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http://en.wikipedia.org/wiki/Hardware_overlay

这个说的比较明白，包括overlay的由来，为什么用他，有哪些功能

Secondary displays

Many newer graphics cards can support more than one monitor and/or a TV screen as output devices. Typically one of these output devices has to be declared the "primary" device, and only the primary device can display hardware overlays. There are exceptions: Intel writes in the FAQ for their Embedded Graphics Drivers that the overlay can be attached to either one of the displays but not to both (note: the Intel 945, G33-Q965 chipsets now have dual hardware overlay, and are capable of good quality mpeg2 on secondary monitors when appropriate software is installed), and some newer Matrox graphics card support overlay on both displays (for example, the Parhelia Series).

Secondary displays require both hardware and driver support; some graphics cards may support overlay on the second display while their drivers may not yet support it (note: recent (July 2008) graphics chipset driver bugs can cause most video formats apart from mpeg2 to work on both monitors, and mpeg2 only on the primary with most players).

Some users note that DVD movies display properly on a laptop screen but don't display on a TV connected to the laptop; in these cases it may be possible to designate the TV as the primary display. Sometimes, the use of hardware overlays may have to be disabled in the media player. However, some graphics cards have the option to completely redirect hardware overlay to the TV screen. In this case, starting a DVD player on the main screen with overlay enabled would result in video being displayed on the attached TV screen.

疑问：针对下面的一段，intel clovertail是不是就是这个样子的，secondary（hdmi）不支持 movies？

2段矛盾的话：

6.7.5.7 Blenders

In SoC, there is one overlay which resides in display Pipe A.

???

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