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Commit 5e7705df authored by Chris Metcalf's avatar Chris Metcalf
Browse files

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: default avatarChris Metcalf <cmetcalf@tilera.com>
parent bdb82054

arch/tile/include/asm/pci.h

+5 −0
Original line number Diff line number Diff line
@@ -122,6 +122,11 @@ static inline void pci_iounmap(struct pci_dev *dev, void __iomem *addr) {} 
 * the CPA plus TILE_PCI_MEM_MAP_BASE_OFFSET. To support 32-bit

 * devices, we create a separate map region that handles the low

 * 4GB.

 *

 * This design lets us avoid the "PCI hole" problem where the host bridge

 * won't pass DMA traffic with target addresses that happen to fall within the

 * BAR space. This enables us to use all the physical memory for DMA, instead

 * of wasting the same amount of physical memory as the BAR window size.

 */

#define	TILE_PCI_MEM_MAP_BASE_OFFSET	(1ULL << CHIP_PA_WIDTH())