, all drivers registered to the system will also
 228become unbound.  This means that fbcon and individual framebuffer drivers
 229can be unloaded or reloaded at will. Reloading the drivers or fbcon will
 230automatically bind the console, fbcon and the drivers together. Unloading
 231all the drivers without unloading fbcon will make it impossible for the
 232console to bind fbcon.
 233
 234Notes for vesafb users:
 235=======================
 236
 237Unfortunately, if your bootline includes a vga=xxx parameter that sets the
 238hardware in graphics mode, such as when loading vesafb, vgacon will not load.
 239Instead, vgacon will replace the default boot console with dummycon, and you
 240won't get any display after detaching fbcon. Your machine is still alive, so
 241you can reattach vesafb. However, to reattach vesafb, you need to do one of
 242the following:
 243
 244Variation 1:
 245
 246    a. Before detaching fbcon, do
 247
 248       vbetool vbemode save > <vesa state file> # do once for each vesafb mode,
 249                                                # the file can be reused
 250
 251    b. Detach fbcon as in step 5.
 252
 253    c. Attach fbcon
 254
 255        vbetool vbestate restore < <vesa state file> && \
 256        echo 1 > /sys/class/vtconsole/vtcon1/bind
 257
 258Variation 2:
 259
 260    a. Before detaching fbcon, do:
 261        echo <ID> > /sys/class/tty/console/bind
 262
 263
 264       vbetool vbemode get
 265
 266    b. Take note of the mode number
 267
 268    b. Detach fbcon as in step 5.
 269
 270    c. Attach fbcon:
 271
 272       vbetool vbemode set <mode number> && \
 273       echo 1 > /sys/class/vtconsole/vtcon1/bind
 274
 275Samples:
 276========
 277
 278Here are 2 sample bash scripts that you can use to bind or unbind the
 279framebuffer console driver if you are in an X86 box:
 280
 281---------------------------------------------------------------------------
 282#!/bin/bash
 283# Unbind fbcon
 284
 285# Change this to where your actual vgastate file is located
 286# Or Use VGASTATE=$1 to indicate the state file at runtime
 287VGASTATE=/tmp/vgastate
 288
 289# path to vbetool
 290VBETOOL=/usr/local/bin
 291
 292
 293for (( i = 0; i < 16; i++))
 294do
 295  if test -x /sys/class/vtconsole/vtcon$i; then
 296      if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
 297           = 1 ]; then
 298            if test -x $VBETOOL/vbetool; then
 299               echo Unbinding vtcon$i
 300               $VBETOOL/vbetool vbestate restore < $VGASTATE
 301               echo 0 > /sys/class/vtconsole/vtcon$i/bind
 302            fi
 303      fi
 304  fi
 305done
 306
 307---------------------------------------------------------------------------
 308#!/bin/bash
 309# Bind fbcon
 310
 311for (( i = 0; i < 16; i++))
 312do
 313  if test -x /sys/class/vtconsole/vtcon$i; then
 314      if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
 315           = 1 ]; then
 316          echo Unbinding vtcon$i
 317          echo 1 > /sys/class/vtconsole/vtcon$i/bind
 318      fi
 319  fi
 320done
 321---------------------------------------------------------------------------
 322
 323--
 324Antonino Daplas <adaplas@pol.net>
 325
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