[PATCH 1/2] drm/amdgpu: Enable scatter gather display support

[Marek Olšák]

On Wed, Mar 21, 2018 at 2:15 PM, Christian König <christian.koenig at amd.com>
wrote:

> Am 21.03.2018 um 19:04 schrieb Marek Olšák:
>
> On Wed, Mar 21, 2018 at 10:07 AM, Christian König <
> christian.koenig at amd.com> wrote:
>
>> Am 21.03.2018 um 14:57 schrieb Marek Olšák:
>>
>> On Wed, Mar 21, 2018 at 4:13 AM, Christian König <
>> ckoenig.leichtzumerken at gmail.com> wrote:
>>
>>> Am 21.03.2018 um 06:08 schrieb Marek Olšák:
>>>
>>> On Tue, Mar 20, 2018 at 4:16 PM, Christian König <
>>> christian.koenig at amd.com> wrote:
>>>
>>>> That's what I meant with use up the otherwise unused VRAM. I don't see
>>>> any disadvantage of always setting GTT as second domain on APUs.
>>>>
>>>> My assumption was that we dropped this in userspace for displayable
>>>> surfaces, but Marek proved that wrong.
>>>>
>>>> So what we should do is actually to add GTT as fallback to all BOs on
>>>> APUs in Mesa and only make sure that the kernel is capable of handling GTT
>>>> with optimal performance (e.g. have user huge pages etc..).
>>>>
>>>
>>> VRAM|GTT is practically as good as GTT. VRAM with BO priorities and
>>> eviction throttling is the true "VRAM|GTT".
>>>
>>> I don't know how else to make use of VRAM intelligently.
>>>
>>>
>>> Well why not set VRAM|GTT as default on APUs? That should still save
>>> quite a bunch of moves even with throttling.
>>>
>>
>> I explained why: VRAM|GTT is practically as good as GTT.
>>
>>
>>>
>>> I mean there really shouldn't be any advantage to use VRAM any more
>>> except that we want to use it up as long as it is available.
>>>
>>
>> Why are you suggesting to use VRAM|GTT then? Let's just only use GTT on
>> all APUs.
>>
>>
>> Then we don't use the memory stolen for VRAM.
>>
>> See we want to get to a point where we have any ~16MB of stolen VRAM on
>> APUs and everything else in GTT.
>>
>> But we still have to support cases where we have 1GB stolen VRAM, and
>> wasting those 1GB would suck a bit.
>>
>
> BO priorities and BO move throttling should take care of optimal VRAM
> usage regardless of the VRAM size. We can adjust the throttling for small
> VRAM, but that's about all we can do.
>
>
> Well at least on APUs move throttling is complete nonsense. VRAM should
> expose the same performance as GTT.
>
> So the only usage we have for VRAM is for special cases like page tables
> and to allow to actually use the stolen memory.
>
> VRAM|GTT doesn't guarantee that VRAM will be used usefully. In fact, it
> doesn't guarantee anything about VRAM.
>
>
> Why not? VRAM|GTT means that we should use VRAM as long as it is available
> and if it is used up fallback to GTT.
>
> When BOs are evicted from VRAM they are never moved back into it. As far
> as I can see that is exactly what we need on APUs.
>

I see. You don't want to use VRAM usefully. You just want to fill it up
with something (anything) so that it's not unused.

Marek
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