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Date: Fri, 16 Feb 2024 11:00:17 +0200
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Subject: Re: [PATCH 1/2] drm/tidss: Fix initial plane zpos values
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To: Daniel Stone <daniel@fooishbar.org>,
 Marius Vlad <marius.vlad@collabora.com>
Cc: Pekka Paalanen <pekka.paalanen@haloniitty.fi>,
 Jyri Sarha <jyri.sarha@iki.fi>,
 Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
 Maxime Ripard <mripard@kernel.org>, Thomas Zimmermann <tzimmermann@suse.de>,
 David Airlie <airlied@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
 Sam Ravnborg <sam@ravnborg.org>, Devarsh Thakkar <devarsht@ti.com>,
 Aradhya Bhatia <a-bhatia1@ti.com>, Francesco Dolcini <francesco@dolcini.it>,
 Tomi Valkeinen <tomi.valkeinen@ti.com>, dri-devel@lists.freedesktop.org,
 linux-kernel@vger.kernel.org,
 "wayland-devel@lists.freedesktop.org" <wayland-devel@lists.freedesktop.org>,
 Randolph Sapp <rs@ti.com>
References: <20240213-tidss-fixes-v1-0-d709e8dfa505@ideasonboard.com>
 <20240213-tidss-fixes-v1-1-d709e8dfa505@ideasonboard.com>
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From: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
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Hi,

On 13/02/2024 13:39, Daniel Stone wrote:
> Hi,
> 
> On Tue, 13 Feb 2024 at 10:18, Marius Vlad <marius.vlad@collabora.com> wrote:
>> On Tue, Feb 13, 2024 at 11:57:59AM +0200, Tomi Valkeinen wrote:
>>> I haven't. I'm quite unfamiliar with Weston, and Randolph from TI (cc'd) has
>>> been working on the Weston side of things. I also don't know if there's
>>> something TI specific here, as the use case is with non-mainline GPU drivers
>>> and non-mainline Mesa. I should have been a bit clearer in the patch
>>> description, as I didn't mean that upstream Weston has a bug (maybe it has,
>>> maybe it has not).
> 
> Don't worry about it. We've had bugs in the past and I'm sure we'll
> have more. :) Either way, it's definitely better to have the kernel
> expose sensible behaviour rather than weird workarounds, unless
> they've been around for so long that they're basically baked into ABI.

Yeah, that's always a worry. I do hope that no user of tidss expects the 
plane zpos values to be the current funny ones. But we'll probably find 
out when I merge this =).

>>> The issue seen is that when Weston decides to use DRM planes for
>>> composition, the plane zpositions are not configured correctly (or at all?).
>>> Afaics, this leads to e.g. weston showing a window with a DRM "overlay"
>>> plane that is behind the "primary" root plane, so the window is not visible.
>>> And as Weston thinks that the area supposedly covered by the overlay plane
>>> does not need to be rendered on the root plane, there are also artifacts on
>>> that area.
>>>
>>> Also, the Weston I used is a bit older one (10.0.1), as I needed to go back
>>> in my buildroot versions to get all that non-mainline GPU stuff compiled and
>>> working. A more recent Weston may behave differently.
>>
>> Right after Weston 10, we had a few minor changes related to the
>> zpos-sorting list of planes and how we parse the plan list without having
>> a temporary zpos ordered list to pick planes from.
>>
>> And there's another fix for missing out to set out the zpos for scanout
>> to the minimum available - which seems like a good candidate to explain
>> what happens in the issue described above. So if trying Weston again,
>> please try with at least Weston 12, which should have those changes
>> in.
> 
> Specifically, you probably want commits 4cde507be6a1 and 58dde0e0c000.
> I think the window of breakage was small enough that - assuming either
> those commits or an upgrade to Weston 12/13 fixes it - we can just ask
> people to upgrade to a fixed Weston.
> 
>>> Presuming this is not related to any TI specific code, I guess it's a
>>> regression in the sense that at some point Weston added the support to use
>>> planes for composition, so previously with only a single plane per display
>>> there was no issue.
> 
> That point was 12 years ago, so not that novel. ;)

Hmm, so do I understand it right, the plane code from 12 years back 
supposedly works ok, but somewhere around Weston 10 something broke, but 
was fixed with the commits you mention above?

  Tomi