Re: [PATCH 8/8] mm: comment arm64's usage of 'enum zone_type'

[Christoph Hellwig]

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.."