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
