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authorStephen Warren <swarren@nvidia.com>2014-02-19 11:43:26 -0700
committerLinus Walleij <linus.walleij@linaro.org>2014-02-27 10:13:47 +0100
commit51e8afc1c43c75b7bc213439206b154a4c8e875f (patch)
tree72a0f0199b185b9698e13c8082603143fa746119
parent3bde4d26f9d7d9ebc87fbd9980d5140ba391b5d4 (diff)
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: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Signed-off-by: Stephen Warren <swarren@nvidia.com> Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
-rw-r--r--Documentation/devicetree/bindings/gpio/gpio.txt60
1 files changed, 52 insertions, 8 deletions
diff --git a/Documentation/devicetree/bindings/gpio/gpio.txt b/Documentation/devicetree/bindings/gpio/gpio.txt
index 0c85bb6e3a80..3fb8f53071b8 100644
--- a/Documentation/devicetree/bindings/gpio/gpio.txt
+++ b/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

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