On 05/12/2018 13:54, Christoph Hellwig wrote:

On Wed, Dec 05, 2018 at 01:48:28PM +0800, Nicolas Boichat wrote:

quoted

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.

quoted

+#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.

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