linux-kernel - Re: [PATCH drm-misc-next v2 5/7] drm/gpuvm: add an abstraction for a VM / BO combination

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Message-ID: <20230907101630.7213c919@collabora.com>
Date:   Thu, 7 Sep 2023 10:16:30 +0200
From:   Boris Brezillon <boris.brezillon@...labora.com>
To:     Danilo Krummrich <dakr@...hat.com>
Cc:     airlied@...il.com, daniel@...ll.ch, matthew.brost@...el.com,
        thomas.hellstrom@...ux.intel.com, sarah.walker@...tec.com,
        donald.robson@...tec.com, christian.koenig@....com,
        faith.ekstrand@...labora.com, dri-devel@...ts.freedesktop.org,
        nouveau@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH drm-misc-next v2 5/7] drm/gpuvm: add an abstraction for
 a VM / BO combination

On Wed,  6 Sep 2023 23:47:13 +0200
Danilo Krummrich <dakr@...hat.com> wrote:

> @@ -812,15 +967,20 @@ EXPORT_SYMBOL_GPL(drm_gpuva_remove);
>  /**
>   * drm_gpuva_link() - link a &drm_gpuva
>   * @va: the &drm_gpuva to link
> + * @vm_bo: the &drm_gpuvm_bo to add the &drm_gpuva to
>   *
> - * This adds the given &va to the GPU VA list of the &drm_gem_object it is
> - * associated with.
> + * This adds the given &va to the GPU VA list of the &drm_gpuvm_bo and the
> + * &drm_gpuvm_bo to the &drm_gem_object it is associated with.
> + *
> + * For every &drm_gpuva entry added to the &drm_gpuvm_bo an additional
> + * reference of the latter is taken.
>   *
>   * This function expects the caller to protect the GEM's GPUVA list against
> - * concurrent access using the GEMs dma_resv lock.
> + * concurrent access using either the GEMs dma_resv lock or a driver specific
> + * lock set through drm_gem_gpuva_set_lock().
>   */
>  void
> -drm_gpuva_link(struct drm_gpuva *va)
> +drm_gpuva_link(struct drm_gpuva *va, struct drm_gpuvm_bo *vm_bo)
>  {
>  	struct drm_gem_object *obj = va->gem.obj;
>  
> @@ -829,7 +989,10 @@ drm_gpuva_link(struct drm_gpuva *va)
>  
>  	drm_gem_gpuva_assert_lock_held(obj);
>  
> -	list_add_tail(&va->gem.entry, &obj->gpuva.list);
> +	drm_gpuvm_bo_get(vm_bo);

Guess we should WARN if vm_obj->obj == obj, at least.

> +	list_add_tail(&va->gem.entry, &vm_bo->list.gpuva);
> +	if (list_empty(&vm_bo->list.entry.gem))
> +		list_add_tail(&vm_bo->list.entry.gem, &obj->gpuva.list);
>  }
>  EXPORT_SYMBOL_GPL(drm_gpuva_link);
>  
> @@ -840,20 +1003,40 @@ EXPORT_SYMBOL_GPL(drm_gpuva_link);
>   * This removes the given &va from the GPU VA list of the &drm_gem_object it is
>   * associated with.
>   *
> + * This removes the given &va from the GPU VA list of the &drm_gpuvm_bo and
> + * the &drm_gpuvm_bo from the &drm_gem_object it is associated with in case
> + * this call unlinks the last &drm_gpuva from the &drm_gpuvm_bo.
> + *
> + * For every &drm_gpuva entry removed from the &drm_gpuvm_bo a reference of
> + * the latter is dropped.
> + *
>   * This function expects the caller to protect the GEM's GPUVA list against
> - * concurrent access using the GEMs dma_resv lock.
> + * concurrent access using either the GEMs dma_resv lock or a driver specific
> + * lock set through drm_gem_gpuva_set_lock().
>   */
>  void
>  drm_gpuva_unlink(struct drm_gpuva *va)
>  {
>  	struct drm_gem_object *obj = va->gem.obj;
> +	struct drm_gpuvm_bo *vm_bo;
>  
>  	if (unlikely(!obj))
>  		return;
>  
>  	drm_gem_gpuva_assert_lock_held(obj);
>  
> +	vm_bo = __drm_gpuvm_bo_find(va->vm, obj);

Could we add a drm_gpuva::vm_bo field so we don't have to search the
vm_bo here, and maybe drop the drm_gpuva::vm and drm_gpuva::obj fields,
since drm_gpuvm_bo contains both the vm and the GEM object. I know that
means adding an extra indirection + allocation for drivers that don't
want to use drm_gpuva_[un]link(), but I wonder if it's not preferable
over having the information duplicated (with potential mismatch)

> +	if (WARN(!vm_bo, "GPUVA doesn't seem to be linked.\n"))
> +		return;
> +
>  	list_del_init(&va->gem.entry);
> +
> +	/* This is the last mapping being unlinked for this GEM object, hence
> +	 * also remove the VM_BO from the GEM's gpuva list.
> +	 */
> +	if (list_empty(&vm_bo->list.gpuva))
> +		list_del_init(&vm_bo->list.entry.gem);
> +	drm_gpuvm_bo_put(vm_bo);
>  }
>  EXPORT_SYMBOL_GPL(drm_gpuva_unlink);