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: <2a473b06-69c1-f620-7595-b119c639669b@redhat.com>
Date:   Thu, 7 Sep 2023 11:11:39 +0200
From:   Danilo Krummrich <dakr@...hat.com>
To:     Boris Brezillon <boris.brezillon@...labora.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 9/7/23 10:16, Boris Brezillon wrote:
> 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)

I was considering that and I think we can add a drm_gpuva::vm_bo field and
get rid of drm_gpuva::obj. However, I think we need to keep drm_gpuva::vm,
since it is valid for ::obj to be NULL, hence it must be valid for ::vm_bo too.
Null objects are used for sparse mappings / userptr.

> 
>> +	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);
>