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Date: Wed, 5 Dec 2018 14:46:06 +0000
From: Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v4 3/3] iommu/io-pgtable-arm-v7s: Request DMA32 memory,
 and improve debugging
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To: Christoph Hellwig <hch@infradead.org>
Cc: Robin Murphy <robin.murphy@arm.com>, Nicolas Boichat <drinkcat@chromium.org>, Joerg Roedel <joro@8bytes.org>, Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>, David Rientjes <rientjes@google.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Andrew Morton <akpm@linux-foundation.org>, Vlastimil Babka <vbabka@suse.cz>, Michal Hocko <mhocko@suse.com>, Mel Gorman <mgorman@techsingularity.net>, Levin Alexander <Alexander.Levin@microsoft.com>, Huaisheng Ye <yehs1@lenovo.com>, Mike Rapoport <rppt@linux.vnet.ibm.com>, linux-arm-kernel@lists.infradead.org, iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Yong Wu <yong.wu@mediatek.com>, Matthias Brugger <matthias.bgg@gmail.com>, Tomasz Figa <tfiga@google.com>, yingjoe.chen@mediatek.com, Matthew Wilcox <willy@infradead.org>

On Wed, Dec 05, 2018 at 06:43:08AM -0800, Christoph Hellwig wrote:
> On Wed, Dec 05, 2018 at 02:40:06PM +0000, Robin Murphy wrote:
> > 32-bit Arm doesn't have ZONE_DMA32, but has (or at least had at the time) a
> > 2GB ZONE_DMA. Whether we actually need that or not depends on how this all
> > interacts with LPAE and highmem, but I'm not sure of those details off-hand.
> 
> Well, arm32 can't address more than 32-bits in the linear kernel
> mapping, so GFP_KERNEL should be perfectly fine there if the limit
> really is 32-bits and not 31 or smaller because someone stole a bit
> or two somewhere.

I'm not sure that's necessarily true on the physical side. Wasn't there a
keystone SoC with /all/ the coherent memory above 4GB?

Will