linux-kernel - Re: [PATCH v4 3/3] iommu/io-pgtable-arm-v7s: Request DMA32 memory, and improve debugging

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Date:   Wed, 5 Dec 2018 14:40:06 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@...radead.org>,
        Nicolas Boichat <drinkcat@...omium.org>
Cc:     Will Deacon <will.deacon@....com>, Joerg Roedel <joro@...tes.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Levin Alexander <Alexander.Levin@...rosoft.com>,
        Huaisheng Ye <yehs1@...ovo.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Yong Wu <yong.wu@...iatek.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Tomasz Figa <tfiga@...gle.com>, yingjoe.chen@...iatek.com,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v4 3/3] iommu/io-pgtable-arm-v7s: Request DMA32 memory,
 and improve debugging

On 05/12/2018 13:54, Christoph Hellwig wrote:
> On Wed, Dec 05, 2018 at 01:48:28PM +0800, Nicolas Boichat wrote:
>> IOMMUs using ARMv7 short-descriptor format require page tables
>> (level 1 and 2) to be allocated within the first 4GB of RAM, even
>> on 64-bit systems.
> 
>> +#ifdef CONFIG_ZONE_DMA32
>> +#define ARM_V7S_TABLE_GFP_DMA GFP_DMA32
>> +#define ARM_V7S_TABLE_SLAB_CACHE SLAB_CACHE_DMA32
>> +#else
>> +#define ARM_V7S_TABLE_GFP_DMA GFP_DMA
>> +#define ARM_V7S_TABLE_SLAB_CACHE SLAB_CACHE_DMA
>> +#endif
> 
> How does using GFP_DMA make sense based on the above?  If the system
> has more than 32-bits worth of RAM it should be using GFP_DMA32, else
> GFP_KERNEL, not GFP_DMA for an arch defined small addressability pool.

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.

Robin.