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

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Date:   Tue, 27 Mar 2018 09:07:43 +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 11:01:53PM +0800, Chen-Yu Tsai wrote:
> On Mon, Mar 26, 2018 at 10:45 PM, Maxime Ripard
> <maxime.ripard@...tlin.com> wrote:
> > 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.
> 
> Looks like it's 32x32 pixels with an 8-bit (max) palette, with full RGBA
> for the colors in the palette. I don't see the 1 bit alpha you mentioned.
> Looks like this needs some extra work for building the palette and copying
> the cursor image.

Indeed, you're right. I'm still not sure how it could be turned into
something useful though.

Maxime

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

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