linux-kernel - Re: [PATCH 8/8] mm: comment arm64's usage of 'enum zone

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Message-ID: <20190801140820.GC23435@lst.de>
Date:   Thu, 1 Aug 2019 16:08:20 +0200
From:   Christoph Hellwig <hch@....de>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     catalin.marinas@....com, hch@....de, wahrenst@....net,
        marc.zyngier@....com, Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, phill@...pberryi.org,
        f.fainelli@...il.com, will@...nel.org, robh+dt@...nel.org,
        eric@...olt.net, mbrugger@...e.com, akpm@...ux-foundation.org,
        frowand.list@...il.com, m.szyprowski@...sung.com,
        linux-rpi-kernel@...ts.infradead.org
Subject: Re: [PATCH 8/8] mm: comment arm64's usage of 'enum zone_type'

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.."