Linux Framebuffer(一) - 概述

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我是靠谱客的博主感性吐司，最近开发中收集的这篇文章主要介绍Linux Framebuffer(一) - 概述，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

了解Linux framebuffer.

1.Framebuffer Intro

FrameBuffer是出现在 2.2.xx 内核当中的一种驱动程序接口。Linux是工作在保护模式下，所以用户态进程是无法象DOS那样使用显卡BIOS里提供的中断调用来实现直接写屏，Linux抽象出 FrameBuffer这个设备来供用户态进程实现直接写屏。

Framebuffer机制模仿显卡的功能，将显卡硬件结构抽象掉，可以通过Framebuffer的读写直接对显存进行操作。用户可以将Framebuffer看成是显示内存的一个映像，将其映射到进程地址空间之后，就可以直接进行读写操作，而写操作可以立即反应在屏幕上。这种操作是抽象的，统一的。用户不必关心物理显存的位置、换页机制等等具体细节。这些都是由Framebuffer设备驱动来完成的。

但Framebuffer本身不具备任何运算数据的能力，就只好比是一个暂时存放水的水池。CPU将运算后的结果放到这个水池，水池再将结果流到显示器，中间不会对数据做处理，应用程序也可以直接读写这个水池的内容。在这种机制下，尽管Framebuffer需要真正的显卡驱动的支持，但所有显示任务都有CPU完成，因此CPU 负担很重。

2.Accessing Graphics

There are 3 different ways that graphics can be performed:

Using a kernel framebuffer driver
Using a kernel DRM driver
Mapping the card’s control registers into user-space (via mmap() calls on /dev/mem), and using user-space code to access the card (very complex)

3.What is a Framebuffer Driver?

It is a Linux kernel driver which exposes a file /dev/fb{N} for a graphics card it is responsible for. Through that file, userspace apps can perform reads/writes to directly access the video card framebuffer - ie the pixel values being displayed on the screen. Via IOCTL calls on that file, other functionality can be accessed including:

setting the graphics mode (width, height, pixel-depth, etc)
passing bitmaps to be copied into video memory
passing coordinates of a rectangle to be filled with a specified colour

The framebuffer interface does not offer much in the way of acceleration other than the bitblits and fills mentioned above.

There is a fairly generic “vesafb” driver that is capable of controlling any graphics card compatible with the VESA standard. Linux also comes with dozens of card-specific framebuffer drivers that support products of specific manufacturers. On boot, Linux uses the PCI ID of a graphics card to load the most appropriate framebuffer driver, which in turn creates the /dev/fb{n} control file.

4.Why Use a Framebuffer Driver?

Embedded systems use them because they often don’t have complex graphics requirements, want a small kernel, and don’t want an X server running in userspace. While X can use a framebuffer to display graphics, so can other simpler graphical libraries.

For many graphics chips, there is a framebuffer driver available but no specific X driver, and no 3D driver.

Graphics display during startup is also a candidate for a framebuffer driver.

And linux text consoles can use a framebuffer driver to display text at nice resolutions (better than available via the BIOS) without otherwise actually wanting to display graphics.

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