pinconf-generic: add BIAS_BUS_HOLD pinconf

Add a new PIN_CONFIG_BIAS_BUS_HOLD pin configuration for a bus holder
pin mode (also known as bus keeper, or repeater). This is a weak latch
which drives the last value on a tristate bus. Another device on the bus
can drive the bus high or low before going tristate to change the value
driven by the pin.
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/pinconf-generic.c

@@ -37,6 +37,7 @@ struct pin_config_item {
 static struct pin_config_item conf_items[] = {
 	PCONFDUMP(PIN_CONFIG_BIAS_DISABLE, "input bias disabled", NULL),
 	PCONFDUMP(PIN_CONFIG_BIAS_HIGH_IMPEDANCE, "input bias high impedance", NULL),
+	PCONFDUMP(PIN_CONFIG_BIAS_BUS_HOLD, "input bias bus hold", NULL),
 	PCONFDUMP(PIN_CONFIG_BIAS_PULL_UP, "input bias pull up", NULL),
 	PCONFDUMP(PIN_CONFIG_BIAS_PULL_DOWN, "input bias pull down", NULL),
 	PCONFDUMP(PIN_CONFIG_DRIVE_PUSH_PULL, "output drive push pull", NULL),

include/linux/pinctrl/pinconf-generic.h

@@ -29,6 +29,11 @@
  *	if for example some other pin is going to drive the signal connected
  *	to it for a while. Pins used for input are usually always high
  *	impedance.
+ * @PIN_CONFIG_BIAS_BUS_HOLD: the pin will be set to weakly latch so that it
+ *	weakly drives the last value on a tristate bus, also known as a "bus
+ *	holder", "bus keeper" or "repeater". This allows another device on the
+ *	bus to change the value by driving the bus high or low and switching to
+ *	tristate. The argument is ignored.
  * @PIN_CONFIG_BIAS_PULL_UP: the pin will be pulled up (usually with high
  *	impedance to VDD). If the argument is != 0 pull-up is enabled,
  *	if it is 0, pull-up is disabled.
@@ -78,6 +83,7 @@
 enum pin_config_param {
 	PIN_CONFIG_BIAS_DISABLE,
 	PIN_CONFIG_BIAS_HIGH_IMPEDANCE,
+	PIN_CONFIG_BIAS_BUS_HOLD,
 	PIN_CONFIG_BIAS_PULL_UP,
 	PIN_CONFIG_BIAS_PULL_DOWN,
 	PIN_CONFIG_DRIVE_PUSH_PULL,