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

[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.

VRAM|GTT doesn't guarantee that VRAM will be used usefully. In fact, it
doesn't guarantee anything about VRAM.

Marek
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