A few small dma-mapping fixes for Linux 4.17-rc3:

- don't loop to try GFP_DMA allocations if ZONE_DMA is not actually
    enabled (regression in 4.16)
  - don't try to do virt_to_page before we know we actuall have a
    valid page in dma_common_mmap
  - a comment fixup related to the above fix
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Merge tag 'dma-mapping-4.17-3' of git://git.infradead.org/users/hch/dma-mapping

Pull dma-mapping fixes from Christoph Hellwig:
 "A few small dma-mapping fixes for Linux 4.17-rc3:

   - don't loop to try GFP_DMA allocations if ZONE_DMA is not actually
     enabled (regression in 4.16)

   - don't try to do virt_to_page before we know we actuall have a valid
     page in dma_common_mmap

   - a comment fixup related to the above fix"

* tag 'dma-mapping-4.17-3' of git://git.infradead.org/users/hch/dma-mapping:
  dma-mapping: postpone cpu addr translation on mmap
  dma-coherent: clarify dma_mmap_from_dev_coherent documentation
  dma-direct: don't retry allocation for no-op GFP_DMA

drivers/base/dma-coherent.c

@@ -312,8 +312,9 @@ static int __dma_mmap_from_coherent(struct dma_coherent_mem *mem,
  * This checks whether the memory was allocated from the per-device
  * coherent memory pool and if so, maps that memory to the provided vma.
  *
- * Returns 1 if we correctly mapped the memory, or 0 if the caller should
- * proceed with mapping memory from generic pools.
+ * Returns 1 if @vaddr belongs to the device coherent pool and the caller
+ * should return @ret, or 0 if they should proceed with mapping memory from
+ * generic areas.
  */
 int dma_mmap_from_dev_coherent(struct device *dev, struct vm_area_struct *vma,
 			   void *vaddr, size_t size, int *ret)

drivers/base/dma-mapping.c

@@ -226,7 +226,6 @@ int dma_common_mmap(struct device *dev, struct vm_area_struct *vma,
 #ifndef CONFIG_ARCH_NO_COHERENT_DMA_MMAP
 	unsigned long user_count = vma_pages(vma);
 	unsigned long count = PAGE_ALIGN(size) >> PAGE_SHIFT;
-	unsigned long pfn = page_to_pfn(virt_to_page(cpu_addr));
 	unsigned long off = vma->vm_pgoff;
 
 	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
@@ -234,12 +233,11 @@ int dma_common_mmap(struct device *dev, struct vm_area_struct *vma,
 	if (dma_mmap_from_dev_coherent(dev, vma, cpu_addr, size, &ret))
 		return ret;
 
-	if (off < count && user_count <= (count - off)) {
+	if (off < count && user_count <= (count - off))
 		ret = remap_pfn_range(vma, vma->vm_start,
-				      pfn + off,
+				      page_to_pfn(virt_to_page(cpu_addr)) + off,
 				      user_count << PAGE_SHIFT,
 				      vma->vm_page_prot);
-	}
 #endif	/* !CONFIG_ARCH_NO_COHERENT_DMA_MMAP */
 
 	return ret;

lib/dma-direct.c

@@ -84,7 +84,8 @@ again:
 		__free_pages(page, page_order);
 		page = NULL;
 
-		if (dev->coherent_dma_mask < DMA_BIT_MASK(32) &&
+		if (IS_ENABLED(CONFIG_ZONE_DMA) &&
+		    dev->coherent_dma_mask < DMA_BIT_MASK(32) &&
 		    !(gfp & GFP_DMA)) {
 			gfp = (gfp & ~GFP_DMA32) | GFP_DMA;
 			goto again;