linux-kernel - Re: [linux-sunxi] Preferring cursor plane over overlay plane

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Date:   Mon, 26 Mar 2018 16:45:07 +0200
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     Joonas Kylmälä <joonas.kylmala@....fi>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        David Airlie <airlied@...ux.ie>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jernej Škrabec <jernej.skrabec@...l.net>,
        linux-sunxi <linux-sunxi@...glegroups.com>
Subject: Re: [linux-sunxi] Preferring cursor plane over overlay plane

On Mon, Mar 26, 2018 at 10:22:45PM +0800, Chen-Yu Tsai wrote:
> On Mon, Mar 26, 2018 at 10:14 PM, Joonas Kylmälä <joonas.kylmala@....fi> wrote:
> > Hi DRM subsystem developers,
> >
> > I ran into this patch where overlay plane was switched to cursor plane
> > because there was no proper cursor plane available on the display
> > hardware: <https://lkml.org/lkml/2018/3/17/120>. Can we discuss whether
> > to have a policy of using a normal plane for cursor plane in case a
> > dedicated HW cursor plane is missing?
> >
> > Daniel Vetter suggests that it might be fine to use normal plane for
> > cursor plane because how to use the plane would be only "a hint to
> > userspace" (see the email linked).
> >
> > My motivation for having this discussion is that the newer Allwinner
> > SoCs don't have dedicated HW cursor plane and the sun4i DRM driver
> > currently uses the extra planes as overlay planes which makes moving the
> > cursor on Xfce4 DE a terrible experience. To have better cursor moving
> > experience one overlay plane would need to be sacrificed.
> 
> If you look at the development history, we've never supported cursor planes.

X can use an overlay to put the cursor though.

> At the beginning we supported one main plane and one overlay plane. That was
> it. The Display Engine 1.0 does have support for an extra hardware cursor,
> but we haven't done the work to support it yet. I don't know about the
> Display Engine 2.0 though.

An issue with supporting the hardware cursor we have is that as far as
I understood, the cursor plane in DRM has the assumption that it would
be an ARGB format. In the first display engine, the format is actually
an 8-bit palette with 1 bit of alpha iirc.

Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com

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