[PATCH 1/2] drm/blend: Account also the primary plane of the crtc for normalized_zpos

Ville Syrjälä ville.syrjala at linux.intel.com
Fri Dec 22 10:12:52 UTC 2017

On Fri, Dec 22, 2017 at 11:16:47AM +0200, Tomi Valkeinen wrote:
> On 21/12/17 17:12, Ville Syrjälä wrote:
> >> If the userspace knows this, then it knows which primary plane is for
> >> which crtc, right?
> >>
> >> And if it doesn't know this, then the userspace cannot associate any
> >> plane to any crtc (except what it configures itself).
> >>
> >> So... If using legacy modesetting (and non-universal planes), there's no
> >> problem, primary planes are fixed and at low zpos. If using atomic
> >> modesetting and universal planes, there's really no (default)
> >> association between planes and crtcs, and "primary plane" doesn't have
> >> much meaning. Is that correct?
> >>
> >> If so... Then I guess the atomic modesetting client essentially randomly
> >> picks which plane it uses as a "root plane" (if it even needs such), and
> >> which planes as overlay planes? If that's the case, then this patch
> >> doesn't quite fix the issue...
> > 
> > I'm not sure anyone has really thought how userspace would/should assign
> > planes to crtcs. My only idea so far has been the crtc and their
> > preferred primary planes should be registered in the same order. But
> > someone should then also implement that same policy in userspace when
> > it's trying to figure out which plane to use. There are other
> > heuristics it should probably use, like preferring to pick a primary
> > plane for any fullscreen surface.
> > 
> > I guess from functional point of view it shouldn't matter which plane
> > you pick as long as the plane's user visible capabilities are
> > sufficient. But there might be user invisible power saving features and
> > whatnot that only work with specific planes. So from that point of view
> > maybe the order in which the planes get registered is important. Or we
> > could maybe come up with some kind of plane usage hints in the uapi
> > which could help userspace decide?
> I was thinking about this, and... maybe there isn't even any problem (at 
> least for OMAP). So lets say we set the default plane zpos to the plane 
> index, and that the primary planes are reserved first (i.e. have lower 
> zpos than overlay planes).
> We have three different cases:
> Legacy modesetting: No problems, primary plane is always at the bottom. 
> If the userspace uses 2+ overlay planes, it can expect the zpos to be 
> based on the plane id, or it can set the zpos.
> Atomic+Universal, the application uses one primary plane, and zero or 
> more overlay planes: No problems here, the situation is the same as for 
> legacy.
> Atomic+Universal, the application uses more than one primary plane, and 
> zero or more overlay planes: in this case the app _has_ to manage zpos, 
> because using two primary planes in a single screen, and expecting it to 
> work by default, doesn't make sense.
> If the above "rules" are valid, then there's no need for this patch.
> But one question I don't have a good answer is that why would we want to 
> normalize the zpos, instead of returning an error? If the above rules 
> are valid, I think returning an error is better than normalizing and 
> hiding the bad configuration.

IIRC I argued against the normalization, but some people really
wanted it for whatever reason.

Ville Syrjälä
Intel OTC

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