linux-kernel - Re: [PATCH v3 3/6] dt-bindings: add binding for Sitronix ST7586 display panels

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Date:   Fri, 4 Aug 2017 10:51:37 -0500
From:   David Lechner <david@...hnology.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        dri-devel@...ts.freedesktop.org
Cc:     devicetree@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Kevin Hilman <khilman@...nel.org>,
        Sekhar Nori <nsekhar@...com>, linux-kernel@...r.kernel.org,
        Rob Herring <robh+dt@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 3/6] dt-bindings: add binding for Sitronix ST7586
 display panels

On 08/04/2017 09:54 AM, Laurent Pinchart wrote:
> Hi David,
> 
> Thank you for the patch.
> 
> On Thursday 03 Aug 2017 17:33:47 David Lechner wrote:
>> This adds a new binding for Sitronix ST7586 display panels.
>>
>> Using lego as the vendor prefix in the compatible string because the display
>> panel I am working with is an integral part of the LEGO MINDSTORMS EV3.
>>
>> Signed-off-by: David Lechner <david@...hnology.com>
>> ---
>>   .../bindings/display/sitronix,st7586.txt           | 26 +++++++++++++++++++
>>   1 file changed, 26 insertions(+)
>>   create mode 100644
>> Documentation/devicetree/bindings/display/sitronix,st7586.txt
>>
>> diff --git a/Documentation/devicetree/bindings/display/sitronix,st7586.txt
>> b/Documentation/devicetree/bindings/display/sitronix,st7586.txt new file
>> mode 100644
>> index 0000000..dfb0b7b
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/display/sitronix,st7586.txt
>> @@ -0,0 +1,26 @@
>> +Sitronix ST7586 display panel
>> +
>> +Required properties:
>> +- compatible:	"lego,ev3-lcd".
>> +
>> +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:
>> +- dc-gpios:	D/C pin. The presence/absence of this GPIO determines
>> +		the panel interface operation mode (IF[3:1] pins):
>> +		- present: IF=011 4-wire 8-bit data serial interface
>> +		- absent:  IF=010 3-wire 9-bit data serial interface
> 
> How does this work ? Do you have a single GPIO on your system connected to
> IF[1], with IF[3:2] hardwired to 01 ?

LEGO has not made the internals of the display publicly available, so I 
cannot say for sure. But I assume that IF[3:1] is hardwired to 011. This 
causes pin D1 to assigned to the signal A0, which is what we are calling 
the dc gpio here.

If IF[3:1] were hardwired to 010, then pin D1 would be not not used and 
there would be no A0 signal.

So, basically, we can infer the state of IF[3:1] by the fact that we 
have a dc pin or not.

> 
>> +- reset-gpios:	Reset pin
>> +- power-supply:	A regulator node for the supply voltage.
>> +- backlight:	phandle of the backlight device attached to the panel
>> +- rotation:	panel rotation in degrees counter clockwise (0,90,180,270)
> 
> Please use the OF graph DT bindings (a.k.a. ports) to describe the connection
> between the panel and its source.

I am afraid that I do not understand this request. What would the source 
of the panel be? There is nothing like a SoC LCD controller that is 
driving this panel.

> 
>> +Example:
>> +	display@0{
>> +		compatible = "lego,ev3-lcd";
>> +		reg = <0>;
>> +		spi-max-frequency = <10000000>;
>> +		dc-gpios = <&gpio 43 GPIO_ACTIVE_HIGH>;
>> +		reset-gpios = <&gpio 80 GPIO_ACTIVE_HIGH>;
>> +	};
>