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 =======================
 The Framebuffer Console
 =======================
 
 The framebuffer console (fbcon), as its name implies, is a text
 console running on top of the framebuffer device. It has the functionality of
 any standard text console driver, such as the VGA console, with the added
 features that can be attributed to the graphical nature of the framebuffer.
 
 In the x86 architecture, the framebuffer console is optional, and
 some even treat it as a toy. For other architectures, it is the only available
 display device, text or graphical.
 
 What are the features of fbcon?  The framebuffer console supports
 high resolutions, varying font types, display rotation, primitive multihead,
 etc. Theoretically, multi-colored fonts, blending, aliasing, and any feature
 made available by the underlying graphics card are also possible.
 
 A. Configuration
 ================
 
 The framebuffer console can be enabled by using your favorite kernel
 configuration tool.  It is under Device Drivers->Graphics Support->
 Console display driver support->Framebuffer Console Support.
 Select 'y' to compile support statically or 'm' for module support.  The
 module will be fbcon.
 
 In order for fbcon to activate, at least one framebuffer driver is
 required, so choose from any of the numerous drivers available. For x86
 systems, they almost universally have VGA cards, so vga16fb and vesafb will
 always be available. However, using a chipset-specific driver will give you
 more speed and features, such as the ability to change the video mode
 dynamically.
 
 To display the penguin logo, choose any logo available in Graphics
 support->Bootup logo.
 
 Also, you will need to select at least one compiled-in font, but if
 you don't do anything, the kernel configuration tool will select one for you,
 usually an 8x16 font.
 
 GOTCHA: A common bug report is enabling the framebuffer without enabling the
 framebuffer console.  Depending on the driver, you may get a blanked or
 garbled display, but the system still boots to completion.  If you are
 fortunate to have a driver that does not alter the graphics chip, then you
 will still get a VGA console.
 
 B. Loading
 ==========
 
 Possible scenarios:
 
 1. Driver and fbcon are compiled statically
 
          Usually, fbcon will automatically take over your console. The notable
          exception is vesafb.  It needs to be explicitly activated with the
          vga= boot option parameter.
 
 2. Driver is compiled statically, fbcon is compiled as a module
 
          Depending on the driver, you either get a standard console, or a
          garbled display, as mentioned above.  To get a framebuffer console,
          do a 'modprobe fbcon'.
 
 3. Driver is compiled as a module, fbcon is compiled statically
 
          You get your standard console.  Once the driver is loaded with
          'modprobe xxxfb', fbcon automatically takes over the console with
          the possible exception of using the fbcon=map:n option. See below.
 
 4. Driver and fbcon are compiled as a module.
 
          You can load them in any order. Once both are loaded, fbcon will take
          over the console.
 
 C. Boot options
 
          The framebuffer console has several, largely unknown, boot options
          that can change its behavior.
 
 1. fbcon=font:<name>
 
         Select the initial font to use. The value 'name' can be any of the
         compiled-in fonts: 10x18, 6x10, 6x8, 7x14, Acorn8x8, MINI4x6,
         PEARL8x8, ProFont6x11, SUN12x22, SUN8x16, TER16x32, VGA8x16, VGA8x8.
 
         Note, not all drivers can handle font with widths not divisible by 8,
         such as vga16fb.
 
 
 2. fbcon=map:<0123>
 
         This is an interesting option. It tells which driver gets mapped to
         which console. The value '0123' is a sequence that gets repeated until
         the total length is 64 which is the number of consoles available. In
         the above example, it is expanded to 012301230123... and the mapping
         will be::
 
                 tty | 1 2 3 4 5 6 7 8 9 ...
                 fb  | 0 1 2 3 0 1 2 3 0 ...
 
                 ('cat /proc/fb' should tell you what the fb numbers are)
 
         One side effect that may be useful is using a map value that exceeds
         the number of loaded fb drivers. For example, if only one driver is
         available, fb0, adding fbcon=map:1 tells fbcon not to take over the
         console.
 
         Later on, when you want to map the console the to the framebuffer
         device, you can use the con2fbmap utility.
 
 3. fbcon=vc:<n1>-<n2>
 
         This option tells fbcon to take over only a range of consoles as
         specified by the values 'n1' and 'n2'. The rest of the consoles
         outside the given range will still be controlled by the standard
         console driver.
 
         NOTE: For x86 machines, the standard console is the VGA console which
         is typically located on the same video card.  Thus, the consoles that
         are controlled by the VGA console will be garbled.
 
 4. fbcon=rotate:<n>
 
         This option changes the orientation angle of the console display. The
         value 'n' accepts the following:
 
             - 0 - normal orientation (0 degree)
             - 1 - clockwise orientation (90 degrees)
             - 2 - upside down orientation (180 degrees)
             - 3 - counterclockwise orientation (270 degrees)
 
         The angle can be changed anytime afterwards by 'echoing' the same
         numbers to any one of the 2 attributes found in
         /sys/class/graphics/fbcon:
 
                 - rotate     - rotate the display of the active console
                 - rotate_all - rotate the display of all consoles
 
         Console rotation will only become available if Framebuffer Console
         Rotation support is compiled in your kernel.
 
         NOTE: This is purely console rotation.  Any other applications that
         use the framebuffer will remain at their 'normal' orientation.
         Actually, the underlying fb driver is totally ignorant of console
         rotation.
 
 5. fbcon=margin:<color>
 
         This option specifies the color of the margins. The margins are the
         leftover area at the right and the bottom of the screen that are not
         used by text. By default, this area will be black. The 'color' value
         is an integer number that depends on the framebuffer driver being used.
 
 6. fbcon=nodefer
 
         If the kernel is compiled with deferred fbcon takeover support, normally
         the framebuffer contents, left in place by the firmware/bootloader, will
         be preserved until there actually is some text is output to the console.
         This option causes fbcon to bind immediately to the fbdev device.
 
 7. fbcon=logo-pos:<location>
 
         The only possible 'location' is 'center' (without quotes), and when
         given, the bootup logo is moved from the default top-left corner
         location to the center of the framebuffer. If more than one logo is
         displayed due to multiple CPUs, the collected line of logos is moved
         as a whole.
 
 8. fbcon=logo-count:<n>
 
         The value 'n' overrides the number of bootup logos. 0 disables the
         logo, and -1 gives the default which is the number of online CPUs.
 
 C. Attaching, Detaching and Unloading
 
 Before going on to how to attach, detach and unload the framebuffer console, an
 illustration of the dependencies may help.
 
 The console layer, as with most subsystems, needs a driver that interfaces with
 the hardware. Thus, in a VGA console::
 
         console ---> VGA driver ---> hardware.
 
 Assuming the VGA driver can be unloaded, one must first unbind the VGA driver
 from the console layer before unloading the driver.  The VGA driver cannot be
 unloaded if it is still bound to the console layer. (See
 Documentation/driver-api/console.rst for more information).
 
 This is more complicated in the case of the framebuffer console (fbcon),
 because fbcon is an intermediate layer between the console and the drivers::
 
         console ---> fbcon ---> fbdev drivers ---> hardware
 
 The fbdev drivers cannot be unloaded if bound to fbcon, and fbcon cannot
 be unloaded if it's bound to the console layer.
 
 So to unload the fbdev drivers, one must first unbind fbcon from the console,
 then unbind the fbdev drivers from fbcon.  Fortunately, unbinding fbcon from
 the console layer will automatically unbind framebuffer drivers from
 fbcon. Thus, there is no need to explicitly unbind the fbdev drivers from
 fbcon.
 
 So, how do we unbind fbcon from the console? Part of the answer is in
 Documentation/driver-api/console.rst. To summarize:
 
 Echo a value to the bind file that represents the framebuffer console
 driver. So assuming vtcon1 represents fbcon, then::
 
   echo 1 > /sys/class/vtconsole/vtcon1/bind - attach framebuffer console to
                                              console layer
   echo 0 > /sys/class/vtconsole/vtcon1/bind - detach framebuffer console from
                                              console layer
 
 If fbcon is detached from the console layer, your boot console driver (which is
 usually VGA text mode) will take over.  A few drivers (rivafb and i810fb) will
 restore VGA text mode for you.  With the rest, before detaching fbcon, you
 must take a few additional steps to make sure that your VGA text mode is
 restored properly. The following is one of the several methods that you can do:
 
 1. Download or install vbetool.  This utility is included with most
    distributions nowadays, and is usually part of the suspend/resume tool.
 
 2. In your kernel configuration, ensure that CONFIG_FRAMEBUFFER_CONSOLE is set
    to 'y' or 'm'. Enable one or more of your favorite framebuffer drivers.
 
 3. Boot into text mode and as root run::
 
         vbetool vbestate save > <vga state file>
 
    The above command saves the register contents of your graphics
    hardware to <vga state file>.  You need to do this step only once as
    the state file can be reused.
 
 4. If fbcon is compiled as a module, load fbcon by doing::
 
        modprobe fbcon
 
 5. Now to detach fbcon::
 
        vbetool vbestate restore < <vga state file> && \
        echo 0 > /sys/class/vtconsole/vtcon1/bind
 
 6. That's it, you're back to VGA mode. And if you compiled fbcon as a module,
    you can unload it by 'rmmod fbcon'.
 
 7. To reattach fbcon::
 
        echo 1 > /sys/class/vtconsole/vtcon1/bind
 
 8. Once fbcon is unbound, all drivers registered to the system will also
 become unbound.  This means that fbcon and individual framebuffer drivers
 can be unloaded or reloaded at will. Reloading the drivers or fbcon will
 automatically bind the console, fbcon and the drivers together. Unloading
 all the drivers without unloading fbcon will make it impossible for the
 console to bind fbcon.
 
 Notes for vesafb users:
 =======================
 
 Unfortunately, if your bootline includes a vga=xxx parameter that sets the
 hardware in graphics mode, such as when loading vesafb, vgacon will not load.
 Instead, vgacon will replace the default boot console with dummycon, and you
 won't get any display after detaching fbcon. Your machine is still alive, so
 you can reattach vesafb. However, to reattach vesafb, you need to do one of
 the following:
 
 Variation 1:
 
     a. Before detaching fbcon, do::
 
         vbetool vbemode save > <vesa state file> # do once for each vesafb mode,
                                                  # the file can be reused
 
     b. Detach fbcon as in step 5.
 
     c. Attach fbcon::
 
         vbetool vbestate restore < <vesa state file> && \
         echo 1 > /sys/class/vtconsole/vtcon1/bind
 
 Variation 2:
 
     a. Before detaching fbcon, do::
 
         echo <ID> > /sys/class/tty/console/bind
 
         vbetool vbemode get
 
     b. Take note of the mode number
 
     b. Detach fbcon as in step 5.
 
     c. Attach fbcon::
 
         vbetool vbemode set <mode number> && \
         echo 1 > /sys/class/vtconsole/vtcon1/bind
 
 Samples:
 ========
 
 Here are 2 sample bash scripts that you can use to bind or unbind the
 framebuffer console driver if you are on an X86 box::
 
   #!/bin/bash
   # Unbind fbcon
 
   # Change this to where your actual vgastate file is located
   # Or Use VGASTATE=$1 to indicate the state file at runtime
   VGASTATE=/tmp/vgastate
 
   # path to vbetool
   VBETOOL=/usr/local/bin
 
 
   for (( i = 0; i < 16; i++))
   do
     if test -x /sys/class/vtconsole/vtcon$i; then
         if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
              = 1 ]; then
             if test -x $VBETOOL/vbetool; then
                echo Unbinding vtcon$i
                $VBETOOL/vbetool vbestate restore < $VGASTATE
                echo 0 > /sys/class/vtconsole/vtcon$i/bind
             fi
         fi
     fi
   done
 
 ---------------------------------------------------------------------------
 
 ::
 
   #!/bin/bash
   # Bind fbcon
 
   for (( i = 0; i < 16; i++))
   do
     if test -x /sys/class/vtconsole/vtcon$i; then
         if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
              = 1 ]; then
           echo Unbinding vtcon$i
           echo 1 > /sys/class/vtconsole/vtcon$i/bind
         fi
     fi
   done
 
 Antonino Daplas <adaplas@pol.net>

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