gpio: document polarity flag best practices

Document what we (Laurent and I, following a mailing list dicussion)
believe are best practices for the polarity flag in a GPIO specifier.

While touching the doc, I made a few minor editing changes to other
areas.
Suggested-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NStephen Warren <swarren@nvidia.com>
Acked-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/gpio.txt

@@ -13,11 +13,11 @@ properties, each containing a 'gpio-list':
 	gpio-specifier : Array of #gpio-cells specifying specific gpio
 			 (controller specific)
 
-GPIO properties should be named "[<name>-]gpios".  Exact
+GPIO properties should be named "[<name>-]gpios". The exact
 meaning of each gpios property must be documented in the device tree
 binding for each device.
 
-For example, the following could be used to describe gpios pins to use
+For example, the following could be used to describe GPIO pins used
 as chip select lines; with chip selects 0, 1 and 3 populated, and chip
 select 2 left empty:
 
@@ -44,35 +44,79 @@ whether pin is open-drain and whether pin is logically inverted.
 Exact meaning of each specifier cell is controller specific, and must
 be documented in the device tree binding for the device.
 
-Example of the node using GPIOs:
+Example of a node using GPIOs:
 
 	node {
 		gpios = <&qe_pio_e 18 0>;
 	};
 
 In this example gpio-specifier is "18 0" and encodes GPIO pin number,
-and empty GPIO flags as accepted by the "qe_pio_e" gpio-controller.
+and GPIO flags as accepted by the "qe_pio_e" gpio-controller.
+
+1.1) GPIO specifier best practices
+----------------------------------
+
+A gpio-specifier should contain a flag indicating the GPIO polarity; active-
+high or active-low. If it does, the follow best practices should be followed:
+
+The gpio-specifier's polarity flag should represent the physical level at the
+GPIO controller that achieves (or represents, for inputs) a logically asserted
+value at the device. The exact definition of logically asserted should be
+defined by the binding for the device. If the board inverts the signal between
+the GPIO controller and the device, then the gpio-specifier will represent the
+opposite physical level than the signal at the device's pin.
+
+When the device's signal polarity is configurable, the binding for the
+device must either:
+
+a) Define a single static polarity for the signal, with the expectation that
+any software using that binding would statically program the device to use
+that signal polarity.
+
+The static choice of polarity may be either:
+
+a1) (Preferred) Dictated by a binding-specific DT property.
+
+or:
+
+a2) Defined statically by the DT binding itself.
+
+In particular, the polarity cannot be derived from the gpio-specifier, since
+that would prevent the DT from separately representing the two orthogonal
+concepts of configurable signal polarity in the device, and possible board-
+level signal inversion.
+
+or:
+
+b) Pick a single option for device signal polarity, and document this choice
+in the binding. The gpio-specifier should represent the polarity of the signal
+(at the GPIO controller) assuming that the device is configured for this
+particular signal polarity choice. If software chooses to program the device
+to generate or receive a signal of the opposite polarity, software will be
+responsible for correctly interpreting (inverting) the GPIO signal at the GPIO
+controller.
 
 2) gpio-controller nodes
 ------------------------
 
-Every GPIO controller node must both an empty "gpio-controller"
-property, and have #gpio-cells contain the size of the gpio-specifier.
+Every GPIO controller node must contain both an empty "gpio-controller"
+property, and a #gpio-cells integer property, which indicates the number of
+cells in a gpio-specifier.
 
 Example of two SOC GPIO banks defined as gpio-controller nodes:
 
 	qe_pio_a: gpio-controller@1400 {
-		#gpio-cells = <2>;
 		compatible = "fsl,qe-pario-bank-a", "fsl,qe-pario-bank";
 		reg = <0x1400 0x18>;
 		gpio-controller;
+		#gpio-cells = <2>;
 	};
 
 	qe_pio_e: gpio-controller@1460 {
-		#gpio-cells = <2>;
 		compatible = "fsl,qe-pario-bank-e", "fsl,qe-pario-bank";
 		reg = <0x1460 0x18>;
 		gpio-controller;
+		#gpio-cells = <2>;
 	};
 
 2.1) gpio- and pin-controller interaction