[PATCH 14/25] drm/amdkfd: Populate DRM render device minor

[Christian König]

Am 14.02.2018 um 17:35 schrieb Felix Kuehling:
> [SNIP]
>>>>>> I don't see how you can have separate TTM objects referring to the
>>>>>> same memory.
>>>>> Well that is trivial, we do this all the time with prime and I+A
>>>>> laptops.
>>>> As I understand it, you use DMABuf to export/import buffers on multiple
>>>> devices. I believe all devices share a single amdgpu_bo, which contains
>>>> the ttm_buffer_object.
>> That's incorrect as well. Normally multiple devices have multiple
>> ttm_buffer_object, one for each device.
>> Going a bit higher that actually makes sense because the status of
>> each BO is deferent for each device. E.g. one device could have the BO
>> in access while it could be idle on another device.
> Can you point me where this is done? I'm looking at
> amdgpu_gem_prime_foreign_bo. It is used if an AMDGPU BO is imported into
> a different AMDGPU device. It creates a new GEM object, with a reference
> to the same amdgpu BO (gobj->bo = amdgpu_bo_ref(bo)). To me this looks
> very much like the same amdgpu_bo, and cosequently the same TTM BO being
> shared by two GEM objects and two devices.

As Michel pointed out as well that stuff isn't upstream and judging from 
the recent requirements it will never go upstream.

>> If this is enabled by any changes that break existing buffer sharing for
>> A+A or A+I systems, please point it out to me. I'm not aware that this
>> patch series does anything to that effect.
>> As I said it completely breaks scanout with A+I systems.
> Please tell me what "it" is. What in the changes I have posted is
> breaking A+I systems. I don't see it.

Using the same amdgpu_bo structure with multiple devices is what "it" 
means here.

As I said that concept is incompatible with the requirements on A+A 
systems, so we need to find another solution to provide the functionality.

What's on my TODO list anyway is to extend DMA-buf to not require 
pinning and to be able to deal with P2P.

The former is actually rather easy and already mostly done by sharing 
the reservation object between exporter and importer.

The later is a bit more tricky because I need to create the necessary 
P2P infrastructure, but even that is doable in the mid term.

Regards,
Christian.