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

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Date:   Mon, 26 Mar 2018 23:01:53 +0800
From:   Chen-Yu Tsai <wens@...e.org>
To:     Maxime Ripard <maxime.ripard@...tlin.com>
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 10:45 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.

ChenYu