--- /dev/null
+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/fb{x}
+
+ con_rotate - rotate the display of the active console
+ con_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.
+
+---
+Antonino Daplas <adaplas@pol.net>
projects / linux-2.6 / commitdiff