[PATCH v2 11/11] mm: refresh ZONE_DMA and ZONE_DMA32 comments in 'enum zone_type'
From: Nicolas Saenz Julienne <hidden>
Date: 2019-08-20 14:58:56
Also in:
linux-arm-kernel, linux-devicetree, linux-iommu, linux-riscv, lkml
Subsystem:
memory management - core, memory management - mglru (multi-gen lru), the rest · Maintainers:
Andrew Morton, David Hildenbrand, Linus Torvalds
These zones usage has evolved with time and the comments were outdated.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 <redacted> --- Changes in v2: - Try another approach merging both zones explanations into one - Address Christoph's comments - If this approach doesn't get much traction I'll just drop the patch from the series as it's not really essential include/linux/mmzone.h | 46 +++++++++++++++++++++++++----------------- 1 file changed, 27 insertions(+), 19 deletions(-)
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index d77d717c620c..9c150223d41f 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h@@ -356,33 +356,41 @@ 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. + * 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. + * + * - powerpc only uses ZONE_DMA, the size, up to 2G, may vary + * depending on the specific device. * - * Some examples + * - s390 uses ZONE_DMA fixed to the lower 2G. * - * Architecture Limit - * --------------------------- - * parisc, ia64, sparc <4G - * s390, powerpc <2G - * arm Various - * alpha Unlimited or 0-16MB. + * - arm64 uses ZONE_DMA to mark the area addresable by all + * peripherals on the device, and ZONE_DMA32 for the rest of the + * lower 4G. ZONE_DMA32 might be left empty. * - * i386, x86_64 and multiple other arches - * <16M. + * - 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 /*
--
2.22.0