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

[Christian König]

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

The major use case I can currently see are A+A laptop, cause there it 
can avoid the additional buffer move from GTT to VRAM for the APU.

Regards,
Christian.