These zones usage has evolved with time and the comments were outdated.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
---
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
-* alphaUnlimited 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
