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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These zones usage has evolved with time and the comments were outdated.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: 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>
This commit is contained in:
Nicolas Saenz Julienne 2019-09-11 20:25:46 +02:00 committed by Catalin Marinas
parent 1a8e1cef76
commit 734f9246e7
1 changed files with 26 additions and 19 deletions
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45
include/linux/mmzone.h
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@ -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
* Some examples:
*
* Architecture Limit
* ---------------------------
* parisc, ia64, sparc <4G
* s390, powerpc <2G
* arm Various
* alpha Unlimited or 0-16MB.
* - i386 and x86_64 have a fixed 16M ZONE_DMA and ZONE_DMA32 for the
* rest of the lower 4G.
*
* i386, x86_64 and multiple other arches
* <16M.
* - 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.
*
* - 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
/*