pinctrl: generic: Add output-enable property

Add output-enable generic pin configuration property.
This properties allows enabling/disabling pin's output capabilities
without actually driving any value on the line.

Acked-by: Rob Herring <robh@kernel.org>
[Added inline elaborations on buffer enabling/disabling]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/pinctrl-bindings.txt

@@ -251,14 +251,20 @@ drive-push-pull		- drive actively high and low
 drive-open-drain	- drive with open drain
 drive-open-source	- drive with open source
 drive-strength		- sink or source at most X mA
-input-enable		- enable input on pin (no effect on output)
-input-disable		- disable input on pin (no effect on output)
+input-enable		- enable input on pin (no effect on output, such as
+			  enabling an input buffer)
+input-disable		- disable input on pin (no effect on output, such as
+			  disabling an input buffer)
 input-schmitt-enable	- enable schmitt-trigger mode
 input-schmitt-disable	- disable schmitt-trigger mode
 input-debounce		- debounce mode with debound time X
 power-source		- select between different power supplies
 low-power-enable	- enable low power mode
 low-power-disable	- disable low power mode
+output-disable		- disable output on a pin (such as disable an output
+			  buffer)
+output-enable		- enable output on a pin without actively driving it
+			  (such as enabling an output buffer)
 output-low		- set the pin to output mode with low level
 output-high		- set the pin to output mode with high level
 slew-rate		- set the slew rate

drivers/pinctrl/pinconf-generic.c

@@ -44,6 +44,7 @@ static const struct pin_config_item conf_items[] = {
 	PCONFDUMP(PIN_CONFIG_INPUT_SCHMITT, "input schmitt trigger", NULL, false),
 	PCONFDUMP(PIN_CONFIG_INPUT_SCHMITT_ENABLE, "input schmitt enabled", NULL, false),
 	PCONFDUMP(PIN_CONFIG_LOW_POWER_MODE, "pin low power", "mode", true),
+	PCONFDUMP(PIN_CONFIG_OUTPUT_ENABLE, "output enabled", NULL, false),
 	PCONFDUMP(PIN_CONFIG_OUTPUT, "pin output", "level", true),
 	PCONFDUMP(PIN_CONFIG_POWER_SOURCE, "pin power source", "selector", true),
 	PCONFDUMP(PIN_CONFIG_SLEW_RATE, "slew rate", NULL, true),
@@ -172,6 +173,8 @@ static const struct pinconf_generic_params dt_params[] = {
 	{ "input-schmitt-enable", PIN_CONFIG_INPUT_SCHMITT_ENABLE, 1 },
 	{ "low-power-disable", PIN_CONFIG_LOW_POWER_MODE, 0 },
 	{ "low-power-enable", PIN_CONFIG_LOW_POWER_MODE, 1 },
+	{ "output-disable", PIN_CONFIG_OUTPUT_ENABLE, 0 },
+	{ "output-enable", PIN_CONFIG_OUTPUT_ENABLE, 1 },
 	{ "output-high", PIN_CONFIG_OUTPUT, 1, },
 	{ "output-low", PIN_CONFIG_OUTPUT, 0, },
 	{ "power-source", PIN_CONFIG_POWER_SOURCE, 0 },

include/linux/pinctrl/pinconf-generic.h

@@ -73,10 +73,16 @@
  *	operation, if several modes of operation are supported these can be
  *	passed in the argument on a custom form, else just use argument 1
  *	to indicate low power mode, argument 0 turns low power mode off.
- * @PIN_CONFIG_OUTPUT: this will configure the pin as an output. Use argument
- *	1 to indicate high level, argument 0 to indicate low level. (Please
- *	see Documentation/pinctrl.txt, section "GPIO mode pitfalls" for a
- *	discussion around this parameter.)
+ * @PIN_CONFIG_OUTPUT_ENABLE: this will enable the pin's output mode
+ * 	without driving a value there. For most platforms this reduces to
+ * 	enable the output buffers and then let the pin controller current
+ * 	configuration (eg. the currently selected mux function) drive values on
+ * 	the line. Use argument 1 to enable output mode, argument 0 to disable
+ * 	it.
+ * @PIN_CONFIG_OUTPUT: this will configure the pin as an output and drive a
+ * 	value on the line. Use argument 1 to indicate high level, argument 0 to
+ * 	indicate low level. (Please see Documentation/pinctrl.txt, section
+ * 	"GPIO mode pitfalls" for a discussion around this parameter.)
  * @PIN_CONFIG_POWER_SOURCE: if the pin can select between different power
  *	supplies, the argument to this parameter (on a custom format) tells
  *	the driver which alternative power source to use.
@@ -105,6 +111,7 @@ enum pin_config_param {
 	PIN_CONFIG_INPUT_SCHMITT,
 	PIN_CONFIG_INPUT_SCHMITT_ENABLE,
 	PIN_CONFIG_LOW_POWER_MODE,
+	PIN_CONFIG_OUTPUT_ENABLE,
 	PIN_CONFIG_OUTPUT,
 	PIN_CONFIG_POWER_SOURCE,
 	PIN_CONFIG_SLEW_RATE,