[PATCH 1/3] drm/i915: audit bo->resource usage

[Matthew Auld]

On 31/08/2022 13:35, Christian König wrote:
> Am 31.08.22 um 14:06 schrieb Matthew Auld:
>> On 31/08/2022 12:03, Christian König wrote:
>>> Am 31.08.22 um 12:37 schrieb Matthew Auld:
>>>> [SNIP]
>>>>>>
>>>>>> That hopefully just leaves i915_ttm_shrink(), which is swapping 
>>>>>> out shmem ttm_tt and is calling ttm_bo_validate() with empty 
>>>>>> placements to force the pipeline-gutting path, which importantly 
>>>>>> unpopulates the ttm_tt for us (since ttm_tt_unpopulate is not 
>>>>>> exported it seems). But AFAICT it looks like that will now also 
>>>>>> nuke the bo->resource, instead of just leaving it in system 
>>>>>> memory. My assumption is that when later calling 
>>>>>> ttm_bo_validate(), it will just do the bo_move_null() in 
>>>>>> i915_ttm_move(), instead of re-populating the ttm_tt and then 
>>>>>> potentially copying it back to local-memory?
>>>>>
>>>>> Well you do ttm_bo_validate() with something like GTT domain, don't 
>>>>> you? This should result in re-populating the tt object, but I'm not 
>>>>> 100% sure if that really works as expected.
>>>>
>>>> AFAIK for domains we either have system memory (which uses ttm_tt 
>>>> and might be shmem underneath) or local-memory. But perhaps i915 is 
>>>> doing something wrong here, or abusing TTM in some way. I'm not sure 
>>>> tbh.
>>>>
>>>> Anyway, I think we have two cases here:
>>>>
>>>> - We have some system memory only object. After doing 
>>>> i915_ttm_shrink(), bo->resource is now NULL. We then call 
>>>> ttm_bo_validate() at some later point, but here we don't need to 
>>>> copy anything, but it also looks like ttm_bo_handle_move_mem() won't 
>>>> populate the ttm_tt or us either, since mem_type == TTM_PL_SYSTEM. 
>>>> It looks like i915_ttm_move() was taking care of this, but now we 
>>>> just call ttm_bo_move_null().
>>>>
>>>> - We have a local-memory only object, which was evicted to shmem, 
>>>> and then swapped out by the shrinker like above. The bo->resource is 
>>>> NULL. However this time when calling ttm_bo_validate() we need to 
>>>> actually do a copy in i915_ttm_move(), as well as re-populate the 
>>>> ttm_tt. i915_ttm_move() was taking care of this, but now we just 
>>>> call ttm_bo_move_null().
>>>>
>>>> Perhaps i915 is doing something wrong in the above two cases?
>>>
>>> Mhm, as far as I can see that should still work.
>>>
>>> See previously you should got a transition from SYSTEM->GTT in 
>>> i915_ttm_move() to re-create your backing store. Not you get 
>>> NULL->SYSTEM which is handled by ttm_bo_move_null() and then 
>>> SYSTEM->GTT.
>>
>> What is GTT here in TTM world? Also I'm not seeing where there is this 
>> SYSTEM->GTT transition? Maybe I'm blind. Just to be clear, i915 is 
>> only calling ttm_bo_validate() once when acquiring the pages, and we 
>> don't call it again, unless it was evicted (and potentially swapped out).
> 
> Well GTT means TTM_PL_TT.
> 
> And calling it only once is perfectly fine, TTM will internally see that 
> we need two hops to reach TTM_PL_TT and so does the NULL->SYSTEM 
> transition and then SYSTEM->TT.

Ah interesting, so that's what the multi-hop thing does. But AFAICT i915 
is not using either TTM_PL_TT or -EMULTIHOP.

Also what is the difference between TTM_PL_TT and TM_PL_SYSTEM? When 
should you use one over the other?

> 
> As far as I can see that should work like it did before.
> 
> Christian.
> 
>>
>>>
>>> If you just validated to SYSTEM memory before I think the tt object 
>>> wouldn't have been populated either.
>>>
>>> Regards,
>>> Christian.
>>>
>>>>
>>>>>
>>>>> Thanks,
>>>>> Christian.
>>>>>
>>>>>>
>>>>>>>
>>>>>>> I've been considering to replacing the ttm_bo_type with a bunch 
>>>>>>> of behavior flags for a bo. I'm hoping that this will clean 
>>>>>>> things up a bit.
>>>>>>>
>>>>>>> Regards,
>>>>>>> Christian.
>>>>>>>
>>>>>>>>
>>>>>>>>>>       caching = i915_ttm_select_tt_caching(obj);
>>>>>>>>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_ttm_move.c 
>>>>>>>>>> b/drivers/gpu/drm/i915/gem/i915_gem_ttm_move.c
>>>>>>>>>> index 9a7e50534b84bb..c420d1ab605b6f 100644
>>>>>>>>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_ttm_move.c
>>>>>>>>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_ttm_move.c
>>>>>>>>>> @@ -560,7 +560,7 @@ int i915_ttm_move(struct ttm_buffer_object 
>>>>>>>>>> *bo, bool evict,
>>>>>>>>>>       bool clear;
>>>>>>>>>>       int ret;
>>>>>>>>>> -    if (GEM_WARN_ON(!obj)) {
>>>>>>>>>> +    if (GEM_WARN_ON(!obj) || !bo->resource) {
>>>>>>>>>>           ttm_bo_move_null(bo, dst_mem);
>>>>>>>>>>           return 0;
>>>>>>>>>>       }
>>>>>>>>>
>>>>>>>
>>>>>
>>>
>