In the same fashion like in the Linux kernel, we support pre-initialized
framebuffers that were set up by either the BIOS or the bootloader.
These framebuffers can be backed by any kind of video hardware, and are
not tied to VGA hardware at all. Therefore, this code should be in a
separate sub-folder in the Graphics subsystem to indicate this.
The mmap interface was removed when we introduced the DisplayConnector
class, as it was quite unsafe to use and didn't handle switching between
graphical and text modes safely. By using the SharedFramebufferVMObject,
we are able to elegantly coordinate the switch by remapping the attached
mmap'ed-Memory::Region(s) with different mappings, therefore, keeping
WindowServer to think that the mappings it has are still valid, while
they are going to a different physical range until we are back to the
graphical mode (after a switch from text mode).
Most drivers take advantage of the fact that we know where is the actual
framebuffer in physical memory space, the SharedFramebufferVMObject is
created with that information. However, the VirtIO driver is different
in that aspect, because it relies on DMA transactions to show graphics
on the framebuffer, so the SharedFramebufferVMObject is created with
that mindset to support the arbitrary framebuffer location in physical
memory space.
