drm/shmem-helpers: Ensure get_pages is not called on imported dma-buf

Just a bit of light paranoia. Also sprinkle this check over
drm_gem_shmem_get_sg_table, which should only be called when
exporting, same for the pin/unpin functions, on which it relies to
work correctly.
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200511093554.211493-9-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_gem_shmem_helper.c

@@ -170,6 +170,8 @@ int drm_gem_shmem_get_pages(struct drm_gem_shmem_object *shmem)
 {
 	int ret;
 
+	WARN_ON(shmem->base.import_attach);
+
 	ret = mutex_lock_interruptible(&shmem->pages_lock);
 	if (ret)
 		return ret;
@@ -225,6 +227,8 @@ int drm_gem_shmem_pin(struct drm_gem_object *obj)
 {
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
 
+	WARN_ON(shmem->base.import_attach);
+
 	return drm_gem_shmem_get_pages(shmem);
 }
 EXPORT_SYMBOL(drm_gem_shmem_pin);
@@ -240,6 +244,8 @@ void drm_gem_shmem_unpin(struct drm_gem_object *obj)
 {
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
 
+	WARN_ON(shmem->base.import_attach);
+
 	drm_gem_shmem_put_pages(shmem);
 }
 EXPORT_SYMBOL(drm_gem_shmem_unpin);
@@ -510,6 +516,8 @@ static void drm_gem_shmem_vm_open(struct vm_area_struct *vma)
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
 	int ret;
 
+	WARN_ON(shmem->base.import_attach);
+
 	ret = drm_gem_shmem_get_pages(shmem);
 	WARN_ON_ONCE(ret != 0);
 
@@ -611,6 +619,8 @@ struct sg_table *drm_gem_shmem_get_sg_table(struct drm_gem_object *obj)
 {
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
 
+	WARN_ON(shmem->base.import_attach);
+
 	return drm_prime_pages_to_sg(shmem->pages, obj->size >> PAGE_SHIFT);
 }
 EXPORT_SYMBOL_GPL(drm_gem_shmem_get_sg_table);