From 1c108eaeae73a504ac1b2d882bc1fefb91eecf17 Mon Sep 17 00:00:00 2001 From: Nicolas Saenz Julienne 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 These zones usage has evolved with time and the comments were outdated.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 Reviewed-by: Christoph Hellwig Reviewed-by: Catalin Marinas Signed-off-by: Catalin Marinas --- 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 /*