On Wed, Jul 31, 2019 at 05:47:51PM +0200, Nicolas Saenz Julienne wrote:

+	 * Architecture			Limit
+	 * ----------------------------------
+	 * parisc, ia64, sparc, arm64	<4G
+	 * s390, powerpc		<2G
+	 * arm				Various
+	 * alpha			Unlimited or 0-16MB.
 	 *
 	 * i386, x86_64 and multiple other arches
-	 * 			<16M.
+	 *				<16M.

powerpc is also Various now, arm64 isn't really < 4G, ia64 only uses
ZONE_DMA32 these days, and parisc doesn't seem to use neither ZONE_DMA
nor ZONE_DMA32.

Based on that I'm not sure the list really makes much sense.

 	 */
 	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.
+	 * x86_64 and arm64 need two ZONE_DMAs because they support devices
+	 * that are only able to DMA a fraction of the 32 bit addressable
+	 * memory area, but also devices that are limited to that whole 32 bit
+	 * area.
 	 */
 	ZONE_DMA32,

Maybe just say various architectures instead of mentioning specific
ones?  Something like "Some 64-bit platforms need.."

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