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-rw-r--r--include/linux/mmzone.h45
1 files changed, 26 insertions, 19 deletions
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index bda20282746b..b0a36d1580b6 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -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
/*