[PATCH v1 1/2] pinctrl: core: Allow drivers to keep "init" pinctrl state after probe

["Michał Kardaś" <mkmkl@google.com>]

During device probe, pinctrl_bind_pins() binds pins to their "init" state
if specified in Device Tree. When probe finishes, pinctrl_init_done()
automatically transitions the pins from "init" to "default" state.

While this auto-transition works well for devices that are immediately
active upon driver binding, certain peripherals (such as power-sequenced
devices connected over UART, SPI, or other buses) remain unpowered until
userspace explicitly opens the device node or attaches a protocol driver.

On board designs where the connected peripheral is kept unpowered during
boot, auto-selecting "default" or "sleep" pin states (where signals such
as TXD or RTS may be driven high or pulled up) can cause parasitic
back-powering into the unpowered peripheral through its ESD protection
diodes.

Allow drivers to explicitly opt out of the automatic "init" -> "default"
transition by calling pinctrl_keep_init_state(dev) during probe. When this
helper is called, pinctrl_init_done() leaves the pins in their "init"
state upon probe completion. The driver can then transition to the
"default" state when the device is first opened by calling
pinctrl_pm_select_default_state(dev).

Suggested-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Michał Kardaś <mkmkl@google.com>
---
 Documentation/driver-api/pin-control.rst | 10 ++++++----
 drivers/pinctrl/core.c                   | 20 ++++++++++++++++++++
 include/linux/pinctrl/consumer.h         |  6 ++++++
 include/linux/pinctrl/devinfo.h          |  2 ++
 4 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/Documentation/driver-api/pin-control.rst b/Documentation/driver-api/pin-control.rst
index 1f585ecca63c..8d52bf74da6c 100644
--- a/Documentation/driver-api/pin-control.rst
+++ b/Documentation/driver-api/pin-control.rst
@@ -1172,7 +1172,8 @@ Possible standard state names are: "default", "init", "sleep" and "idle".
 
 - if ``init`` and ``default`` are defined in the device tree, the "init"
   state is selected before the driver probe and the "default" state is
-  selected after the driver probe.
+  selected after the driver probe, unless the driver explicitly opts out
+  by calling ``pinctrl_keep_init_state()`` during probe.
 
 - the ``sleep`` and ``idle`` states are for power management and can only
   be selected with the PM API bellow.
@@ -1233,9 +1234,10 @@ operation and going to sleep, moving from the ``PINCTRL_STATE_DEFAULT`` to
 current in sleep mode.
 
 Another case is when the pinctrl needs to switch to a certain mode during
-probe and then revert to the default state at the end of probe. For example
-a PINMUX may need to be configured as a GPIO during probe. In this case, use
-``PINCTRL_STATE_INIT`` to switch state before probe, then move to
+probe and then revert to the default state at the end of probe (or remain
+in the init state until activated if ``pinctrl_keep_init_state()`` is called).
+For example a PINMUX may need to be configured as a GPIO during probe. In this
+case, use ``PINCTRL_STATE_INIT`` to switch state before probe, then move to
 ``PINCTRL_STATE_DEFAULT`` at the end of probe for normal operation.
 
 A driver may request a certain control state to be activated, usually just the
diff --git a/drivers/pinctrl/core.c b/drivers/pinctrl/core.c
index 1675dd36bd5c..fd0c91610338 100644
--- a/drivers/pinctrl/core.c
+++ b/drivers/pinctrl/core.c
@@ -1640,6 +1640,9 @@ int pinctrl_init_done(struct device *dev)
 	if (pins->p->state != pins->init_state)
 		return 0; /* Not at init anyway */
 
+	if (pins->keep_init)
+		return 0; /* Driver explicitly requested to stay in init state */
+
 	if (IS_ERR(pins->default_state))
 		return 0; /* No default state */
 
@@ -1678,6 +1681,23 @@ int pinctrl_select_default_state(struct device *dev)
 }
 EXPORT_SYMBOL_GPL(pinctrl_select_default_state);
 
+/**
+ * pinctrl_keep_init_state() - mark pinctrl handle to stay in init state after probe
+ * @dev: device to keep init state for
+ *
+ * Return: true if the device has a valid init state and keep_init flag was set,
+ *         false otherwise.
+ */
+bool pinctrl_keep_init_state(struct device *dev)
+{
+	if (!dev->pins || IS_ERR(dev->pins->init_state))
+		return false;
+
+	dev->pins->keep_init = true;
+	return true;
+}
+EXPORT_SYMBOL_GPL(pinctrl_keep_init_state);
+
 #ifdef CONFIG_PM
 
 /**
diff --git a/include/linux/pinctrl/consumer.h b/include/linux/pinctrl/consumer.h
index 11b8f0b8da0c..4312a098fb4c 100644
--- a/include/linux/pinctrl/consumer.h
+++ b/include/linux/pinctrl/consumer.h
@@ -47,6 +47,7 @@ int pinctrl_select_state(struct pinctrl *p, struct pinctrl_state *s);
 struct pinctrl * __must_check devm_pinctrl_get(struct device *dev);
 void devm_pinctrl_put(struct pinctrl *p);
 int pinctrl_select_default_state(struct device *dev);
+bool pinctrl_keep_init_state(struct device *dev);
 
 #ifdef CONFIG_PM
 int pinctrl_pm_select_default_state(struct device *dev);
@@ -152,6 +153,11 @@ static inline int pinctrl_select_default_state(struct device *dev)
 	return 0;
 }
 
+static inline bool pinctrl_keep_init_state(struct device *dev)
+{
+	return false;
+}
+
 static inline int pinctrl_pm_select_default_state(struct device *dev)
 {
 	return 0;
diff --git a/include/linux/pinctrl/devinfo.h b/include/linux/pinctrl/devinfo.h
index de4228eea90a..13dac33f2df1 100644
--- a/include/linux/pinctrl/devinfo.h
+++ b/include/linux/pinctrl/devinfo.h
@@ -32,6 +32,7 @@ struct pinctrl;
  * @init_state: the state at probe time, if found
  * @sleep_state: the state at suspend time, if found
  * @idle_state: the state at idle (runtime suspend) time, if found
+ * @keep_init: flag indicating if init state should persist after probe
  */
 struct dev_pin_info {
 	struct pinctrl *p;
@@ -41,6 +42,7 @@ struct dev_pin_info {
 	struct pinctrl_state *sleep_state;
 	struct pinctrl_state *idle_state;
 #endif
+	bool keep_init:1;
 };
 
 extern int pinctrl_init_done(struct device *dev);
-- 
2.55.0.654.g21b8a5bc05-goog