[PATCH 1/2] drm/tidss: Fix initial plane zpos values

[Tomi Valkeinen]

Hi,

On 13/02/2024 11:04, Pekka Paalanen wrote:
> On Tue, 13 Feb 2024 10:16:36 +0200
> Tomi Valkeinen <tomi.valkeinen at ideasonboard.com> wrote:
> 
>> When the driver sets up the zpos property it sets the default zpos value
>> to the HW id of the plane. That is fine as such, but as on many DSS
>> versions the driver arranges the DRM planes in a different order than
>> the HW planes (to keep the non-scalable planes first), this leads to odd
>> initial zpos values. An example is J721e, where the initial zpos values
>> for DRM planes are 1, 3, 0, 2.
>>
>> In theory the userspace should configure the zpos values properly when
>> using multiple planes, and in that sense the initial zpos values
>> shouldn't matter, but there's really no reason not to fix this and help
>> the userspace apps which don't handle zpos perfectly. In particular,
>> Weston seems to have issues dealing with the planes with the current
>> default zpos values.
>>
>> So let's change the zpos values for the DRM planes to 0, 1, 2, 3.
>>
>> Another option would be to configure the planes marked as primary planes
>> to zpos 0. On a two display system this would give us plane zpos values
>> of 0, 0, 1, 2. The end result and behavior would be very similar in this
>> option, and I'm not aware that this would actually help us in any way.
>> So, to keep the code simple, I opted for the 0, 1, 2, 3 values.
>>
>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen at ideasonboard.com>
>> Fixes: 32a1795f57ee ("drm/tidss: New driver for TI Keystone platform Display SubSystem")
> 
> Hi Tomi,
> 
> have you reported this to Weston? What exactly is the problem?

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).

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.

> It doesn't seem like a good idea to work around userspace bugs
> (non-regression, I presume?) with kernel changes.

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.

I agree with you, this patch shouldn't be merged to "fix" issues with 
tidss + Weston. However, the current default zpos values really don't 
make sense, so I think this patch can stand on its own, and should be 
merged just to make the tidss behavior a bit saner.

But even if this patch merged, the issue with Weston should be looked at 
(*poke* Randolph =).

  Tomi

> 
> 
> Thanks,
> pq
> 
>> ---
>>   drivers/gpu/drm/tidss/tidss_plane.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/tidss/tidss_plane.c b/drivers/gpu/drm/tidss/tidss_plane.c
>> index e1c0ef0c3894..68fed531f6a7 100644
>> --- a/drivers/gpu/drm/tidss/tidss_plane.c
>> +++ b/drivers/gpu/drm/tidss/tidss_plane.c
>> @@ -213,7 +213,7 @@ struct tidss_plane *tidss_plane_create(struct tidss_device *tidss,
>>   
>>   	drm_plane_helper_add(&tplane->plane, &tidss_plane_helper_funcs);
>>   
>> -	drm_plane_create_zpos_property(&tplane->plane, hw_plane_id, 0,
>> +	drm_plane_create_zpos_property(&tplane->plane, tidss->num_planes, 0,
>>   				       num_planes - 1);
>>   
>>   	ret = drm_plane_create_color_properties(&tplane->plane,
>>
>