drm/prime: reject DMA-BUF attach when get_sg_table is missing

drm_gem_map_dma_buf() requires drm_gem_object_funcs.get_sg_table
to be implemented, or else WARNs.

Allow drivers to leave this hook unimplemented to implement purely
local DMA-BUFs (ie, DMA-BUFs which cannot be imported anywhere
else but the device which allocated them). In that case, reject
imports to other devices in drm_gem_map_attach().

v2: new patch

v3: use ENOSYS

Signed-off-by: Simon Ser <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Tian Tao <[e-mail]>
Cc: Maxime Ripard <[e-mail]>
Cc: Hans de Goede <[e-mail]>
Acked-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_prime.c

···
  * Optional pinning of buffers is handled at dma-buf attach and detach time in
  * drm_gem_map_attach() and drm_gem_map_detach(). Backing storage itself is
  * handled by drm_gem_map_dma_buf() and drm_gem_unmap_dma_buf(), which relies on
- * &drm_gem_object_funcs.get_sg_table.
+ * &drm_gem_object_funcs.get_sg_table. If &drm_gem_object_funcs.get_sg_table is
+ * unimplemented, exports into another device are rejected.
  *
  * For kernel-internal access there's drm_gem_dmabuf_vmap() and
  * drm_gem_dmabuf_vunmap(). Userspace mmap support is provided by
···
 		       struct dma_buf_attachment *attach)
 {
 	struct drm_gem_object *obj = dma_buf->priv;
+
+	if (!obj->funcs->get_sg_table)
+		return -ENOSYS;
 
 	return drm_gem_pin(obj);
 }