Question about implicit fence sync in display DRM drivers

[Qiang Yu]

Hi Eric and Wladimir,

GPU driver like VC4, Etnaviv and Lima only do rendering and left display
to other DRM display drivers. So there should be a render/display sync
problem for these drivers.

As I know, there are two ways: implicit and explicit described here:
https://www.collabora.com/news-and-blog/blog/2016/09/13/mainline-explicit-fencing-part-1/

The implicit way need the display DRM driver wait on the reservation object
associated with that framebuffer before display it, the reservation
object contains
fence which is added by render DRM driver to indicate render finish:
https://www.phoronix.com/scan.php?page=news_item&px=AMDGPU-Intel-4.9-PRIME-Ex-Patch

I investigate a bit into these DRM display drivers using CMA GEM helpers,
seems there's no reservation object at all, so it should be impossible to
support implicit sync.

Although explicit way is better, there are still some apps not support it like
kodi-gbm. So how do VC4+PL111 and Etnaviv+IMX solve this sync problem,
and do these drivers support implicit sync?

Thanks,
Qiang