Re: [PATCH v2 2/3] drm/xe: share bo dma-resv with backup object

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Wed, 2025-04-16 at 16:09 +0100, Matthew Auld wrote:
> We end up needing to grab both locks together anyway and keep them
> held
> until we complete the copy or add the fence. Plus the backup_obj is
> short lived and tied to the parent object, so seems reasonable to
> share
> the same dma-resv. This will simplify the locking here, and in follow
> up patches.
> 
> v2:
>   - Hold reference to the parent bo to be sure the shared dma-resv
> can't
>     go out of scope too soon. (Thomas)
> 
> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>

Another thought: What if someone releases the last "ordinary" reference
on an object that has a backup object so that the only remaining
reference is the backup object. Can that happen?

I have a vague recollection of having to deal with that for i915...

/Thomas


> ---
>  drivers/gpu/drm/xe/xe_bo.c       | 28 +++++++++++++---------------
>  drivers/gpu/drm/xe/xe_bo_types.h |  2 ++
>  2 files changed, 15 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
> index c337790c81ae..79adaee5a0e9 100644
> --- a/drivers/gpu/drm/xe/xe_bo.c
> +++ b/drivers/gpu/drm/xe/xe_bo.c
> @@ -1120,13 +1120,15 @@ int xe_bo_evict_pinned(struct xe_bo *bo)
>  	if (bo->flags & XE_BO_FLAG_PINNED_NORESTORE)
>  		goto out_unlock_bo;
>  
> -	backup = xe_bo_create_locked(xe, NULL, NULL, bo->size,
> ttm_bo_type_kernel,
> -				     XE_BO_FLAG_SYSTEM |
> XE_BO_FLAG_NEEDS_CPU_ACCESS |
> -				     XE_BO_FLAG_PINNED);
> +	backup = ___xe_bo_create_locked(xe, NULL, NULL, bo-
> >ttm.base.resv, NULL, bo->size,
> +					DRM_XE_GEM_CPU_CACHING_WB,
> ttm_bo_type_kernel,
> +					XE_BO_FLAG_SYSTEM |
> XE_BO_FLAG_NEEDS_CPU_ACCESS |
> +					XE_BO_FLAG_PINNED);
>  	if (IS_ERR(backup)) {
>  		ret = PTR_ERR(backup);
>  		goto out_unlock_bo;
>  	}
> +	backup->parent_obj = xe_bo_get(bo); /* Released by
> bo_destroy */
>  
>  	if (xe_bo_is_user(bo) || (bo->flags &
> XE_BO_FLAG_PINNED_LATE_RESTORE)) {
>  		struct xe_migrate *migrate;
> @@ -1177,7 +1179,6 @@ int xe_bo_evict_pinned(struct xe_bo *bo)
>  
>  out_backup:
>  	xe_bo_vunmap(backup);
> -	xe_bo_unlock(backup);
>  	if (ret)
>  		xe_bo_put(backup);
>  out_unlock_bo:
> @@ -1212,17 +1213,12 @@ int xe_bo_restore_pinned(struct xe_bo *bo)
>  	if (!backup)
>  		return 0;
>  
> -	xe_bo_lock(backup, false);
> +	xe_bo_lock(bo, false);
>  
>  	ret = ttm_bo_validate(&backup->ttm, &backup->placement,
> &ctx);
>  	if (ret)
>  		goto out_backup;
>  
> -	if (WARN_ON(!dma_resv_trylock(bo->ttm.base.resv))) {
> -		ret = -EBUSY;
> -		goto out_backup;
> -	}
> -
>  	if (xe_bo_is_user(bo) || (bo->flags &
> XE_BO_FLAG_PINNED_LATE_RESTORE)) {
>  		struct xe_migrate *migrate;
>  		struct dma_fence *fence;
> @@ -1271,15 +1267,14 @@ int xe_bo_restore_pinned(struct xe_bo *bo)
>  
>  	bo->backup_obj = NULL;
>  
> +out_backup:
> +	xe_bo_vunmap(backup);
> +	if (!bo->backup_obj)
> +		xe_bo_put(backup);
>  out_unlock_bo:
>  	if (unmap)
>  		xe_bo_vunmap(bo);
>  	xe_bo_unlock(bo);
> -out_backup:
> -	xe_bo_vunmap(backup);
> -	xe_bo_unlock(backup);
> -	if (!bo->backup_obj)
> -		xe_bo_put(backup);
>  	return ret;
>  }
>  
> @@ -1532,6 +1527,9 @@ static void xe_ttm_bo_destroy(struct
> ttm_buffer_object *ttm_bo)
>  	if (bo->vm && xe_bo_is_user(bo))
>  		xe_vm_put(bo->vm);
>  
> +	if (bo->parent_obj)
> +		xe_bo_put(bo->parent_obj);
> +
>  	mutex_lock(&xe->mem_access.vram_userfault.lock);
>  	if (!list_empty(&bo->vram_userfault_link))
>  		list_del(&bo->vram_userfault_link);
> diff --git a/drivers/gpu/drm/xe/xe_bo_types.h
> b/drivers/gpu/drm/xe/xe_bo_types.h
> index 81396181aaea..eb5e83c5f233 100644
> --- a/drivers/gpu/drm/xe/xe_bo_types.h
> +++ b/drivers/gpu/drm/xe/xe_bo_types.h
> @@ -30,6 +30,8 @@ struct xe_bo {
>  	struct ttm_buffer_object ttm;
>  	/** @backup_obj: The backup object when pinned and suspended
> (vram only) */
>  	struct xe_bo *backup_obj;
> +	/** @parent_obj: Ref to parent bo if this a backup_obj */
> +	struct xe_bo *parent_obj;
>  	/** @size: Size of this buffer object */
>  	size_t size;
>  	/** @flags: flags for this buffer object */