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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
It's possible to detach/attach the framebuffer console from the vt layer by
echoing anything to the following sysfs attributes found
/sys/class/graphics/fbcon.
attach - attach framebuffer console to vt layer
detach - detach framebuffer console to vt layer
If fbcon is detached from the vt 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 1 > /sys/class/graphics/fbcon/detach
6. That's it, you're back to VGA mode. And if you compiled your drivers as
modules, you can unload them at will. So if you want to change your driver
from xxxfb to yyyfb, you can do this:
detach fbcon
rmmod xxxfb
modprobe yyyfb
Of course, con2fbmap can do the same thing but will not work if xxxfb
and yyyfb are not compatible (ie, cannot be loaded at the same time).
7. To reattach fbcon:
echo 1 > /sys/class/graphics/fbcon/attach
8. Once the framebuffer console is detached, and if it is compiled as a module,
the module can be unloaded with 'rmmod fbcon'. This feature is great for
developers.
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/graphics/fbcon/attach
Variation 2:
a. Before detaching fbcon, do:
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/graphics/fbcon/attach
--
Antonino Daplas <adaplas@pol.net>
|
