linux-kernel - [PATCH 6/6] arm64: document the choice of page attributes for pgprot

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Date:   Fri, 16 Aug 2019 09:07:54 +0200
From:   Christoph Hellwig <hch@....de>
To:     iommu@...ts.linux-foundation.org
Cc:     Guan Xuetao <gxt@....edu.cn>, Shawn Anastasio <shawn@...stas.io>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-mips@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-m68k@...ts.linux-m68k.org, linux-kernel@...r.kernel.org
Subject: [PATCH 6/6] arm64: document the choice of page attributes for pgprot_dmacoherent

Based on an email from Will Deacon.

Signed-off-by: Christoph Hellwig <hch@....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