Documentation/fb/fbcon.rst - linux - Git at Google

 =======================
 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.

 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.

 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>
	=======================
	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.

	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.

	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>