+int drm_gem_cma_mmap_noncoherent(struct drm_gem_object *obj,
+				 struct vm_area_struct *vma)
+{
+	struct drm_gem_cma_object *cma_obj;
+	unsigned long pfn;
+	int ret;
+
+	/*
+	 * Clear the VM_PFNMAP flag that was set by drm_gem_mmap(), and set the
+	 * vm_pgoff (used as a fake buffer offset by DRM) to 0 as we want to map
+	 * the whole buffer.
+	 */
+	vma->vm_pgoff -= drm_vma_node_start(&obj->vma_node);
+	vma->vm_flags &= ~VM_PFNMAP;
+	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
+
+	cma_obj = to_drm_gem_cma_obj(obj);
+
+	pfn = PHYS_PFN(dma_to_phys(cma_obj->base.dev->dev, cma_obj->paddr));
+
+	ret = remap_pfn_range(vma, vma->vm_start, pfn,
+			      vma->vm_end - vma->vm_start,
+			      vma->vm_page_prot);

dma_to_phys must not be used by drivers.

I have a proper helper for this waiting for users:

http://git.infradead.org/users/hch/misc.git/commitdiff/96a546e7229ec53aadbdb7936d1e5e6cb5958952

If you can confirm the helpers works for you I can try to still sneak
it to Linus for 5.12 to ease the merge pain.