linux-kernel - Re: [PATCH 21/22] arm64: replace ZONE_DMA with ZONE

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Date:   Wed, 10 Jan 2018 16:55:17 +0100
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org,
        linux-arch@...r.kernel.org, linux-mips@...ux-mips.org,
        Michal Simek <monstr@...str.eu>, linux-ia64@...r.kernel.org,
        Christian König 
        <ckoenig.leichtzumerken@...il.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad@...nok.org>,
        Guan Xuetao <gxt@...c.pku.edu.cn>,
        linuxppc-dev@...ts.ozlabs.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 21/22] arm64: replace ZONE_DMA with ZONE_DMA32

On Wed, Jan 10, 2018 at 12:58:14PM +0000, Robin Murphy wrote:
> On 10/01/18 08:09, Christoph Hellwig wrote:
>> arm64 uses ZONE_DMA for allocations below 32-bits.  These days we
>> name the zone for that ZONE_DMA32, which will allow to use the
>> dma-direct and generic swiotlb code as-is, so rename it.
>
> I do wonder if we could also "upgrade" GFP_DMA to GFP_DMA32 somehow when 
> !ZONE_DMA - there are almost certainly arm64 drivers out there using a 
> combination of GFP_DMA and streaming mappings which will no longer get the 
> guaranteed 32-bit addresses they expect after this. I'm not sure quite how 
> feasible that is, though :/

I can't find anything obvious in the tree. The alternative would be
to keep ZONE_DMA and set ARCH_ZONE_DMA_BITS.

> That said, I do agree that this is an appropriate change (the legacy of 
> GFP_DMA is obviously horrible), so, provided we get plenty of time to find 
> and fix the fallout when it lands:
>
> Reviewed-by: Robin Murphy <robin.murphy@....com>

I was hoping to get this into 4.15.  What would be proper time to
fix the fallout?