=======================

  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->Frame
  buffer Devices->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, 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=scrollback:<value>[k]

  	The scrollback buffer is memory that is used to preserve display
  	contents that has already scrolled past your view.  This is accessed
  	by using the Shift-PageUp key combination.  The value 'value' is any
  	integer. It defaults to 32KB.  The 'k' suffix is optional, and will
  	multiply the 'value' by 1024.

  
  3. 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.
  
  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  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>