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

[Robin Murphy]

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.