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These zones usage has evolved with time and the comments were outdated.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>

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index bda20282746b92d055a573c7855b70e070fcd41f..b0a36d1580b62d8f5ba60be5c8b1acf40d525dff 100644 (file)
--- 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
        /*