[PATCH] drm/ttm: Explain why ttm_bo_add_move_fence uses a shared slot

[Huang Rui]

On Wed, May 19, 2021 at 04:24:09PM +0800, Daniel Vetter wrote:
> Motivated because I got confused and Christian confirmed why this
> works. I think this is non-obvious enough that it merits a slightly
> longer comment.
> 
> Cc: Christian König <ckoenig.leichtzumerken at gmail.com>
> Cc: Christian Koenig <christian.koenig at amd.com>
> Cc: Huang Rui <ray.huang at amd.com>
> Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>

Acked-by: Huang Rui <ray.huang at amd.com>

> ---
>  drivers/gpu/drm/ttm/ttm_bo.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
> index ca1b098b6a56..51a94fd63bd7 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> @@ -682,7 +682,9 @@ int ttm_mem_evict_first(struct ttm_device *bdev,
>  }
>  
>  /*
> - * Add the last move fence to the BO and reserve a new shared slot.
> + * Add the last move fence to the BO and reserve a new shared slot. We only use
> + * a shared slot to avoid unecessary sync and rely on the subsequent bo move to
> + * either stall or use an exclusive fence respectively set bo->moving.
>   */
>  static int ttm_bo_add_move_fence(struct ttm_buffer_object *bo,
>  				 struct ttm_resource_manager *man,
> -- 
> 2.31.0
>