Re: [PATCH] drm/mediatek: Declare Z Position for all planes

[AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>]

Il 18/07/24 13:15, Daniel Stone ha scritto:
> Hi,
> 
> On Thu, 18 Jul 2024 at 09:25, AngeloGioacchino Del Regno
> <angelogioacchino.delregno@collabora.com> wrote:
>> MediaTek SoCs support multiple planes, one of which is the primary
>> and all the others are overlays (and CURSOR is the last overlay).
>>
>> In all currently supported SoCs, the Z order of the overlays can't
>> be changed with any fast muxing action, and can only be changed by
>> swapping the contents of the entire register set of one overlay
>> with the other to internally reorder the layer properties, which
>> is indeed feasible, but probably more expensive than desired.
>>
>> Declare the Z position for all planes with an immutable property
>> at least for now, so that the userspace can take its decisions
>> accordingly.
> 
> Thanks a lot for this fix!
> 
> If I understand your middle paragraph correctly, please don't ever do
> that though. I think what you're suggesting by 'swapping the contents
> of the entire register set' is:
> * plane ID 40 (normally) controls OVL1
> * plane ID 41 controls OVL2
> * userspace configures planes 40 & 41 with a zpos suggesting that 41
> should be below 40
> * the DRM driver 'swaps the contents of the entire register set' by
> making plane 40 control OVL2 instead and plane 41 control OVL1 instead

Yeah, that's exactly what I mean.

> 
> If so - please no. Just declare an immutable zpos, because that
> actually matches the hardware, and then leave userspace to configure
> the planes in a way which makes sense. 

I had a hunch... and that's a perfect confirmation - thank you!

> Looking at the zpos property is
> already required in order to handle overlapping overlay planes, and
> any userspace which supports overlays (including Weston) already looks
> at zpos, handles immutable properties, and will dtrt.
> 
> Anyway, this is:
> Acked-by: Daniel Stone <daniels@collabora.com>

Thumbs up! :-)

Cheers!

> 
> Cheers,
> Daniel