New PowerVR Support Introduced to ARM Cortex-A Embeded Processors
The state of drivers for embedded graphics is, to say the least, poor.

The major embedded graphics vendors are:


Imagination Technology: PowerVR - TI OMAP and Sitara, Early Intel Atoms
ARM: Mali - Samsung Exynos??
Vivante - Frescale i.mx6 
NVIDIA - Tegra
Qualcomm Adreno (Formerly AMD) - Freescale i.mx53
Intel HD4000 derivatives - Bay Trail and later


Only the Intel graphics driver is fully open source and thus it was the natural choice for our next industrial screen project.

All the other range from being a fully closed kernel binary blob (PowerVR) through to having a open source kernel shim and closed userspace library (Vivante and Mali).

Often, you'll have a "free" (as in beer) drivers supplied with a board support package, often tied to a specific kernel version and of dubious quality (been there, done that).  

Without naming names, the silicon vendor will sometimes ask you to purchase a premium support package and only then will they supply something that you can compile with your own kernel (been  there, done that).  Or, they will ask for a royalty for the driver which is roughly the same price as the chip itself (been there, done that too!).

Now, this generally applies to 3D acceleration only. 2D acceleration (or at least a frame buffer) is usually open source and will happily talk HDMI/DVI/VGA or whatever interface you require.

There are open source efforts to reverse engineer and/or write FOSS drivers for the above.  However, these are not yet production quality, unless there has been a major recent advance I am not aware of.

So yes, the drivers are closed source and the SoC graphics hardware specification is proprietary.  Fun and games for all concerned :(