))It looks like Xorg drivers on Linux platform are NOT kernel modules. Could 
someone explain, or better point some brief document - what these Xorg drivers 
are and how they are implemented ?

Sorry about the top-posting, let's try that again :)

The xorg drivers run in userspace, called by the X server. Where things get 
confusing is that they can be written to operate in at least different ways.

Basic 2d-only drivers can directly access the hardware from userspace for both 
modesetting and acceleration,  and can run without a kernel driver. This was 
the norm 10-15 years ago and many drivers can still fall back to this mode.

Drivers from a few years ago tend to access modesetting hardware from userspace 
but go through a kernel driver for acceleration, allowing the acceleration 
hardware to be shared with a separate 3d driver. Using the kernel driver to 
coordinate access to acceleration hardware is part of the Direct Rendering 
Infrastructure (DRI).

The most recent xorg drivers use the kernel driver for essentially all hardware 
accesses, including modesetting. Moving modesetting into the kernel driver was 
an initiative called (predictably) Kernel ModeSetting or KMS.

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