[PATCH 2/2] Documentation: gpio: board: describe the con_id parameter

[Dirk Behme <dirk.behme@xxxxxxxxx>]

The con_id parameter has to match the GPIO description and is automatically
extended by the GPIO suffix if not NULL. I had to look into the code to
understand this and properly find the GPIO I've been looking for, so document
this.

Signed-off-by: Dirk Behme <dirk.behme@xxxxxxxxx>
---
 Documentation/gpio/board.txt    | 36 ++++++++++++++++++++++++++++++++++++
 Documentation/gpio/consumer.txt |  3 +++
 2 files changed, 39 insertions(+)

diff --git a/Documentation/gpio/board.txt b/Documentation/gpio/board.txt
index 7605773..eac78fc 100644
--- a/Documentation/gpio/board.txt
+++ b/Documentation/gpio/board.txt
@@ -48,6 +48,42 @@ This property will make GPIOs 15, 16 and 17 available to the driver under the
 The led GPIOs will be active-high, while the power GPIO will be active-low (i.e.
 gpiod_is_active_low(power) will be true).
 
+If in the consumer device's node the property doesn't have a <function>- prefix,
+the second parameter (con_id) of the gpiod_get*() functions has to be NULL:
+
+E.g. if in the above device tree example the GPIO array is just called 'gpios'
+
+	foo_device {
+		compatible = "acme,foo";
+		...
+		gpios = <&gpio 15 GPIO_ACTIVE_HIGH>, /* red */
+			<&gpio 16 GPIO_ACTIVE_HIGH>, /* green */
+			<&gpio 17 GPIO_ACTIVE_HIGH>; /* blue */
+	};
+
+the gpiod_get*() calls will be:
+
+	struct gpio_desc *red, *green, *blue, *power;
+
+	red = gpiod_get_index(dev, NULL, 0, GPIOD_OUT_HIGH);
+	green = gpiod_get_index(dev, NULL, 1, GPIOD_OUT_HIGH);
+	blue = gpiod_get_index(dev, NULL, 2, GPIOD_OUT_HIGH);
+
+To summarize:
+
+The con_id string parameter has to be either NULL or the <function>-prefix
+of the GPIO suffixes ("gpios" or "gpio", automatically looked up by the
+gpiod functions internally):
+
+* If the GPIO description is just named with one of the GPIO suffixes
+  ("gpios" or "gpio"), use NULL.
+* If the GPIO description is prefixed with anything, e.g. "led-gpios", use the
+  prefix without the "-" as con_id parameter (in this example "led").
+
+In case con_id is not NULL, the GPIO subsystem prefixes the GPIO suffix
+("gpios" or "gpio") with the string passed in con_id to get the resulting string
+(snprintf(... "%s-%s", con_id, gpio_suffixes[]).
+
 ACPI
 ----
 ACPI also supports function names for GPIOs in a similar fashion to DT.
diff --git a/Documentation/gpio/consumer.txt b/Documentation/gpio/consumer.txt
index 75542b9..47ce4fd 100644
--- a/Documentation/gpio/consumer.txt
+++ b/Documentation/gpio/consumer.txt
@@ -39,6 +39,9 @@ device that displays digits), an additional index argument can be specified:
 					  const char *con_id, unsigned int idx,
 					  enum gpiod_flags flags)
 
+For a more detailed description of the con_id parameter in the DeviceTree case
+see Documentation/gpio/board.txt
+
 The flags parameter is used to optionally specify a direction and initial value
 for the GPIO. Values can be:
 
-- 
2.4.6

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