drm: Fix for GEM buffers with write-combine memory

The previous commit wrongly assumed that dma_mmap_wc() could be replaced
by pgprot_writecombine() + dma_mmap_pages(). It did work on my setup,
but did not work everywhere.

Use dma_mmap_wc() when the buffer has the write-combine cache attribute,
and dma_mmap_pages() when it has the non-coherent cache attribute.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reported-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Tested-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Fixes: cf8ccbc7 ("drm: Add support for GEM buffers backed by non-coherent memory")
Link: https://patchwork.freedesktop.org/patch/msgid/20210527230334.151947-1-paul@crapouillou.net

drivers/gpu/drm/drm_gem_cma_helper.c

@@ -514,13 +514,16 @@ int drm_gem_cma_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
 
 	cma_obj = to_drm_gem_cma_obj(obj);
 
+	if (cma_obj->map_noncoherent) {
 		vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
-	if (!cma_obj->map_noncoherent)
-		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
 
 		ret = dma_mmap_pages(cma_obj->base.dev->dev,
 				     vma, vma->vm_end - vma->vm_start,
 				     virt_to_page(cma_obj->vaddr));
+	} else {
+		ret = dma_mmap_wc(cma_obj->base.dev->dev, vma, cma_obj->vaddr,
+				  cma_obj->paddr, vma->vm_end - vma->vm_start);
+	}
 	if (ret)
 		drm_gem_vm_close(vma);