dma-buf: Make mmap callback actually optional

The docs state the callback is optional but it is not, make it optional.
Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190329165201.16233-1-afd@ti.com

drivers/dma-buf/dma-buf.c

@@ -90,6 +90,10 @@ static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
 
 	dmabuf = file->private_data;
 
+	/* check if buffer supports mmap */
+	if (!dmabuf->ops->mmap)
+		return -EINVAL;
+
 	/* check for overflowing the buffer's size */
 	if (vma->vm_pgoff + vma_pages(vma) >
 	    dmabuf->size >> PAGE_SHIFT)
@@ -404,8 +408,7 @@ struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info)
 			  || !exp_info->ops
 			  || !exp_info->ops->map_dma_buf
 			  || !exp_info->ops->unmap_dma_buf
-			  || !exp_info->ops->release
-			  || !exp_info->ops->mmap)) {
+			  || !exp_info->ops->release)) {
 		return ERR_PTR(-EINVAL);
 	}
 
@@ -906,6 +909,10 @@ int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
 	if (WARN_ON(!dmabuf || !vma))
 		return -EINVAL;
 
+	/* check if buffer supports mmap */
+	if (!dmabuf->ops->mmap)
+		return -EINVAL;
+
 	/* check for offset overflow */
 	if (pgoff + vma_pages(vma) < pgoff)
 		return -EOVERFLOW;