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tile PCI RC: add comment about "PCI hole" problem
Explain the rationale of not overlapping the 64-bit DMA window with the PA range. Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
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@ -122,6 +122,11 @@ static inline void pci_iounmap(struct pci_dev *dev, void __iomem *addr) {}
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* the CPA plus TILE_PCI_MEM_MAP_BASE_OFFSET. To support 32-bit
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* devices, we create a separate map region that handles the low
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* 4GB.
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*
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* This design lets us avoid the "PCI hole" problem where the host bridge
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* won't pass DMA traffic with target addresses that happen to fall within the
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* BAR space. This enables us to use all the physical memory for DMA, instead
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* of wasting the same amount of physical memory as the BAR window size.
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*/
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#define TILE_PCI_MEM_MAP_BASE_OFFSET (1ULL << CHIP_PA_WIDTH())
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