[PATCH v3 7/8] dt-bindings: display: allwinner: Split H616 DE33 layer reg space

[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.

This change is intentionally not backward compatible and the old binding
will not be supported by the driver, for the following reasons:

- It can't describe the hardware. Plane register space is shared
  between all mixers, but the old binding puts it in the mixer node, so
  only one mixer can ever claim it. The second mixer can't be supported
  with the old binding at all. It also has no way to reference plane to
  mixer mapping registers, which live in display clocks register space.

- Supporting it would mean keeping a second, mixer-local plane
  management path in the driver, with hardcoded plane mapping, which
  doesn't match the hardware and which can't be tested, since nothing
  uses it.

- It was never used. Upstream H616 DT has no Display Engine nodes and
  the rest of H616 display pipeline (TCON, HDMI, PHY) is not supported
  upstream yet, so the old binding never produced a working display.
  Vendor BSP kernels use a completely different DT structure and no
  other known DT consumer (U-Boot, *BSD, Zephyr, TF-A, ...) implements
  H616 display support. Only out of tree WIP patches used it and those
  are updated together with the kernel.

The driver will print an error, asking to update the device tree, when
the old binding is detected.

Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
---
 .../display/allwinner,sun8i-a83t-de2-mixer.yaml  | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

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
-- 
2.43.0