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

From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Peter Ujfalusi <peter.ujfalusi@ti.com>,
	airlied@linux.ie, dri-devel@lists.freedesktop.org,
	laurent.pinchart@ideasonboard.com, jsarha@ti.com
Subject: Re: [PATCH 1/2] drm/blend: Account also the primary plane of the crtc for normalized_zpos
Date: Thu, 21 Dec 2017 16:20:15 +0200	[thread overview]
Message-ID: <20171221142015.GL10981@intel.com> (raw)
In-Reply-To: <7a4284cd-5408-9977-0a9b-1a26f2bb056a@ti.com>

On Thu, Dec 21, 2017 at 03:44:56PM +0200, Tomi Valkeinen wrote:
> On 21/12/17 14:55, Ville Syrjälä wrote:
> > On Thu, Dec 21, 2017 at 02:11:00PM +0200, Peter Ujfalusi wrote:
> >> Make sure that the primary plane will get normalized_zpos=0 if it's zpos is
> >> set to 0, avoiding other planes to be placed in the background.
> > 
> > Can you describe the actual "bad" configuration?
> > 
> > Without knowing the details this looks like it's making things
> > more complicated for no particularly good reason. If you're worried
> > about clients that don't set zpos, then I think you should just
> > assign the default zpos values better and/or allocate the planes
> > in a better order. But it's definitely possible I'm missing the
> > reason why you're doing this.
> 
> Let's say we have two displays and two planes. First display will use 
> crtc0 and plane0 as primary plane, the second display crtc1, plane1. The 
> zpos of primary planes will be set to 0 by default.
> 
> The userspace wants to use the second display, with an overlay plane. So 
> it takes the plane0 and moves it to crtc1. Now the second display has 
> two planes with zpos 0, and normalize_zpos will fix it according to the 
> plane id, i.e. the primary plane of the second display will be on top of 
> the "overlay" plane.
> 
> I don't think other default zpos values and/or allocating planes in 
> better order can solve this.

Hmm. True. OTOH this messes up the simple "plane id is used to resolve
zpos conflicts" logic.

Also since you have multiple primary planes on the same crtc, which
primary plane is the "real primary" for the crtc? How would userspace
even determine that? I guess the rule could be that the primary planes
have to be registered in the same order as the crtcs?

> 
> If the userspace is required to understand and set zpos, then this patch 
> is not needed. But at least in my test scripts I've hit this a few times =).

I think it would be nice if we can just make it a rule that any
userspace that moves planes between crtcs has to know about zpos.
Otherwise it's pretty much pure luck what stacking order you would
get.

And my idea for planes that can move between crtcs is that the default
zpos should be assigned globally, probably matching the plane id
order as well. Ie. no two planes would default to the same zpos
value. And only in case of hardware that has no planes that can
move between crtcs you would allocate the default zpos per-crtc.

-- 
Ville Syrjälä
Intel OTC
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