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From: Christoph Hellwig <hch@lst.de>
To: iommu@lists.linux-foundation.org
Cc: Shawn Anastasio <shawn@anastas.io>, Will Deacon <will@kernel.org>,
	linux-m68k@lists.linux-m68k.org, Guan Xuetao <gxt@pku.edu.cn>,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	Russell King <linux@armlinux.org.uk>,
	linux-mips@vger.kernel.org, Paul Burton <paul.burton@mips.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	James Hogan <jhogan@kernel.org>,
	Robin Murphy <robin.murphy@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: [PATCH 6/6] arm64: document the choice of page attributes for pgprot_dmacoherent
Date: Fri, 16 Aug 2019 09:07:54 +0200
Message-ID: <20190816070754.15653-7-hch@lst.de> (raw)
In-Reply-To: <20190816070754.15653-1-hch@lst.de>

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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Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-16  7:07 cleanup the dma_pgprot handling Christoph Hellwig
2019-08-16  7:07 ` [PATCH 1/6] MIPS: remove support for DMA_ATTR_WRITE_COMBINE Christoph Hellwig
2019-08-16  7:07 ` [PATCH 2/6] unicore32: remove the unused pgprot_dmacoherent define Christoph Hellwig
2019-08-16  7:07 ` [PATCH 3/6] arm-nommu: " Christoph Hellwig
2019-08-16  7:07 ` [PATCH 4/6] dma-mapping: remove arch_dma_mmap_pgprot Christoph Hellwig
2019-08-16  7:43   ` Geert Uytterhoeven
2019-08-16  7:07 ` [PATCH 5/6] dma-mapping: make dma_atomic_pool_init self-contained Christoph Hellwig
2019-08-16  7:07 ` Christoph Hellwig [this message]
2019-08-16 17:31   ` [PATCH 6/6] arm64: document the choice of page attributes for pgprot_dmacoherent Will Deacon
2019-08-16 17:36     ` Mark Rutland
2019-08-16 17:59     ` Christoph Hellwig
2019-08-16 18:05       ` Will Deacon
2019-08-23 21:58 ` cleanup the dma_pgprot handling Paul Burton
2019-08-24 22:34   ` Christoph Hellwig

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