haiku/docs/develop/busses/agp_gart

AGP (and PCI-express) Graphics Address Re-Mapping Table
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The GART is an IO-MMU allowing the videocard and CPU to share some
memory. Either the CPU can access the video RAM directly (“aperture”),
or the video card can access the system RAM using DMA access.

The GART converts between physical addresses and virtual addresses on
the video card side. Of course, the CPU must then map these physical
addresses in its own address space to use them (using the MMU).

The GART works as you’d expect from an MMU. It has a page table (called
GTT) in RAM and walks it to figure out mappings. Since there cannot be
page misses (that would require exception handling on the GPU side),
access to missing pages are instead sent to a dedicated “scratch” page
which is not used for anything else.

Our driver implements the GART and GTT for Intel graphics card only, so
far. Since our videodrivers are only doing modesetting, they do not need
much support and other drivers implemented GTT management directly on
their own (it is usually enough to make the framebuffer accessible to
the CPU). However, this could be generalized into a more flexible iommu
bus protocol.