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

[Michel Dänzer]

On 2018-03-06 07:23 PM, Christian König wrote:
> Am 06.03.2018 um 19:15 schrieb Michel Dänzer:
>> On 2018-03-06 07:02 PM, Christian König wrote:
>>> Am 06.03.2018 um 18:51 schrieb Michel Dänzer:
>>>> On 2018-03-06 06:44 PM, Christian König wrote:
>>>>> Am 06.03.2018 um 18:22 schrieb Li, Samuel:
>>>>>>> addition to that I agree with Michel that the module parameter is
>>>>>>> overkill.
>>>>>> That I already explained. Currently SG display feature needs to
>>>>>> provide options for all kinds of use cases. All amd drivers so far
>>>>>> provides options, and the default configuration is actually to
>>>>>> allocate everything from GTT when possible.
>>>>> That isn't job of the kernel to have this parameter. Saying that we
>>>>> should probably add a DDX and/or environment option to control that.
>>>> Why would we need that? It's the kernel driver's job to handle this as
>>>> needed.
>>> Buffer placement is specified by the DDX/Mesa, when we override that in
>>> the kernel we just force unnecessary buffer moves.
>> Userspace just needs a way to know which domains can/should be used for
>> scanout buffers, e.g. via an info ioctl.
> 
> Yeah, but why shouldn't they then make the decision where to place it as
> well?
> 
> See when we enforce a certain placement in the kernel we will just get
> an unnecessary buffer move with old user space components.
> 
> In other words the kernel just needs to advertise that the hw/sw
> combination allows scanout from GTT as well and then an updated
> userspace can actually make use of that placement or decide to not use
> it in certain situations.

I think we'll need to always pin BOs to GTT for scanout in some cases,
because some hardware has issues when transitioning between scanout from
VRAM and GTT.

How about:

Userspace can query from the kernel which domain(s) can be used for
scanout: only VRAM / VRAM or GTT / only GTT. Userspace sets the allowed
domains correspondingly for scanout BOs.

If you can think of a scenario where this won't work, please
specifically describe it and how you propose addressing it.


> E.g. the last time I tested it placing things into GTT still resulted
> in quite a performance penalty for rendering.

FWIW, I think the penalty is most likely IOMMU related. Last time I
tested, I couldn't measure a big difference with IOMMU disabled.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer