commit 
Author: 
Date: 

    drm/ttm: make ttm_bo_unpin more defensive
    
    [ Upstream commit 6c5403173a13a08ff61dbdafa4c0ed4a9dedbfe0 ]
    
    We seem to have some more driver bugs than thought.
    
    Signed-off-by: Christian König <christian.koenig@amd.com>
    Fixes: deb0814b43f3 ("drm/ttm: add ttm_bo_pin()/ttm_bo_unpin() v2")
    Acked-by: Matthew Auld <matthew.auld@intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20210312093810.2202-1-christian.koenig@amd.com

diff --git a/include/drm/ttm/ttm_bo_api.h b/include/drm/ttm/ttm_bo_api.h
index 2564e66e67d7..eb231cfbe37a 100644
--- a/include/drm/ttm/ttm_bo_api.h
+++ b/include/drm/ttm/ttm_bo_api.h
@@ -613,7 +613,10 @@ static inline void ttm_bo_unpin(struct ttm_buffer_object *bo)
 {
 	dma_resv_assert_held(bo->base.resv);
 	WARN_ON_ONCE(!bo->pin_count);
-	--bo->pin_count;
+	if (bo->pin_count)
+		--bo->pin_count;
+	else
+		WARN_ON_ONCE(true);
 }
 
 int ttm_mem_evict_first(struct ttm_bo_device *bdev,