Am 13.02.2018 um 18:18 schrieb Felix Kuehling:
On 2018-02-13 12:06 PM, Christian König wrote:
Ah, yeah that is also a point I wanted to to talk about with you.

The approach of using the same buffer object with multiple amdgpu
devices doesn't work in general.

We need separate TTM object for each BO in each device or otherwise we
break A+A laptops.
I think it broke for VRAM BOs because we enabled P2P on systems that
didn't support it properly. But at least system memory BOs can be shared
quite well between devices and we do it all the time.

Sharing VRAM BOs is one issue, but the problem goes deeper than just that.

Starting with Carizzo we can scanout from system memory to avoid the extra copy on A+A laptops. For this to work we need the BO mapped to GART (and I mean a real VMID0 mapping, not just in the GTT domain). And for this to work in turn we need a TTM object per device and not a global one.

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.

That is also the reason we had to disable this feature again in the
hybrid branches.
What you disabled on hybrid branches was P2P, which only affects
large-BAR systems. Sharing of system memory BOs between devices still
works fine.

No, it doesn't. It completely breaks any scanout on Carizzo, Stoney and Raven. Additional to that we found that it breaks some aspects of the user space interface.

So end result is that we probably need to revert it and find a different solution. I'm already working on this for a couple of weeks now and should have something ready after I'm done with the PASID handling.

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