Re: [PATCH 02/20] kernel/dma/direct: refine dma_direct_alloc zone selection

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Mon, 2018-07-30 at 18:38 +0200, Christoph Hellwig wrote:
> We need to take the DMA offset and encryption bit into account when selecting
> a zone.  Add a helper that takes those into account and use it.

That whole "encryption" stuff seems to be completely specific to the
way x86 does memory encryption, or am I mistaken ? It's not clear to me
what that does in practice and how it relates to DMA mappings.

I'm also not sure about that whole business with ZONE_DMA and
ARCH_ZONE_DMA_BITS...

On ppc64, unless you enable swiotlb (which we only do currently on
some embedded platforms), you have all of memory in ZONE_DMA.

[    0.000000] Zone ranges:
[    0.000000]   DMA      [mem 0x0000000000000000-0x0000001fffffffff]
[    0.000000]   DMA32    empty
[    0.000000]   Normal   empty
[    0.000000]   Device   empty

I'm not sure how this will work with that dma direct code.

I also see a number of tests against a 64-bit mask rather than the
top of memory...

Ben.

> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  kernel/dma/direct.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index d32d4f0d2c0c..c2c1df8827f2 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -58,6 +58,14 @@ static bool dma_coherent_ok(struct device *dev, phys_addr_t phys, size_t size)
>  	return addr + size - 1 <= dev->coherent_dma_mask;
>  }
>  
> +static bool dma_coherent_below(struct device *dev, u64 mask)
> +{
> +	dma_addr_t addr = force_dma_unencrypted() ?
> +		__phys_to_dma(dev, mask) : phys_to_dma(dev, mask);
> +
> +	return dev->coherent_dma_mask <= addr;
> +}
> +
>  void *dma_direct_alloc(struct device *dev, size_t size, dma_addr_t *dma_handle,
>  		gfp_t gfp, unsigned long attrs)
>  {
> @@ -70,9 +78,9 @@ void *dma_direct_alloc(struct device *dev, size_t size, dma_addr_t *dma_handle,
>  	gfp &= ~__GFP_ZERO;
>  
>  	/* GFP_DMA32 and GFP_DMA are no ops without the corresponding zones: */
> -	if (dev->coherent_dma_mask <= DMA_BIT_MASK(ARCH_ZONE_DMA_BITS))
> +	if (dma_coherent_below(dev, DMA_BIT_MASK(ARCH_ZONE_DMA_BITS)))
>  		gfp |= GFP_DMA;
> -	if (dev->coherent_dma_mask <= DMA_BIT_MASK(32) && !(gfp & GFP_DMA))
> +	if (dma_coherent_below(dev, DMA_BIT_MASK(32) && !(gfp & GFP_DMA)))
>  		gfp |= GFP_DMA32;
>  
>  again:
> @@ -92,14 +100,14 @@ void *dma_direct_alloc(struct device *dev, size_t size, dma_addr_t *dma_handle,
>  		page = NULL;
>  
>  		if (IS_ENABLED(CONFIG_ZONE_DMA32) &&
> -		    dev->coherent_dma_mask < DMA_BIT_MASK(64) &&
> +		    dma_coherent_below(dev, DMA_BIT_MASK(64)) &&
>  		    !(gfp & (GFP_DMA32 | GFP_DMA))) {
>  			gfp |= GFP_DMA32;
>  			goto again;
>  		}
>  
>  		if (IS_ENABLED(CONFIG_ZONE_DMA) &&
> -		    dev->coherent_dma_mask < DMA_BIT_MASK(32) &&
> +		    dma_coherent_below(dev, DMA_BIT_MASK(32)) &&
>  		    !(gfp & GFP_DMA)) {
>  			gfp = (gfp & ~GFP_DMA32) | GFP_DMA;
>  			goto again;