dt-bindings: touchscreen: add touch-overlay property

The touch-overlay encompasses a number of touch areas that define a
clipped touchscreen area and/or buttons with a specific functionality.

A clipped touchscreen area avoids getting events from regions that are
physically hidden by overlay frames.

For touchscreens with printed overlay buttons, sub-nodes with a suitable
key code can be defined to report key events instead of the original
touch events.

Reviewed-by: Jeff LaBundy <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Javier Carrasco <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

Documentation/devicetree/bindings/input/touchscreen/touchscreen.yaml

···
   touchscreen-y-plate-ohms:
     description: Resistance of the Y-plate in Ohms
 
+  touch-overlay:
+    description: |
+      List of nodes defining segments (touch areas) on the touchscreen.
+
+      This object can be used to describe a series of segments to restrict
+      the region within touch events are reported or buttons with a specific
+      functionality.
+
+      This is of special interest if the touchscreen is shipped with a physical
+      overlay on top of it with a frame that hides some part of the original
+      touchscreen area. Printed buttons on that overlay are also a typical
+      use case.
+
+      A new touchscreen area is defined as a sub-node without a key code. If a
+      key code is defined in the sub-node, it will be interpreted as a button.
+
+      The x-origin and y-origin properties of a touchscreen area define the
+      offset of a new origin from where the touchscreen events are referenced.
+      This offset is applied to the events accordingly. The x-size and y-size
+      properties define the size of the touchscreen effective area.
+
+      The following example shows a new touchscreen area with the new origin
+      (0',0') for the touch events generated by the device.
+
+                   Touchscreen (full area)
+         ┌────────────────────────────────────────┐
+         │    ┌───────────────────────────────┐   │
+         │    │                               │   │
+         │    ├ y-size                        │   │
+         │    │                               │   │
+         │    │       touchscreen area        │   │
+         │    │         (no key code)         │   │
+         │    │                               │   │
+         │    │            x-size             │   │
+         │   ┌└──────────────┴────────────────┘   │
+         │(0',0')                                 │
+        ┌└────────────────────────────────────────┘
+      (0,0)
+
+      where (0',0') = (0+x-origin,0+y-origin)
+
+      Sub-nodes with key codes report the touch events on their surface as key
+      events instead.
+
+      The following example shows a touchscreen with a single button on it.
+
+              Touchscreen (full area)
+        ┌───────────────────────────────────┐
+        │                                   │
+        │                                   │
+        │   ┌─────────┐                     │
+        │   │button 0 │                     │
+        │   │KEY_POWER│                     │
+        │   └─────────┘                     │
+        │                                   │
+        │                                   │
+       ┌└───────────────────────────────────┘
+      (0,0)
+
+      Segments defining buttons and clipped toushcreen areas can be combined
+      as shown in the following example.
+      In that case only the events within the touchscreen area are reported
+      as touch events. Events within the button areas report their associated
+      key code. Any events outside the defined areas are ignored.
+
+                  Touchscreen (full area)
+        ┌─────────┬──────────────────────────────┐
+        │         │                              │
+        │         │    ┌───────────────────────┐ │
+        │ button 0│    │                       │ │
+        │KEY_POWER│    │                       │ │
+        │         │    │                       │ │
+        ├─────────┤    │   touchscreen area    │ │
+        │         │    │     (no key code)     │ │
+        │         │    │                       │ │
+        │ button 1│    │                       │ │
+        │ KEY_INFO│   ┌└───────────────────────┘ │
+        │         │(0',0')                       │
+       ┌└─────────┴──────────────────────────────┘
+      (0,0)
+
+    type: object
+
+    patternProperties:
+      '^segment-':
+        type: object
+        description:
+          Each segment is represented as a sub-node.
+        properties:
+          x-origin:
+            description: horizontal origin of the node area
+            $ref: /schemas/types.yaml#/definitions/uint32
+
+          y-origin:
+            description: vertical origin of the node area
+            $ref: /schemas/types.yaml#/definitions/uint32
+
+          x-size:
+            description: horizontal resolution of the node area
+            $ref: /schemas/types.yaml#/definitions/uint32
+
+          y-size:
+            description: vertical resolution of the node area
+            $ref: /schemas/types.yaml#/definitions/uint32
+
+          label:
+            description: descriptive name of the segment
+            $ref: /schemas/types.yaml#/definitions/string
+
+          linux,code: true
+
+        required:
+          - x-origin
+          - y-origin
+          - x-size
+          - y-size
+
+        unevaluatedProperties: false
+
 dependencies:
   touchscreen-size-x: [ touchscreen-size-y ]
   touchscreen-size-y: [ touchscreen-size-x ]