android_kernel_xiaomi_sm8350/Documentation/fb/fbcon.txt

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>