Commit 734f9246 authored by Nicolas Saenz Julienne's avatar Nicolas Saenz Julienne Committed by Catalin Marinas
Browse files

mm: refresh ZONE_DMA and ZONE_DMA32 comments in 'enum zone_type' 



These zones usage has evolved with time and the comments were outdated.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: default avatarNicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Reviewed-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
parent 1a8e1cef

include/linux/mmzone.h

+26 −19
Original line number Diff line number Diff line
@@ -359,33 +359,40 @@ 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.

	 * 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.

	 *

	 * Some examples

	 *  - powerpc only uses ZONE_DMA, the size, up to 2G, may vary

	 *    depending on the specific device.

	 *

	 * Architecture		Limit

	 * ---------------------------

	 * parisc, ia64, sparc	<4G

	 * s390, powerpc	<2G

	 * arm			Various

	 * alpha		Unlimited or 0-16MB.

	 *  - s390 uses ZONE_DMA fixed to the lower 2G.

	 *

	 * 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

	/*

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