Re: [RFC] Initial state for GPIOs

[Frank Rowand <frowand.list@gmail.com>]

+frank (me)

On 6/20/19 6:16 AM, Martyn Welch wrote:
> Hi Rob, Mark,
> 
> Attempts have been made to define an approach for describing the
> initial state of gpios (direction and value when driven as an output) a
> number of times in the past, but a concensus on the approach to take
> seems to have never been reached.
> 
> The aim is to be able to describe GPIOs which a definitive use exists
> (i.e. are routed from an SoC to a pin on another device with a
> definitive purpose) and which the desired, and possibly required, state
> of the pin is known. This differs from gpio-hog in that there is an
> expectation that a consumer of the gpio may appear at a later date,
> which may take the form of the GPIO being exported to user space.
> 
> Previous attempts have suggested a variation of the gpio-hogs[1][2].
> "gpio-hogs" uses a node for each GPIO containing the "gpio-hogs"
> property, with which the Linux kernel will act as a consumer,
> statically setting the provided state on the GPIO line, for example:
> 
>         qe_pio_a: gpio-controller@1400 {
>                 compatible = "fsl,qe-pario-bank-a", 
> 			     "fsl,qe-pario-bank";
>                 reg = <0x1400 0x18>;
>                 gpio-controller;
>                 #gpio-cells = <2>;
> 
>                 line_b {
>                         gpio-hog;
>                         gpios = <6 0>;
>                         output-low;
>                         line-name = "foo-bar-gpio";
>                 };
>         };
> 
> It had been suggested to either replace "gpio-hogs" with "gpio-initval" 
> or to include a node without the "gpio-hogs" property to set an inital
> state, but allow another consumer to come along at a later date.
> 
> A previous related attempt to upstream a "gpio-switch" consumer[3] also
> took the approach of defining nodes in the device tree. The
> conversation pointed towards a suggestion of using nodes with
> compatible properties, for example:
> 
>         &gpiochip {
>                 some_led {
>                         compatible = "gpio-leds";
>                         default-state = "on";
>                         gpios = <3 0>;
>                         line-name = "leda";
>                 };
> 
>                 some_switch {
>                         compatible = "gpio-switch", "gpio-initval";
>                         gpios = <4 0>;
>                         line-name = "switch1";
> 
>                         /*
> 			 * This is used by gpio-initval in case 
> 			 * gpio-switch is not implemented
> 			 */
>                         output-low;
>                 };
> 
>                 some_interrupt {
>                         gpios = <5 0>;
>                         line-name = "some_interrupt_line";
>                 };
> 
>                 line_b {
>                         gpios = <6 0>;
>                         line-name = "line-b";
>                 };
>         };
> 
> An alternative that has been briefly raised[4] when I approached the
> subject recently on the GPIO mailing list is to add a property to the
> controller node, rather than child nodes, that listed the expected
> initial states of the pins as an array, much like the line names are
> handled through "gpio-line-names". I'm not quite sure how it would best
> to treat offsets where no special initial state is required (gpio-line-
> names uses empty strings). Something like this?:
> 
> --- gpio.h
>         /* Bit 4 express initial state */
>         #define GPIO_INPUT 0
>         #define GPIO_OUTPUT 16
> 
>         /* Bit 5 express initial state */
>         #define GPIO_INITIAL_LOW 0
>         #define GPIO_INITIAL_HIGH 32
>         
>         #define GPIO_OUTPUT_LOW (GPIO_OUTPUT | GPIO_INITIAL_LOW)
>         #define GPIO_OUTPUT_HIGH (GPIO_OUTPUT | GPIO_INITIAL_HIGH)
> ---
> 
> --- device tree
>         &gpiochip {
>                 gpio-line-names = "", "", "", "widget_en",
> 			"widget_signal";
>                 gpio-initial-states = <>, <>, <>,
> 			<GPIO_OUTPUT_HIGH | GPIO_LINE_OPEN_DRAIN>,
> 			<GPIO_INPUT | GPIO_ACTIVE_LOW>;
>         };
> ---        
> 
> An alternative option may be to provide the offset as the first item
> (though this is then different from "gpio-line-names"), so:
> 
> --- device tree
>         &gpiochip {
>                 gpio-line-names = "", "", "", "widget_en",
> 			"widget_signal";
>                 gpio-initial-states =
> 			<3 GPIO_OUTPUT_HIGH | GPIO_LINE_OPEN_DRAIN>,
> 			<4 GPIO_INPUT | GPIO_ACTIVE_LOW>;
>         };
> ---        
> 
> I'm interested in understanding what form would be acceptable as part
> of the device tree binding.
> 
> Thanks in advance,
> 
> Martyn
> 
> [1] https://marc.info/?l=devicetree&m=145621411916777&w=2
> [2] https://patchwork.ozlabs.org/patch/545493/
> [3] https://lore.kernel.org/patchwork/patch/624195/
> [4] https://www.spinics.net/lists/linux-gpio/msg39810.html
> 
>