amdgpu doesn't do implicit sync, requires drivers to do it in IBs

[Christian König]

Am 28.05.20 um 12:06 schrieb Michel Dänzer:
> On 2020-05-28 11:11 a.m., Christian König wrote:
>> Well we still need implicit sync [...]
> Yeah, this isn't about "we don't want implicit sync", it's about "amdgpu
> doesn't ensure later jobs fully see the effects of previous implicitly
> synced jobs", requiring userspace to do pessimistic flushing.

Yes, exactly that.

For the background: We also do this flushing for explicit syncs. And 
when this was implemented 2-3 years ago we first did the flushing for 
implicit sync as well.

That was immediately reverted and then implemented differently because 
it caused severe performance problems in some use cases.

I'm not sure of the root cause of this performance problems. My 
assumption was always that we then insert to many pipeline syncs, but 
Marek doesn't seem to think it could be that.

On the one hand I'm rather keen to remove the extra handling and just 
always use the explicit handling for everything because it simplifies 
the kernel code quite a bit. On the other hand I don't want to run into 
this performance problem again.

Additional to that what the kernel does is a "full" pipeline sync, e.g. 
we busy wait for the full hardware pipeline to drain. That might be 
overkill if you just want to do some flushing so that the next shader 
sees the stuff written, but I'm not an expert on that.

Regards,
Christian.