Re: [PATCH] arc: Implement arch-specific dma_map_ops.mmap

[Catalin Marinas]

On Wed, Nov 02, 2016 at 03:19:43PM +0300, Alexey Brodkin wrote:
> diff --git a/arch/arc/mm/dma.c b/arch/arc/mm/dma.c
> index 20afc65e22dc..034ec6a8a764 100644
> --- a/arch/arc/mm/dma.c
> +++ b/arch/arc/mm/dma.c
> @@ -105,6 +105,31 @@ static void arc_dma_free(struct device *dev, size_t 
> size, void *vaddr,
>   __free_pages(page, get_order(size));
>  }
>  
> +static int arc_dma_mmap(struct device *dev, struct vm_area_struct *vma,
> + void *cpu_addr, dma_addr_t dma_addr, size_t size,
> + unsigned long attrs)
> +{
> + unsigned long user_count = vma_pages(vma);
> + unsigned long count = PAGE_ALIGN(size) >> PAGE_SHIFT;
> + unsigned long pfn = __phys_to_pfn(dma_addr);

I don't think that's correct in all situations. Better as (for arc):

unsigned long pfn = __phys_to_pfn(plat_dma_to_phys(dma_addr));

Other than that:

Reviewed-by: Catalin Marinas 

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[PATCH] arc: Implement arch-specific dma_map_ops.mmap

[Alexey Brodkin]

We used to use generic implementation of dma_map_ops.mmap which is
dma_common_mmap() but that only worked for simpler cached mappings when
vaddr = paddr.

If a driver requests uncached DMA buffer kernel maps it to virtual
address so that MMU gets involved and page uncached status takes into
account. In that case usage of dma_common_mmap() lead to mapping of
vaddr to vaddr for user-space which is obviously wrong. For more detals
please refer to verbose explanation here [1].

So here we implement our own version of mmap() which always deals
with dma_addr and maps underlying memory to user-space properly
(note that DMA buffer mapped to user-space is always uncached
because there's no way to properly manage cache from user-space).

[1] https://lkml.org/lkml/2016/10/26/973

Signed-off-by: Alexey Brodkin 
Cc: Catalin Marinas 
Cc: Marek Szyprowski 
Cc: sta...@vger.kernel.org
---
 arch/arc/mm/dma.c | 26 ++
 1 file changed, 26 insertions(+)

diff --git a/arch/arc/mm/dma.c b/arch/arc/mm/dma.c
index 20afc65e22dc..034ec6a8a764 100644
--- a/arch/arc/mm/dma.c
+++ b/arch/arc/mm/dma.c
@@ -105,6 +105,31 @@ static void arc_dma_free(struct device *dev, size_t size, 
void *vaddr,
__free_pages(page, get_order(size));
 }
 
+static int arc_dma_mmap(struct device *dev, struct vm_area_struct *vma,
+   void *cpu_addr, dma_addr_t dma_addr, size_t size,
+   unsigned long attrs)
+{
+   unsigned long user_count = vma_pages(vma);
+   unsigned long count = PAGE_ALIGN(size) >> PAGE_SHIFT;
+   unsigned long pfn = __phys_to_pfn(dma_addr);
+   unsigned long off = vma->vm_pgoff;
+   int ret = -ENXIO;
+
+   vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+
+   if (dma_mmap_from_coherent(dev, vma, cpu_addr, size, ))
+   return ret;
+
+   if (off < count && user_count <= (count - off)) {
+   ret = remap_pfn_range(vma, vma->vm_start,
+ pfn + off,
+ user_count << PAGE_SHIFT,
+ vma->vm_page_prot);
+   }
+
+   return ret;
+}
+
 /*
  * streaming DMA Mapping API...
  * CPU accesses page via normal paddr, thus needs to explicitly made
@@ -193,6 +218,7 @@ static int arc_dma_supported(struct device *dev, u64 
dma_mask)
 struct dma_map_ops arc_dma_ops = {
.alloc  = arc_dma_alloc,
.free   = arc_dma_free,
+   .mmap   = arc_dma_mmap,
.map_page   = arc_dma_map_page,
.map_sg = arc_dma_map_sg,
.sync_single_for_device = arc_dma_sync_single_for_device,
-- 
2.7.4


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