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These zones usage has evolved with time and the comments were outdated.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