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Date:   Wed, 5 Dec 2018 14:46:06 +0000
From:   Will Deacon <will.deacon@arm.com>
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>
Subject: Re: [PATCH v4 3/3] iommu/io-pgtable-arm-v7s: Request DMA32 memory,
 and improve debugging
Message-ID: <20181205144605.GA16171@arm.com>
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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