Re: [PATCH v5 2/3] iommu/io-pgtable-arm-v7s: Request DMA32 memory, and improve debugging

[Nicolas Boichat]

On Fri, Dec 7, 2018 at 4:05 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Fri, Dec 07, 2018 at 02:16:19PM +0800, Nicolas Boichat wrote:
> > +#ifdef CONFIG_ZONE_DMA32
> > +#define ARM_V7S_TABLE_GFP_DMA GFP_DMA32
> > +#define ARM_V7S_TABLE_SLAB_CACHE SLAB_CACHE_DMA32
>
> This name doesn't make any sense.  Why not ARM_V7S_TABLE_SLAB_FLAGS ?

Sure, will fix in v6 then.

> > +#else
> > +#define ARM_V7S_TABLE_GFP_DMA GFP_DMA
> > +#define ARM_V7S_TABLE_SLAB_CACHE SLAB_CACHE_DMA
>
> Can you remind me again why it is, on machines which don't support
> ZONE_DMA32, why we have to allocate from ZONE_DMA?  My understanding
> is that 64-bit machines have ZONE_DMA32 and 32-bit machines don't.
> So shouldn't this rather be GFP_KERNEL?

Sorry I mean to reply on the v4 thread, Christoph raised the same
question (https://patchwork.kernel.org/patch/10713025/).

I don't know, and I don't have all the hardware needed to test this
,-( Robin and Will both didn't seem sure.

I'd rather not introduce a new regression, this patch series tries to
fix a known arm64 regression, where we _need_ tables to be in DMA32.
If we want to change 32-bit hardware to use GFP_KERNEL, and we're sure
it works, that's fine by me, but it should be in another patch set.

Hope this makes sense,

Thanks,

> Actually, maybe we could centralise this in gfp.h:
>
> #ifdef CONFIG_64BIT
> # ifdef CONFIG_ZONE_DMA32
> #define GFP_32BIT       GFP_DMA32
> # else
> #define GFP_32BIT       GFP_DMA
> #else /* 32-bit */
> #define GFP_32BIT       GFP_KERNEL
> #endif
>