Question about implicit fence sync in display DRM drivers

[Daniel Vetter]

On Sun, Nov 18, 2018 at 08:44:57PM +0800, Qiang Yu wrote:
>  add ".prepare_fb = drm_gem_fb_prepare_fb," in "drm_plane_helper_funcs"
> solve my flicker problem on Allwinner A64.

Yeah, some drivers fail to set this. Would be great if you could submit
this bugfix.

Thanks, Daniel

> 
> Thanks,
> Qiang
> On Sun, Nov 18, 2018 at 8:14 PM Qiang Yu <yuq825 at gmail.com> wrote:
> >
> > I find the reservation object will be created when: dma_buf_export
> > if no reservation object created when buffer object creation.
> >
> > And the implicit fence can be synced before display in: drm_gem_fb_prepare_fb
> > but seems most display driver doesn't call this function.
> >
> > Regards,
> > Qiang
> > On Sun, Nov 18, 2018 at 10:43 AM Qiang Yu <yuq825 at gmail.com> wrote:
> > >
> > > 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
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch