Messages in this thread Patch in this message | | | From | Christoph Hellwig <> | Subject | [PATCH 6/6] arm64: document the choice of page attributes for pgprot_dmacoherent | Date | Fri, 16 Aug 2019 09:07:54 +0200 |
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Based on an email from Will Deacon.
Signed-off-by: Christoph Hellwig <hch@lst.de> --- arch/arm64/include/asm/pgtable.h | 8 ++++++++ 1 file changed, 8 insertions(+)
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h index 6700371227d1..6ff221d9a631 100644 --- a/arch/arm64/include/asm/pgtable.h +++ b/arch/arm64/include/asm/pgtable.h @@ -435,6 +435,14 @@ static inline pmd_t pmd_mkdevmap(pmd_t pmd) __pgprot_modify(prot, PTE_ATTRINDX_MASK, PTE_ATTRINDX(MT_NORMAL_NC) | PTE_PXN | PTE_UXN) #define pgprot_device(prot) \ __pgprot_modify(prot, PTE_ATTRINDX_MASK, PTE_ATTRINDX(MT_DEVICE_nGnRE) | PTE_PXN | PTE_UXN) +/* + * DMA allocations for non-coherent devices use what the Arm architecture calls + * "Normal non-cacheable" memory, which permits speculation, unaligned accesses + * and merging of writes. This is different from "Strongly Ordered" memory + * which is intended for MMIO and thus forbids speculation, preserves access + * size, requires strict alignment and also forces write responses to come from + * the endpoint. + */ #define pgprot_dmacoherent(prot) \ __pgprot_modify(prot, PTE_ATTRINDX_MASK, \ PTE_ATTRINDX(MT_NORMAL_NC) | PTE_PXN | PTE_UXN) -- 2.20.1
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