linux-kernel - Re: [PATCH 5/8] arm64: use ZONE_DMA on DMA addressing limited devices

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Message-ID: <20190731170742.GC17773@arrakis.emea.arm.com>
Date:   Wed, 31 Jul 2019 18:07:42 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     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,
        Will Deacon <will@...nel.org>, phill@...pberryi.org,
        f.fainelli@...il.com, linux-kernel@...r.kernel.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 5/8] arm64: use ZONE_DMA on DMA addressing limited devices

On Wed, Jul 31, 2019 at 05:47:48PM +0200, Nicolas Saenz Julienne wrote:
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 1c4ffabbe1cb..f5279ef85756 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -50,6 +50,13 @@
>  s64 memstart_addr __ro_after_init = -1;
>  EXPORT_SYMBOL(memstart_addr);
>  
> +/*
> + * We might create both a ZONE_DMA and ZONE_DMA32. ZONE_DMA is needed if there
> + * are periferals unable to address the first naturally aligned 4GB of ram.
> + * ZONE_DMA32 will be expanded to cover the rest of that memory. If such
> + * limitations doesn't exist only ZONE_DMA32 is created.
> + */

Shouldn't we instead only create ZONE_DMA to cover the whole 32-bit
range and leave ZONE_DMA32 empty? Can__GFP_DMA allocations fall back
onto ZONE_DMA32?

-- 
Catalin