Kernel driver, is pinctrl-0 property always needed with GPIO overlay?

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I'm creating a kernel driver for SPI controlled display, which is meant to be working with Raspberry PI. I'm not sure about a one thing that I see sometimes in device tree overlays created by other people - pinctrl properties.



The display is controlled through SPI - as mentioned above - but it also has 3 additional control lines: BUSY, RST and DC. In order to has a possibility of controlling these lines, besides the spi overlay my DTS has to include another one, which clearly is: gpio.



fragment@0 
target = <&spi0>;
__overlay__
#address-cells = <1>;
#size-cells = <0>;

status = "okay";

spidev@0
status = "disabled";
;

epd0: epd@0
compatible = "waveshare,epd";
reg = <0>;

pinctrl-names = "default";
pinctrl-0 = <&epd_pins>;

spi-max-frequency = <1000000>;

width = <128>;
height = <296>;

dc-gpios = <&gpio 16 0>;
reset-gpios = <&gpio 20 0>;
busy-gpios = <&gpio 21 0>;

status = "okay";
;
;
;

fragment@1
target = <&gpio>;
__overlay__
epd_pins: epd_pins
brcm,pins = <16 20 21>; /* DC RST BUSY */
brcm,function = <1 1 0>; /* out out in */
;
;
;


That DTS works perfectly fine and I didn't expect any troubles. But there is one thing I'm not sure about:



pinctrl-names = "default";
pinctrl-0 = <&epd_pins>;


I've seen lines like that in other's DTs with gpio overlays, but not always; sometimes they are, sometimes they're not. If I comment out these two lines, it seems like nothing changes, and my driver still works as it should.



I have two questions:



  1. What is the purpose of those pinctrl lines? I'm aware of pin controller subsystem, but I'm asking strictly in context of my DT.

  2. Why do I need to declare the gpio overlay? I can set IN or OUT function directly from my driver code and my gpio numbers are defined in spi overlay (dc-gpios, reset-gpios, busy-gpios).









share|improve this question




























    0














    I'm creating a kernel driver for SPI controlled display, which is meant to be working with Raspberry PI. I'm not sure about a one thing that I see sometimes in device tree overlays created by other people - pinctrl properties.



    The display is controlled through SPI - as mentioned above - but it also has 3 additional control lines: BUSY, RST and DC. In order to has a possibility of controlling these lines, besides the spi overlay my DTS has to include another one, which clearly is: gpio.



    fragment@0 
    target = <&spi0>;
    __overlay__
    #address-cells = <1>;
    #size-cells = <0>;

    status = "okay";

    spidev@0
    status = "disabled";
    ;

    epd0: epd@0
    compatible = "waveshare,epd";
    reg = <0>;

    pinctrl-names = "default";
    pinctrl-0 = <&epd_pins>;

    spi-max-frequency = <1000000>;

    width = <128>;
    height = <296>;

    dc-gpios = <&gpio 16 0>;
    reset-gpios = <&gpio 20 0>;
    busy-gpios = <&gpio 21 0>;

    status = "okay";
    ;
    ;
    ;

    fragment@1
    target = <&gpio>;
    __overlay__
    epd_pins: epd_pins
    brcm,pins = <16 20 21>; /* DC RST BUSY */
    brcm,function = <1 1 0>; /* out out in */
    ;
    ;
    ;


    That DTS works perfectly fine and I didn't expect any troubles. But there is one thing I'm not sure about:



    pinctrl-names = "default";
    pinctrl-0 = <&epd_pins>;


    I've seen lines like that in other's DTs with gpio overlays, but not always; sometimes they are, sometimes they're not. If I comment out these two lines, it seems like nothing changes, and my driver still works as it should.



    I have two questions:



    1. What is the purpose of those pinctrl lines? I'm aware of pin controller subsystem, but I'm asking strictly in context of my DT.

    2. Why do I need to declare the gpio overlay? I can set IN or OUT function directly from my driver code and my gpio numbers are defined in spi overlay (dc-gpios, reset-gpios, busy-gpios).









    share|improve this question


























      0












      0








      0







      I'm creating a kernel driver for SPI controlled display, which is meant to be working with Raspberry PI. I'm not sure about a one thing that I see sometimes in device tree overlays created by other people - pinctrl properties.



      The display is controlled through SPI - as mentioned above - but it also has 3 additional control lines: BUSY, RST and DC. In order to has a possibility of controlling these lines, besides the spi overlay my DTS has to include another one, which clearly is: gpio.



      fragment@0 
      target = <&spi0>;
      __overlay__
      #address-cells = <1>;
      #size-cells = <0>;

      status = "okay";

      spidev@0
      status = "disabled";
      ;

      epd0: epd@0
      compatible = "waveshare,epd";
      reg = <0>;

      pinctrl-names = "default";
      pinctrl-0 = <&epd_pins>;

      spi-max-frequency = <1000000>;

      width = <128>;
      height = <296>;

      dc-gpios = <&gpio 16 0>;
      reset-gpios = <&gpio 20 0>;
      busy-gpios = <&gpio 21 0>;

      status = "okay";
      ;
      ;
      ;

      fragment@1
      target = <&gpio>;
      __overlay__
      epd_pins: epd_pins
      brcm,pins = <16 20 21>; /* DC RST BUSY */
      brcm,function = <1 1 0>; /* out out in */
      ;
      ;
      ;


      That DTS works perfectly fine and I didn't expect any troubles. But there is one thing I'm not sure about:



      pinctrl-names = "default";
      pinctrl-0 = <&epd_pins>;


      I've seen lines like that in other's DTs with gpio overlays, but not always; sometimes they are, sometimes they're not. If I comment out these two lines, it seems like nothing changes, and my driver still works as it should.



      I have two questions:



      1. What is the purpose of those pinctrl lines? I'm aware of pin controller subsystem, but I'm asking strictly in context of my DT.

      2. Why do I need to declare the gpio overlay? I can set IN or OUT function directly from my driver code and my gpio numbers are defined in spi overlay (dc-gpios, reset-gpios, busy-gpios).









      share|improve this question















      I'm creating a kernel driver for SPI controlled display, which is meant to be working with Raspberry PI. I'm not sure about a one thing that I see sometimes in device tree overlays created by other people - pinctrl properties.



      The display is controlled through SPI - as mentioned above - but it also has 3 additional control lines: BUSY, RST and DC. In order to has a possibility of controlling these lines, besides the spi overlay my DTS has to include another one, which clearly is: gpio.



      fragment@0 
      target = <&spi0>;
      __overlay__
      #address-cells = <1>;
      #size-cells = <0>;

      status = "okay";

      spidev@0
      status = "disabled";
      ;

      epd0: epd@0
      compatible = "waveshare,epd";
      reg = <0>;

      pinctrl-names = "default";
      pinctrl-0 = <&epd_pins>;

      spi-max-frequency = <1000000>;

      width = <128>;
      height = <296>;

      dc-gpios = <&gpio 16 0>;
      reset-gpios = <&gpio 20 0>;
      busy-gpios = <&gpio 21 0>;

      status = "okay";
      ;
      ;
      ;

      fragment@1
      target = <&gpio>;
      __overlay__
      epd_pins: epd_pins
      brcm,pins = <16 20 21>; /* DC RST BUSY */
      brcm,function = <1 1 0>; /* out out in */
      ;
      ;
      ;


      That DTS works perfectly fine and I didn't expect any troubles. But there is one thing I'm not sure about:



      pinctrl-names = "default";
      pinctrl-0 = <&epd_pins>;


      I've seen lines like that in other's DTs with gpio overlays, but not always; sometimes they are, sometimes they're not. If I comment out these two lines, it seems like nothing changes, and my driver still works as it should.



      I have two questions:



      1. What is the purpose of those pinctrl lines? I'm aware of pin controller subsystem, but I'm asking strictly in context of my DT.

      2. Why do I need to declare the gpio overlay? I can set IN or OUT function directly from my driver code and my gpio numbers are defined in spi overlay (dc-gpios, reset-gpios, busy-gpios).






      drivers kernel-modules device-tree






      share|improve this question















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      edited Dec 19 '18 at 17:25

























      asked Dec 19 '18 at 17:20









      itachi

      1013




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