Date:   Sun, 10 Dec 2017 16:10:32 -0600
From:   David Lechner <david@...hnology.com>
To:     dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org
Cc:     David Lechner <david@...hnology.com>,
        Noralf Trønnes <noralf@...nnes.org>,
        limor@...yada.net, Linus Walleij <linus.walleij@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/2] dt-bindings: Add binding for Sitronix ST7735R display panels

This adds a new device tree binding for Sitronix ST7735R display panels,
such as the Adafruit 1.8" TFT.

Signed-off-by: David Lechner <david@...hnology.com>
Acked-by: Rob Herring <robh@...nel.org>
---

v2: changes:
* None, but...

I'm wondering about my choice of compatible here. I chose the name
"sitronix,st7735r-jd-t18003-t01" to mean a Sitronix ST7735R controller
connected to a JD-T18003-T01 LCD screen.

What I am actually using, though, is an Adafruit 1.8" TFT breakout [1]. It
has some additional electronics which just pass through signals to the
controller. The bare display is also available from Adafruit [2].

Limor brought up an interesting point in an off-list discussion. The
same controller can be wired to the same LCD in different ways. This is
evident in the fbftf drivers in staging. There is an "adafruit18" and an
"adafruit18_green" in fbftf_devices.c where apparently, two otherwise
identical displays were wired slightly differently at the factory so that
on one, the on-board GRAM word 0 does not correspond to pixel 0,0 on the
LCD. It requires a special offset to the GRAM starting address in order to
have the image displayed correctly.

Additionally, fbtft supports a SainSmart 1.8" TFT [3] that uses the same
controller, but it appears that these have different gamma curves (perhaps
they use different LCDs?). The available pins are exactly the same as the
Adafruit display though.

As I am writing this, I am looking at the JD-1800 datasheet [4] again for
the Adafruit display and I see that in addition to specifying the size of
the display it says "IC: ST7735B". So, I am starting to think that "jianda,
jd-t18003-t01" would be a better compatible string since it tells you
everything you need to know about this display. Just using the controller
name by itself ("sitronix,st7735r") is not enough because it does not tell
you things like the number of pixels or the physical size of the LCD.

What I am not sure about, though, is how to represent the Adafruit display
that requires the offset in the device tree. Would it be a different
compatible string or should we add an extra property to indicate this
quirk?

On a related note, I recently submitted device tree bindings for a similar
SPI display breakout board [5][6]. After more digging though, I found a
datasheet for that display [7], so I'm thinking a compatible string of
"vot,v220hf01a-t" would be better by the same reasoning above.

And, I know this is getting long, but one more thing...

There was a binding acked recently for the LCD on a D-Link DIR-685 Wireless
N Storage Router [8]. This uses the compound compatible string of "dlink,
dir-685-panel", "ilitek,ili9322". If we want to try to keep things 
consistent, perhaps I should be adopting this pattern as well? And perhaps
it would be better to use the better known vendor name instead of the
obscure vendors from the datasheets that I have found? For example,
"adafruit,618" "sitronix,st7735r" instead of "jianda,jd-t18003-t01"?

[1]: https://www.adafruit.com/product/358
[2]: https://www.adafruit.com/product/618
[3]: https://www.sainsmart.com/products/1-8-tft-spi-lcd-screen-with-microsd-socket
[4]: http://www.adafruit.com/datasheets/JD-T1800.pdf
[5]: https://patchwork.freedesktop.org/patch/187038/
[6]: https://patchwork.freedesktop.org/patch/189233/
[7]: http://www.hotmcu.com/22-176x220-tft-lcd-with-spi-interface-p-316.html
[8]: https://patchwork.freedesktop.org/patch/191462/

 .../bindings/display/sitronix,st7735r.txt          | 35 ++++++++++++++++++++++
 1 file changed, 35 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/display/sitronix,st7735r.txt

diff --git a/Documentation/devicetree/bindings/display/sitronix,st7735r.txt b/Documentation/devicetree/bindings/display/sitronix,st7735r.txt
new file mode 100644
index 0000000..bbb8ba6
--- /dev/null
+++ b/Documentation/devicetree/bindings/display/sitronix,st7735r.txt
@@ -0,0 +1,35 @@
+Sitronix ST7735R display panels
+
+This binding is for display panels using a Sitronix ST7735R controller in SPI
+mode.
+
+Required properties:
+- compatible:	"sitronix,st7735r-jd-t18003-t01"
+- dc-gpios:	Display data/command selection (D/CX)
+- reset-gpios:	Reset signal (RSTX)
+
+The node for this driver must be a child node of a SPI controller, hence
+all mandatory properties described in ../spi/spi-bus.txt must be specified.
+
+Optional properties:
+- rotation:	panel rotation in degrees counter clockwise (0,90,180,270)
+- backlight:	phandle of the backlight device attached to the panel
+
+Example:
+
+	backlight: backlight {
+		compatible = "gpio-backlight";
+		gpios = <&gpio 44 GPIO_ACTIVE_HIGH>;
+	}
+
+	...
+
+	display@0{
+		compatible = "sitronix,st7735r-jd-t18003-t01";
+		reg = <0>;
+		spi-max-frequency = <32000000>;
+		dc-gpios = <&gpio 43 GPIO_ACTIVE_HIGH>;
+		reset-gpios = <&gpio 80 GPIO_ACTIVE_HIGH>;
+		rotation = <270>;
+		backlight = &backlight;
+	};
-- 
2.7.4

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