diff mbox series

[v2,7/8] dt-bindings: display: allwinner: Split H616 DE33 layer reg space

Message ID 20260509190015.79086-8-jernej.skrabec@siol.net (mailing list archive)
State New
Headers
Series drm/sun4i: update DE33 support |

Commit Message

Jernej Škrabec May 9, 2026, 7 p.m. UTC 
From: Jernej Skrabec <jernej.skrabec@gmail.com>

As it turns out, current H616 DE33 binding was written based on
incomplete understanding of DE33 design. Namely, planes are shared
resource and not tied to specific mixer, which was the case for previous
generations of Display Engine (DE3 and earlier).

This means that current DE33 binding doesn't properly reflect HW and
using it would mean that second mixer (used for second display output)
can't be supported.

Remove layer register space, which will be represented with additional
node, and replace it with phandle, which will point to that new, shared
node. That way, all mixers can share same layers.

There is no user of this binding yet, so changes can be made safely,
without breaking any backward compatibility.

Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
---
Changes in v1:
- update commit subject
- reword commit message

 .../display/allwinner,sun8i-a83t-de2-mixer.yaml  | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

Comments

Krzysztof Kozlowski May 14, 2026, 12:04 p.m. UTC | #1 
On Sat, May 09, 2026 at 09:00:14PM +0200, Jernej Skrabec wrote:
> From: Jernej Skrabec <jernej.skrabec@gmail.com>
> 
> As it turns out, current H616 DE33 binding was written based on
> incomplete understanding of DE33 design. Namely, planes are shared
> resource and not tied to specific mixer, which was the case for previous
> generations of Display Engine (DE3 and earlier).
> 
> This means that current DE33 binding doesn't properly reflect HW and
> using it would mean that second mixer (used for second display output)
> can't be supported.
> 
> Remove layer register space, which will be represented with additional
> node, and replace it with phandle, which will point to that new, shared
> node. That way, all mixers can share same layers.
> 
> There is no user of this binding yet, so changes can be made safely,
> without breaking any backward compatibility.

There is user. git grep gives me:
drivers/gpu/drm/sun4i/sun8i_mixer.c

which means this is a released ABI. As I understood, the old code was
working fine but just did not support all use cases. Why this cannot be
kept backwards compatible?

Best regards,
Krzysztof
Chen-Yu Tsai May 24, 2026, 9:20 p.m. UTC | #2 
Hi,

On Thu, May 14, 2026 at 2:04 PM Krzysztof Kozlowski <krzk@kernel.org> wrote:
>
> On Sat, May 09, 2026 at 09:00:14PM +0200, Jernej Skrabec wrote:
> > From: Jernej Skrabec <jernej.skrabec@gmail.com>
> >
> > As it turns out, current H616 DE33 binding was written based on
> > incomplete understanding of DE33 design. Namely, planes are shared
> > resource and not tied to specific mixer, which was the case for previous
> > generations of Display Engine (DE3 and earlier).
> >
> > This means that current DE33 binding doesn't properly reflect HW and
> > using it would mean that second mixer (used for second display output)
> > can't be supported.
> >
> > Remove layer register space, which will be represented with additional
> > node, and replace it with phandle, which will point to that new, shared
> > node. That way, all mixers can share same layers.
> >
> > There is no user of this binding yet, so changes can be made safely,
> > without breaking any backward compatibility.
>
> There is user. git grep gives me:
> drivers/gpu/drm/sun4i/sun8i_mixer.c
>
> which means this is a released ABI. As I understood, the old code was

We held off on merging the DT changes so that we could rework this.
I can't find the actual request though. It was probably over IRC.

> working fine but just did not support all use cases. Why this cannot be
> kept backwards compatible?

AFAIK the "planes" block is shared between two display mixers. As the
commit message explains, this prevents using the second mixer, since
only one of them can claim and map the register space. And on the H700
(which is the same die as the H616 discussed here but with more exposed
interfaces), there could actually be a use case for the second mixer.

Hope that explains things.


ChenYu
Chen-Yu Tsai May 24, 2026, 9:33 p.m. UTC | #3 
Hi,

(resent from new email)

On Thu, May 14, 2026 at 2:04 PM Krzysztof Kozlowski <krzk@kernel.org> wrote:
>
> On Sat, May 09, 2026 at 09:00:14PM +0200, Jernej Skrabec wrote:
> > From: Jernej Skrabec <jernej.skrabec@gmail.com>
> >
> > As it turns out, current H616 DE33 binding was written based on
> > incomplete understanding of DE33 design. Namely, planes are shared
> > resource and not tied to specific mixer, which was the case for previous
> > generations of Display Engine (DE3 and earlier).
> >
> > This means that current DE33 binding doesn't properly reflect HW and
> > using it would mean that second mixer (used for second display output)
> > can't be supported.
> >
> > Remove layer register space, which will be represented with additional
> > node, and replace it with phandle, which will point to that new, shared
> > node. That way, all mixers can share same layers.
> >
> > There is no user of this binding yet, so changes can be made safely,
> > without breaking any backward compatibility.
>
> There is user. git grep gives me:
> drivers/gpu/drm/sun4i/sun8i_mixer.c
>
> which means this is a released ABI. As I understood, the old code was

We held off on merging the DT changes so that we could rework this.
I can't find the actual request though. It was probably over IRC.

> working fine but just did not support all use cases. Why this cannot be
> kept backwards compatible?

AFAIK the "planes" block is shared between two display mixers. As the
commit message explains, this prevents using the second mixer, since
only one of them can claim and map the register space. And on the H700
(which is the same die as the H616 discussed here but with more exposed
interfaces), there could actually be a use case for the second mixer.

Hope that explains things.


ChenYu
Krzysztof Kozlowski May 25, 2026, 12:10 p.m. UTC | #4 
On 24/05/2026 23:33, Chen-Yu Tsai wrote:
> Hi,
> 
> (resent from new email)
> 
> On Thu, May 14, 2026 at 2:04 PM Krzysztof Kozlowski <krzk@kernel.org> wrote:
>>
>> On Sat, May 09, 2026 at 09:00:14PM +0200, Jernej Skrabec wrote:
>>> From: Jernej Skrabec <jernej.skrabec@gmail.com>
>>>
>>> As it turns out, current H616 DE33 binding was written based on
>>> incomplete understanding of DE33 design. Namely, planes are shared
>>> resource and not tied to specific mixer, which was the case for previous
>>> generations of Display Engine (DE3 and earlier).
>>>
>>> This means that current DE33 binding doesn't properly reflect HW and
>>> using it would mean that second mixer (used for second display output)
>>> can't be supported.
>>>
>>> Remove layer register space, which will be represented with additional
>>> node, and replace it with phandle, which will point to that new, shared
>>> node. That way, all mixers can share same layers.
>>>
>>> There is no user of this binding yet, so changes can be made safely,
>>> without breaking any backward compatibility.
>>
>> There is user. git grep gives me:
>> drivers/gpu/drm/sun4i/sun8i_mixer.c
>>
>> which means this is a released ABI. As I understood, the old code was
> 
> We held off on merging the DT changes so that we could rework this.
> I can't find the actual request though. It was probably over IRC.
> 
>> working fine but just did not support all use cases. Why this cannot be
>> kept backwards compatible?
> 
> AFAIK the "planes" block is shared between two display mixers. As the
> commit message explains, this prevents using the second mixer, since
> only one of them can claim and map the register space. And on the H700
> (which is the same die as the H616 discussed here but with more exposed
> interfaces), there could actually be a use case for the second mixer.

It explains why you want to make the changes but not why you cannot keep
it backwards compatible.


Best regards,
Krzysztof
diff mbox series

Patch

diff --git a/Documentation/devicetree/bindings/display/allwinner,sun8i-a83t-de2-mixer.yaml b/Documentation/devicetree/bindings/display/allwinner,sun8i-a83t-de2-mixer.yaml
index cbd18fd83e52..064e4ca7e419 100644
--- a/Documentation/devicetree/bindings/display/allwinner,sun8i-a83t-de2-mixer.yaml
+++ b/Documentation/devicetree/bindings/display/allwinner,sun8i-a83t-de2-mixer.yaml
@@ -46,6 +46,10 @@  properties:
   resets:
     maxItems: 1
 
+  allwinner,planes:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description: Phandle of Display Engine 3.3 planes node
+
   ports:
     $ref: /schemas/graph.yaml#/properties/ports
 
@@ -74,22 +78,22 @@  allOf:
       properties:
         reg:
           description: |
-            Registers for controlling individual layers of the display
-            engine (layers), global control (top), and display blending
-            control (display). Names are from Allwinner BSP kernel.
-          maxItems: 3
+            Registers for display blending control (display) and global
+            control (top). Names are from Allwinner BSP kernel.
+          maxItems: 2
         reg-names:
           items:
-            - const: layers
-            - const: top
             - const: display
+            - const: top
       required:
         - reg-names
+        - allwinner,planes
 
     else:
       properties:
         reg:
           maxItems: 1
+        allwinner,planes: false
 
 required:
   - compatible