[PATCH] fbcon: Console Rotation - Add framebuffer console documentation

Add documentation as Documentation/fb/fbcon.txt describing the framebuffer
console and its boot options.

Signed-off-by: Antonino Daplas <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Documentation/fb/fbcon.txt

@@ -0,0 +1,152 @@
+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 <[email protected]>