Documentation/fb/fbcon.rst

Source file repositories/reference/linux-study-clean/Documentation/fb/fbcon.rst

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Documentation/fb/fbcon.rst
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Support Tooling And Documentation: documentation
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Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

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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->
Console display driver support->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 Graphics
support->Bootup logo.

Also, you will need to select at least one compiled-in font, but if
you don't do anything, the kernel configuration tool will select one for you,
usually an 8x16 font.

.. admonition:: 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

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