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24 lines
1.2 KiB
ReStructuredText
24 lines
1.2 KiB
ReStructuredText
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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
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memory. Either the CPU can access the video RAM directly (“aperture”),
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or the video card can access the system RAM using DMA access.
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The GART converts between physical addresses and virtual addresses on
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the video card side. Of course, the CPU must then map these physical
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addresses in its own address space to use them (using the MMU).
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The GART works as you’d expect from an MMU. It has a page table (called
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GTT) in RAM and walks it to figure out mappings. Since there cannot be
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page misses (that would require exception handling on the GPU side),
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access to missing pages are instead sent to a dedicated “scratch” page
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which is not used for anything else.
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Our driver implements the GART and GTT for Intel graphics card only, so
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far. Since our videodrivers are only doing modesetting, they do not need
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much support and other drivers implemented GTT management directly on
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their own (it is usually enough to make the framebuffer accessible to
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the CPU). However, this could be generalized into a more flexible iommu
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bus protocol.
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