From: Christoph Hellwig Date: Sat, 3 Aug 2019 09:38:31 +0000 (+0300) Subject: arm64: document the choice of page attributes for pgprot_dmacoherent X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=3e4e1d3fb89193cc072858e1469d6f2926c603f7;p=linux.git arm64: document the choice of page attributes for pgprot_dmacoherent Based on an email from Will Deacon. Signed-off-by: Christoph Hellwig Acked-by: Will Deacon Acked-by: Mark Rutland --- diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h index 6700371227d1d..fd40fb05eb516 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 "Device-nGnR[nE]" memory which + * is intended for MMIO and thus forbids speculation, preserves access size, + * requires strict alignment and can also force 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)