Re: Fence, timeline and android sync points

[Maarten Lankhorst <maarten.lankhorst@canonical.com>]

Op 14-08-14 om 13:53 schreef Christian König:
>> But because of driver differences I can't implement it as a straight wait queue. Some drivers may not have a reliable interrupt, so they need a custom wait function. (qxl)
>> Some may need to do extra flushing to get fences signaled (vmwgfx), others need some locking to protect against gpu lockup races (radeon, i915??).  And nouveau
>> doesn't use wait queues, but rolls its own (nouveau).
> But when all those drivers need a special wait function how can you still justify the common callback when a fence is signaled?
>
> If I understood it right the use case for this was waiting for any fence of a list of fences from multiple drivers, but if each driver needs special handling how for it's wait how can that work reliable?
TTM doesn't rely on the callbacks. It will call .enable_signaling when .is_signaled is NULL, to make sure that fence_is_signaled returns true sooner.

QXL is completely awful, I've seen some patches to add dma-buf support but I'll make sure it never supports importing from/exporting to other devices. This should reduce insanity factor there.
If I understand QXL correctly, sometimes fences may never signal at all due to virt-hw bugs.

nouveau (pre nv84) has no interrupt for completed work, but it has a reliable is_signaled. So .enable_signaling only checks if fence is signaled here.
A custom waiting function makes sure things work correctly, and also signals all unsignaled fences for that context. I preserved the original wait from before the fence conversion.
Nouveau keeps a global list of unsignaled fences, so they will all signal eventually.
I may have to limit importing/exporting dma-buf's to other devices, or add delayed work that periodically checks all contexts for completed fences for this to work cross-device.

nv84+ has a sane interrupt, so I use it. :-)

Radeon with fence conversion has the delayed work for handling lockups that also checks.

~Maarten