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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->Support for
-framebuffer devices->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 Logo
-Configuration->Boot up logo.
-
- Also, you will need to select at least one compiled-in fonts, 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: VGA8x16, 7x14, 10x18, VGA8x8, MINI4x6, RomanLarge,
- SUN8x16, SUN12x22, ProFont6x11, Acorn8x8, PEARL8x8.
-
- 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 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.
-
-C. Attaching, Detaching and Unloading
-
-Before going on on 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/console/console.txt 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 it's 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/console/console.txt. 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 in 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>