linux-kernel - Re: [PATCH] drm: DON'T require each CRTC to have a unique primary plane

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Message-ID: <20210330095510.4c6432f7@eldfell>
Date:   Tue, 30 Mar 2021 09:55:10 +0300
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Simon Ser <contact@...rsion.fr>,
        Paul Cercueil <paul@...pouillou.net>
Cc:     Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, od@...c.me,
        Maxime Ripard <maxime@...no.tech>, stable@...r.kernel.org
Subject: Re: [PATCH] drm: DON'T require each CRTC to have a unique primary
 plane

On Mon, 29 Mar 2021 15:36:27 +0000
Simon Ser <contact@...rsion.fr> wrote:

> On Monday, March 29th, 2021 at 5:32 PM, Paul Cercueil <paul@...pouillou.net> wrote:
> 
> > Making the second plane an overlay would break the ABI, which is never
> > something I'm happy to do; but I'd prefer to do it now than later.  
> 
> Yeah, I wonder if some user-space depends on this behavior somehow?
> 
> > I still have concerns about the user-space being "clever" enough to
> > know it can disable the primary plane. Can e.g. wlroots handle that?  
> 
> wlroots will always pick the first primary plane, and will never use
> overlays. The plan is to use libliftoff [1] to make use of overlay
> planes. libliftoff should already support the scenario you describe.
> 
> I think Weston supports that too.

Weston supports overlays, but I don't think it will try without "the"
primary plane, IIRC. I'd need to verify.

I'm not quite sure what Weston would do with multiple primary planes.
It probably picks one for a CRTC ahead of time, and then sticks to it,
always using it.

But if Weston never worked with a driver to begin with, it also can't
regress, so you're safe.

Thanks,
pq

> 
> [1]: https://github.com/emersion/libliftoff
> _______________________________________________
> dri-devel mailing list
> dri-devel@...ts.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

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