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85 lines
2.9 KiB
Diff
85 lines
2.9 KiB
Diff
From 1c108eaeae73a504ac1b2d882bc1fefb91eecf17 Mon Sep 17 00:00:00 2001
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From: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
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Date: Wed, 11 Sep 2019 20:25:46 +0200
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Subject: [PATCH] mm: refresh ZONE_DMA and ZONE_DMA32 comments in 'enum
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zone_type'
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commit 734f9246e791d8da278957b2c326d7709b2a97c0 upstream.
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These zones usage has evolved with time and the comments were outdated.
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This joins both ZONE_DMA and ZONE_DMA32 explanation and gives up to date
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examples on how they are used on different architectures.
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Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
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Reviewed-by: Christoph Hellwig <hch@lst.de>
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Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
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Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
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---
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include/linux/mmzone.h | 45 ++++++++++++++++++++++++------------------
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1 file changed, 26 insertions(+), 19 deletions(-)
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--- a/include/linux/mmzone.h
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+++ b/include/linux/mmzone.h
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@@ -358,33 +358,40 @@ struct per_cpu_nodestat {
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#endif /* !__GENERATING_BOUNDS.H */
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enum zone_type {
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-#ifdef CONFIG_ZONE_DMA
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/*
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- * ZONE_DMA is used when there are devices that are not able
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- * to do DMA to all of addressable memory (ZONE_NORMAL). Then we
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- * carve out the portion of memory that is needed for these devices.
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- * The range is arch specific.
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- *
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- * Some examples
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- *
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- * Architecture Limit
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- * ---------------------------
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- * parisc, ia64, sparc <4G
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- * s390, powerpc <2G
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- * arm Various
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- * alpha Unlimited or 0-16MB.
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+ * ZONE_DMA and ZONE_DMA32 are used when there are peripherals not able
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+ * to DMA to all of the addressable memory (ZONE_NORMAL).
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+ * On architectures where this area covers the whole 32 bit address
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+ * space ZONE_DMA32 is used. ZONE_DMA is left for the ones with smaller
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+ * DMA addressing constraints. This distinction is important as a 32bit
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+ * DMA mask is assumed when ZONE_DMA32 is defined. Some 64-bit
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+ * platforms may need both zones as they support peripherals with
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+ * different DMA addressing limitations.
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+ *
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+ * Some examples:
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+ *
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+ * - i386 and x86_64 have a fixed 16M ZONE_DMA and ZONE_DMA32 for the
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+ * rest of the lower 4G.
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+ *
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+ * - arm only uses ZONE_DMA, the size, up to 4G, may vary depending on
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+ * the specific device.
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+ *
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+ * - arm64 has a fixed 1G ZONE_DMA and ZONE_DMA32 for the rest of the
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+ * lower 4G.
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+ *
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+ * - powerpc only uses ZONE_DMA, the size, up to 2G, may vary
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+ * depending on the specific device.
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*
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- * i386, x86_64 and multiple other arches
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- * <16M.
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+ * - s390 uses ZONE_DMA fixed to the lower 2G.
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+ *
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+ * - ia64 and riscv only use ZONE_DMA32.
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+ *
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+ * - parisc uses neither.
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*/
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+#ifdef CONFIG_ZONE_DMA
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ZONE_DMA,
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#endif
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#ifdef CONFIG_ZONE_DMA32
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- /*
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- * x86_64 needs two ZONE_DMAs because it supports devices that are
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- * only able to do DMA to the lower 16M but also 32 bit devices that
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- * can only do DMA areas below 4G.
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- */
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ZONE_DMA32,
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#endif
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/*
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