Subject: Re: [PATCH 2/3] powerpc/dma: Support 32-bit coherent mask with 64-bit dma_mask

From: Scott Wood <[email-masked]>
Date: Tue, 24 Feb 2015 14:34:37 -0600

On Fri, 2015-02-20 at 19:35 +1100, Benjamin Herrenschmidt wrote:
> @@ -149,14 +141,13 @@ static void dma_direct_unmap_sg(struct device *dev, struct scatterlist *sg,
>  
>  static int dma_direct_dma_supported(struct device *dev, u64 mask)
>  {
> -#ifdef CONFIG_PPC64
> -	/* Could be improved so platforms can set the limit in case
> -	 * they have limited DMA windows
> -	 */
> -	return mask >= get_dma_offset(dev) + (memblock_end_of_DRAM() - 1);
> -#else
> -	return 1;
> +	u64 offset = get_dma_offset(dev);
> +	u64 limit = offset + memblock_end_of_DRAM() - 1;
> +
> +#if defined(CONFIG_ZONE_DMA32)
> +	limit = offset + dma_get_zone_limit(ZONE_DMA32);
>  #endif
> +	return mask >= limit;
>  }

I'm confused as to whether dma_supported() is supposed to be testing a
coherent mask or regular mask...  The above suggests coherent, as does
the call to dma_supported() in dma_set_coherent_mask(), but if swiotlb
is used, swiotlb_dma_supported() will only check for a mask that can
accommodate io_tlb_end, without regard for coherent allocations.

>  static u64 dma_direct_get_required_mask(struct device *dev)
> diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> index f146ef0..a7f15e2 100644
> --- a/arch/powerpc/mm/mem.c
> +++ b/arch/powerpc/mm/mem.c
> @@ -277,6 +277,11 @@ int dma_pfn_limit_to_zone(u64 pfn_limit)
>  	return -EPERM;
>  }
>  
> +u64 dma_get_zone_limit(int zone)
> +{
> +	return max_zone_pfns[zone] << PAGE_SHIFT;
> +}

If you must do this in terms of bytes rather than pfn, cast to u64
before shifting -- and even then the result will be PAGE_SIZE - 1 too
small.

-Scott