[PATCH 2/2] drm/amdgpu: Enable clear page functionality

[Christian König]

Am 13.12.23 um 16:39 schrieb Felix Kuehling:
> On 2023-12-13 9:20, Christian König wrote:
>> Am 12.12.23 um 00:32 schrieb Felix Kuehling:
>>>
>>> On 2023-12-11 04:50, Christian König wrote:
>>>> Am 08.12.23 um 20:53 schrieb Alex Deucher:
>>>>> [SNIP]
>>>>>> You also need a functionality which resets all cleared blocks to
>>>>>> uncleared after suspend/resume.
>>>>>>
>>>>>> No idea how to do this, maybe Alex knows of hand.
>>>>> Since the buffers are cleared on creation, is there actually 
>>>>> anything to do?
>>>>
>>>> Well exactly that's the problem, the buffers are no longer always 
>>>> cleared on creation with this patch.
>>>>
>>>> Instead we clear on free, track which areas are cleared and clear 
>>>> only the ones which aren't cleared yet on creation.
>>>
>>> The code I added for clearing-on-free a long time ago, does not 
>>> clear to 0, but to a non-0 poison value. That was meant to make it 
>>> easier to catch applications incorrectly relying on 0-initialized 
>>> memory. Is that being changed? I didn't see it in this patch series.
>>
>> Yeah, Arun stumbled over that as well. Any objections that we fill 
>> with zeros instead or is that poison value something necessary for 
>> debugging?
>
> I don't think it's strictly necessary. But it may encourage sloppy 
> user mode programming to rely on 0-initialized memory that ends up 
> breaking in corner cases or on older kernels.
>
> That said, I see that this patch series adds clearing of memory in the 
> VRAM manager, but it doesn't remove the clearing for 
> AMDGPU_GEM_CREATE_VRAM_WIPE_ON_RELEASE in amdgpu_bo_release_notify and 
> amdgpu_move_blit. This will lead to duplicate work.
>
> I'm also not sure how the clearing added in this patch series will 
> affect free-latency observed in user mode. Will this be synchronous 
> and cause the user mode thread to stall while the memory is being 
> cleared?

Yeah, that's not fully working at the moment. I already pointed out as 
well that Arun should remove the clearing in the VRAM manager.

Regards,
Christian.

>
> Regards,
>   Felix
>
>
>>
>> Regards,
>> Christian.
>>
>>>
>>> Regards,
>>>   Felix
>>>
>>>
>>>>
>>>> So some cases need special handling. E.g. when the engine is not 
>>>> initialized yet or suspend/resume.
>>>>
>>>> In theory after a suspend/resume cycle the VRAM is cleared to 
>>>> zeros, but in practice that's not always true.
>>>>
>>>> Christian.
>>>>
>>>>> Alex
>>