Message-Id: <20230719154450.620410-1-viteosen@gmail.com>
Date:   Wed, 19 Jul 2023 18:44:49 +0300
From:   Viktar Simanenka <viteosen@...il.com>
To:     David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Viktar Simanenka <viteosen@...il.com>,
        dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/2] dt-bindings: add bindings for pcd8544 displays

Signed-off-by: Viktar Simanenka <viteosen@...il.com>

V2: deleted oneOf property from compatible
changed prefix from 'philips' to existing vendor prefix 'nxp'
placed `reg = <0>` right after `compatible`

About parameters exposed for controller:
inverted - I had an unbranded display that after reset had all pixels black,
while register was in 'normal operation'. The display I have now is opposite:
all white after reset in normal mode. This parameter should help user keep
same settings in KMS(?) for diverse displays;
voltage-op & temperature-coeff - adjusts contrast for display. voltage-op
cannot be expressed in real units, because resulting equation looks like
V = a + Vop * b, where a and b varies with ambient temperature. Vop is a
coefficient. same story for temperature-coeff - it shifts resulting voltage
curve depending on display usage invironment;
bias - relates to waveforms for LCD segments. default is 4 and should be
changed only if you use external oscillator for display.

v1 link: https://lore.kernel.org/linux-devicetree/20230719092903.316452-1-viteosen@gmail.com/

---
 .../bindings/display/nxp,pcd8544.yaml         | 94 +++++++++++++++++++
 1 file changed, 94 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/display/nxp,pcd8544.yaml

diff --git a/Documentation/devicetree/bindings/display/nxp,pcd8544.yaml b/Documentation/devicetree/bindings/display/nxp,pcd8544.yaml
new file mode 100644
index 000000000000..52e40fd0eacb
--- /dev/null
+++ b/Documentation/devicetree/bindings/display/nxp,pcd8544.yaml
@@ -0,0 +1,94 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/display/nxp,pcd8544.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Philips Semiconductors PCD8544 LCD Display Controller
+
+maintainers:
+  - Viktar Simanenka <viteosen@...il.com>
+
+description: |
+  Philips Semiconductors PCD8544 LCD Display Controller with SPI control bus.
+  Designed to drive a graphic display of 48 rows and 84 columns,
+  such as Nokia 5110/3310 LCDs.
+
+allOf:
+  - $ref: panel/panel-common.yaml#
+  - $ref: /schemas/spi/spi-peripheral-props.yaml#
+
+properties:
+  compatible:
+    enum:
+      - nxp,pcd8544
+
+  dc-gpios:
+    maxItems: 1
+    description: Data/Command selection pin (D/CX)
+
+  reset-gpios:
+    maxItems: 1
+    description: Display Reset pin (RST)
+
+  nxp,inverted:
+    type: boolean
+    description: Display color inversion
+
+  nxp,voltage-op:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    minimum: 0
+    maximum: 127
+    description: |
+      Liquid crystall voltage operation coefficient. Determines the LCD
+      controlling voltage on the display segments. Should be adjusted
+      depending on the ambient temperature.
+
+  nxp,temperature-coeff:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    minimum: 0
+    maximum: 3
+    description: |
+      Display temperature compensation coefficient. Increases LCD
+      controlling voltage at lower temperatures to maintain optimum
+      contrast.
+
+  nxp,bias:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    minimum: 0
+    maximum: 7
+    description: Display bias system coefficient.
+
+required:
+  - compatible
+  - reg
+  - dc-gpios
+  - reset-gpios
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/gpio/gpio.h>
+
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        display@0 {
+            compatible = "nxp,pcd8544";
+            reg = <0>;
+            spi-max-frequency = <8000000>;
+
+            dc-gpios = <&pio 0 3 GPIO_ACTIVE_HIGH>; /* DC=PA3 */
+            reset-gpios = <&pio 0 1 GPIO_ACTIVE_HIGH>; /* RESET=PA1 */
+            backlight = <&backlight>;
+
+            nxp,inverted;
+            nxp,voltage-op = <0>;
+            nxp,bias = <4>;
+            nxp,temperature-coeff = <0>;
+        };
+    };
+
+...
-- 
2.34.1

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