[PATCH v2] drm/prime: fix drm_prime_gem_destroy comment

[Chris Bainbridge <chris.bainbridge@gmail.com>]

Edit the comments on correct usage of drm_prime_gem_destroy to note
that, if using TTM, drm_prime_gem_destroy must be called in the
ttm_buffer_object.destroy hook, to avoid the dma_buf being freed leaving
a dangling pointer which will be later dereferenced by
ttm_bo_delayed_delete.

Signed-off-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Suggested-by: Christian König <christian.koenig@amd.com>
---
 drivers/gpu/drm/drm_prime.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index 32a8781cfd67..452d5c7cd292 100644
--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -929,7 +929,9 @@ EXPORT_SYMBOL(drm_gem_prime_export);
  * &drm_driver.gem_prime_import_sg_table internally.
  *
  * Drivers must arrange to call drm_prime_gem_destroy() from their
- * &drm_gem_object_funcs.free hook when using this function.
+ * &drm_gem_object_funcs.free hook or &ttm_buffer_object.destroy
+ * hook when using this function, to avoid the dma_buf being freed while the
+ * ttm_buffer_object can still dereference it.
  */
 struct drm_gem_object *drm_gem_prime_import_dev(struct drm_device *dev,
 					    struct dma_buf *dma_buf,
@@ -999,7 +1001,9 @@ EXPORT_SYMBOL(drm_gem_prime_import_dev);
  * implementation in drm_gem_prime_fd_to_handle().
  *
  * Drivers must arrange to call drm_prime_gem_destroy() from their
- * &drm_gem_object_funcs.free hook when using this function.
+ * &drm_gem_object_funcs.free hook or &ttm_buffer_object.destroy
+ * hook when using this function, to avoid the dma_buf being freed while the
+ * ttm_buffer_object can still dereference it.
  */
 struct drm_gem_object *drm_gem_prime_import(struct drm_device *dev,
 					    struct dma_buf *dma_buf)
-- 
2.47.2