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.
Tomi
--
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