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

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

[Daniel Vetter]

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@gmail.com>
Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.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

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

[Christian König]

Am 19.05.21 um 10:24 schrieb Daniel Vetter:
> 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@gmail.com>
> Cc: Christian Koenig <christian.koenig@amd.com>
> Cc: Huang Rui <ray.huang@amd.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

Reviewed-by: Christian König <christian.koenig@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,

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

[Daniel Vetter]

On Wed, May 19, 2021 at 12:43:49PM +0200, Christian König wrote:
> Am 19.05.21 um 10:24 schrieb Daniel Vetter:
> > 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@gmail.com>
> > Cc: Christian Koenig <christian.koenig@amd.com>
> > Cc: Huang Rui <ray.huang@amd.com>
> > Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> 
> Reviewed-by: Christian König <christian.koenig@amd.com>

Applied to drm-misc-next, thanks for reviewing.
-Daniel

> 
> > ---
> >   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,
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [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@gmail.com>
> Cc: Christian Koenig <christian.koenig@amd.com>
> Cc: Huang Rui <ray.huang@amd.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

Acked-by: Huang Rui <ray.huang@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
>