Re: [RFC v2 5/8] drm/fence: add in-fences support

[Gustavo Padovan <gustavo@padovan.org>]

2016-04-27 Daniel Stone <daniel@fooishbar.org>:

> Hi,
> 
> On 26 April 2016 at 21:48, Greg Hackmann <ghackmann@google.com> wrote:
> > On 04/26/2016 01:05 PM, Daniel Vetter wrote:
> >> On Tue, Apr 26, 2016 at 09:55:06PM +0300, Ville Syrjälä wrote:
> >>> What are they doing that can't stuff the fences into an array
> >>> instead of props?
> >>
> >> The hw composer interface is one in-fence per plane. That's really the
> >> major reason why the kernel interface is built to match. And I really
> >> don't think we should diverge just because we have a slight different
> >> color preference ;-)
> >
> > The relationship between layers and fences is only fuzzy and indirect
> > though.  The relationship is really between the buffer you're displaying on
> > that layer, and the fence representing the work done to render into that
> > buffer.  SurfaceFlinger just happens to bundle them together inside the same
> > struct hwc_layer_1 as an API convenience.
> 
> Right, and when using implicit fencing, this comes as a plane
> property, by virtue of plane -> fb -> buffer -> fence.
> 
> > Which is kind of splitting hairs as long as you have a 1-to-1 relationship
> > between layers and DRM planes.  But that's not always the case.
> 
> Can you please elaborate?
> 
> > A (per-CRTC?) array of fences would be more flexible.  And even in the cases
> > where you could make a 1-to-1 mapping between planes and fences, it's not
> > that much more work for userspace to assemble those fences into an array
> > anyway.
> 
> As Ville says, I don't want to go down the path of scheduling CRTC
> updates separately, because that breaks MST pretty badly. If you don't
> want your updates to display atomically, then don't schedule them
> atomically ... ? That's the only reason I can see for making fencing
> per-CRTC, rather than just a pile of unassociated fences appended to
> the request. Per-CRTC fences also forces userspace to merge fences
> before submission when using multiple planes per CRTC, which is pretty
> punitive.
> 
> I think having it semantically attached to the plane is a little bit
> nicer for tracing (why was this request delayed? -> a fence -> which
> buffer was that fence for?) at a glance. Also the 'pile of appended
> fences' model is a bit awkward for more generic userspace, which
> creates a libdrm request and builds it (add a plane, try it out, wind
> back) incrementally. Using properties makes that really easy, but
> without properties, we'd have to add separate codepaths - and thus
> separate ABI, which complicates distribution - to libdrm to account
> for a separate plane array which shares a cursor with the properties.
> So for that reason if none other, I'd really prefer not to go down
> that route.

I also agree to have it as FENCE_FD prop on the plane. Summarizing the
arguments on this thread, they are:

 - implicit fences also needs one fence per plane/fb, so it will be good to     
   match with that.                                                             
 - requires userspace to always merge fences                                    
 - can use standard plane properties, making kernel and userspace life easier,  
   an array brings more work to build the atomic request plus extra checkings   
   on the kernel.                                                               
 - do not need to changes to drivers                                            
 - better for tracing, can identify the buffer/fence promptly

	Gustavo