In-tree phy-can-transceiver driver has already implemented a local
version of devm_mux_state_get_optional.

The omap-i2c driver gets and selects an optional mux in its probe
function without using any helper.

Add new helper functions covering both aforementioned use-cases:

- mux_control_get_optional:
  Get a mux-control if specified in dt, return NULL otherwise.
- devm_mux_state_get_optional:
  Get a mux-state if specified in dt, return NULL otherwise.
- devm_mux_state_get_selected:
  Get and select a mux-state specified in dt, return error otherwise.
- devm_mux_state_get_optional_selected:
  Get and select a mux-state if specified in dt, return error or NULL.

Existing mux_get helper function is changed to take an extra argument
indicating whether the mux is optional.
In this case no error is printed, and NULL returned in case of ENOENT.

Calling code is adapted to handle NULL return case, and to pass optional
argument as required.

To support automatic deselect for _selected helper, a new structure is
created storing an exit pointer similar to clock core which is called on
release.

To facilitate code sharing between optional/mandatory/selected helpers,
a new internal helper function is added to handle quiet (optional) and
verbose (mandatory) errors, as well as storing the correct callback for
devm release: __devm_mux_state_get

Due to this structure devm_mux_state_get_*_selected can no longer print
a useful error message when select fails. Instead callers should print
errors where needed.

Commit e153fdea9db04 ("phy: can-transceiver: Re-instate "mux-states"
property presence check") noted that "mux_get() always prints an error
message in case of an error, including when the property is not present,
confusing the user."

The first error message covers the case that a mux name is not matched
in dt. The second error message is based on of_parse_phandle_with_args
return value.

In optional case no error is printed and NULL is returned.
This ensures that the new helper functions will not confuse the user
either.

With the addition of optional helper functions it became clear that
drivers should compile and link even if CONFIG_MULTIPLEXER was not enabled.
Add stubs for all symbols exported by mux core.

Signed-off-by: Josua Mayer <josua@solid-run.com>
---
 drivers/mux/core.c           | 178 ++++++++++++++++++++++++++++++++++++-------
 include/linux/mux/consumer.h | 108 +++++++++++++++++++++++++-
 2 files changed, 253 insertions(+), 33 deletions(-)

diff --git a/drivers/mux/core.c b/drivers/mux/core.c
index a3840fe0995f..3da5c464e05e 100644
--- a/drivers/mux/core.c
+++ b/drivers/mux/core.c
@@ -46,6 +46,16 @@ static const struct class mux_class = {
 	.name = "mux",
 };
 
+/**
+ * struct devm_mux_state_state -	Tracks managed resources for mux-state objects.
+ * @mux:				Pointer to a mux state.
+ * @exit:				An optional callback to execte before free.
+ */
+struct devm_mux_state_state {
+	struct mux_state *mstate;
+	int (*exit)(struct mux_state *mstate);
+};
+
 static DEFINE_IDA(mux_ida);
 
 static int __init mux_init(void)
@@ -522,11 +532,12 @@ static struct mux_chip *of_find_mux_chip_by_node(struct device_node *np)
  * @mux_name: The name identifying the mux-control.
  * @state: Pointer to where the requested state is returned, or NULL when
  *         the required multiplexer states are handled by other means.
+ * @optional: Whether to return NULL and silence errors when mux doesn't exist.
  *
  * Return: A pointer to the mux-control, or an ERR_PTR with a negative errno.
  */
 static struct mux_control *mux_get(struct device *dev, const char *mux_name,
-				   unsigned int *state)
+				   unsigned int *state, bool optional)
 {
 	struct device_node *np = dev->of_node;
 	struct of_phandle_args args;
@@ -542,7 +553,9 @@ static struct mux_control *mux_get(struct device *dev, const char *mux_name,
 		else
 			index = of_property_match_string(np, "mux-control-names",
 							 mux_name);
-		if (index < 0) {
+		if (index < 0 && optional) {
+			return NULL;
+		} else if (index < 0) {
 			dev_err(dev, "mux controller '%s' not found\n",
 				mux_name);
 			return ERR_PTR(index);
@@ -558,8 +571,12 @@ static struct mux_control *mux_get(struct device *dev, const char *mux_name,
 						 "mux-controls", "#mux-control-cells",
 						 index, &args);
 	if (ret) {
+		if (optional && ret == -ENOENT)
+			return NULL;
+
 		dev_err(dev, "%pOF: failed to get mux-%s %s(%i)\n",
-			np, state ? "state" : "control", mux_name ?: "", index);
+			np, state ? "state" : "control",
+			mux_name ?: "", index);
 		return ERR_PTR(ret);
 	}
 
@@ -617,10 +634,23 @@ static struct mux_control *mux_get(struct device *dev, const char *mux_name,
  */
 struct mux_control *mux_control_get(struct device *dev, const char *mux_name)
 {
-	return mux_get(dev, mux_name, NULL);
+	return mux_get(dev, mux_name, NULL, false);
 }
 EXPORT_SYMBOL_GPL(mux_control_get);
 
+/**
+ * mux_control_get_optional() - Get the optional mux-control for a device.
+ * @dev: The device that needs a mux-control.
+ * @mux_name: The name identifying the mux-control.
+ *
+ * Return: A pointer to the mux-control, an ERR_PTR with a negative errno.
+ */
+struct mux_control *mux_control_get_optional(struct device *dev, const char *mux_name)
+{
+	return mux_get(dev, mux_name, NULL, true);
+}
+EXPORT_SYMBOL_GPL(mux_control_get_optional);
+
 /**
  * mux_control_put() - Put away the mux-control for good.
  * @mux: The mux-control to put away.
@@ -657,8 +687,8 @@ struct mux_control *devm_mux_control_get(struct device *dev,
 	if (!ptr)
 		return ERR_PTR(-ENOMEM);
 
-	mux = mux_control_get(dev, mux_name);
-	if (IS_ERR(mux)) {
+	mux = mux_get(dev, mux_name, NULL, false);
+	if (IS_ERR_OR_NULL(mux)) {
 		devres_free(ptr);
 		return mux;
 	}
@@ -677,20 +707,19 @@ EXPORT_SYMBOL_GPL(devm_mux_control_get);
  *
  * Return: A pointer to the mux-state, or an ERR_PTR with a negative errno.
  */
-static struct mux_state *mux_state_get(struct device *dev, const char *mux_name)
+static struct mux_state *mux_state_get(struct device *dev, const char *mux_name, bool optional)
 {
 	struct mux_state *mstate;
+	struct mux_control *mux;
 
 	mstate = kzalloc(sizeof(*mstate), GFP_KERNEL);
 	if (!mstate)
 		return ERR_PTR(-ENOMEM);
 
-	mstate->mux = mux_get(dev, mux_name, &mstate->state);
-	if (IS_ERR(mstate->mux)) {
-		int err = PTR_ERR(mstate->mux);
-
+	mstate->mux = mux = mux_get(dev, mux_name, &mstate->state, optional);
+	if (IS_ERR_OR_NULL(mux)) {
 		kfree(mstate);
-		return ERR_PTR(err);
+		return ERR_CAST(mux);
 	}
 
 	return mstate;
@@ -710,41 +739,132 @@ static void mux_state_put(struct mux_state *mstate)
 
 static void devm_mux_state_release(struct device *dev, void *res)
 {
-	struct mux_state *mstate = *(struct mux_state **)res;
+	struct devm_mux_state_state *devm_state = res;
 
-	mux_state_put(mstate);
+	if (devm_state->exit)
+		devm_state->exit(devm_state->mstate);
+
+	mux_state_put(devm_state->mstate);
 }
 
 /**
- * devm_mux_state_get() - Get the mux-state for a device, with resource
- *			  management.
- * @dev: The device that needs a mux-control.
- * @mux_name: The name identifying the mux-control.
+ * __devm_mux_state_get() - Get the optional mux-state for a device,
+ *			    with resource management.
+ * @dev: The device that needs a mux-state.
+ * @mux_name: The name identifying the mux-state.
+ * @optional: Whether to return NULL and silence errors when mux doesn't exist.
+ * @init: Optional function pointer for mux-state object initialisation.
+ * @exit: Optional function pointer for mux-state object cleanup on release.
  *
  * Return: Pointer to the mux-state, or an ERR_PTR with a negative errno.
  */
-struct mux_state *devm_mux_state_get(struct device *dev,
-				     const char *mux_name)
+static struct mux_state *__devm_mux_state_get(struct device *dev, const char *mux_name,
+					      bool optional,
+					      int (*init)(struct mux_state *mstate),
+					      int (*exit)(struct mux_state *mstate))
 {
-	struct mux_state **ptr, *mstate;
+	struct devm_mux_state_state *devm_state;
+	struct mux_state *mstate;
+	int ret;
 
-	ptr = devres_alloc(devm_mux_state_release, sizeof(*ptr), GFP_KERNEL);
-	if (!ptr)
+	devm_state = devres_alloc(devm_mux_state_release, sizeof(*devm_state), GFP_KERNEL);
+	if (!devm_state)
 		return ERR_PTR(-ENOMEM);
 
-	mstate = mux_state_get(dev, mux_name);
-	if (IS_ERR(mstate)) {
-		devres_free(ptr);
-		return mstate;
+	mstate = mux_state_get(dev, mux_name, optional);
+	if (IS_ERR_OR_NULL(mstate)) {
+		ret = PTR_ERR(mstate);
+		goto err_mux_state_get;
 	}
 
-	*ptr = mstate;
-	devres_add(dev, ptr);
+	if (init) {
+		ret = init(mstate);
+		if (ret)
+			goto err_mux_state_init;
+	}
+
+	devm_state->mstate = mstate;
+	devm_state->exit = exit;
+	devres_add(dev, devm_state);
 
 	return mstate;
+
+err_mux_state_init:
+	mux_state_put(mstate);
+err_mux_state_get:
+	devres_free(devm_state);
+	return ERR_PTR(ret);
+}
+
+/**
+ * devm_mux_state_get() - Get the mux-state for a device, with resource
+ *			  management.
+ * @dev: The device that needs a mux-control.
+ * @mux_name: The name identifying the mux-control.
+ *
+ * Return: Pointer to the mux-state, or an ERR_PTR with a negative errno.
+ *
+ * The mux-state will automatically be freed on release.
+ */
+struct mux_state *devm_mux_state_get(struct device *dev, const char *mux_name)
+{
+	return __devm_mux_state_get(dev, mux_name, false, NULL, NULL);
 }
 EXPORT_SYMBOL_GPL(devm_mux_state_get);
 
+/**
+ * devm_mux_state_get_optional() - Get the optional mux-state for a device,
+ *				   with resource management.
+ * @dev: The device that needs a mux-state.
+ * @mux_name: The name identifying the mux-state.
+ *
+ * Return: Pointer to the mux-state, or an ERR_PTR with a negative errno.
+ *
+ * The mux-state will automatically be freed on release.
+ */
+struct mux_state *devm_mux_state_get_optional(struct device *dev, const char *mux_name)
+{
+	return __devm_mux_state_get(dev, mux_name, true, NULL, NULL);
+}
+EXPORT_SYMBOL_GPL(devm_mux_state_get_optional);
+
+/**
+ * devm_mux_state_get_selected() - Get the mux-state for a device, with
+ *				   resource management.
+ * @dev: The device that needs a mux-state.
+ * @mux_name: The name identifying the mux-state.
+ *
+ * Return: Pointer to the mux-state, or an ERR_PTR with a negative errno.
+ *
+ * The returned mux-state (if valid) is already selected.
+ *
+ * The mux-state will automatically be deselected and freed on release.
+ */
+struct mux_state *devm_mux_state_get_selected(struct device *dev, const char *mux_name)
+{
+	return __devm_mux_state_get(dev, mux_name, false, mux_state_select, mux_state_deselect);
+}
+EXPORT_SYMBOL_GPL(devm_mux_state_get_selected);
+
+/**
+ * devm_mux_state_get_optional_selected() - Get the optional mux-state for
+ *					    a device, with resource management.
+ * @dev: The device that needs a mux-state.
+ * @mux_name: The name identifying the mux-state.
+ *
+ * Return: Pointer to the mux-state, or an ERR_PTR with a negative errno.
+ *
+ * The returned mux-state (if valid) is already selected.
+ *
+ * The mux-state will automatically be deselected and freed on release.
+ */
+struct mux_state *devm_mux_state_get_optional_selected(struct device *dev,
+						       const char *mux_name)
+{
+	return __devm_mux_state_get(dev, mux_name, true, mux_state_select, mux_state_deselect);
+}
+EXPORT_SYMBOL_GPL(devm_mux_state_get_optional_selected);
+
 /*
  * Using subsys_initcall instead of module_init here to try to ensure - for
  * the non-modular case - that the subsystem is initialized when mux consumers
diff --git a/include/linux/mux/consumer.h b/include/linux/mux/consumer.h
index 2e25c838f831..3ede55b907eb 100644
--- a/include/linux/mux/consumer.h
+++ b/include/linux/mux/consumer.h
@@ -16,6 +16,8 @@ struct device;
 struct mux_control;
 struct mux_state;
 
+#ifdef CONFIG_MULTIPLEXER
+
 unsigned int mux_control_states(struct mux_control *mux);
 int __must_check mux_control_select_delay(struct mux_control *mux,
 					  unsigned int state,
@@ -54,11 +56,109 @@ int mux_control_deselect(struct mux_control *mux);
 int mux_state_deselect(struct mux_state *mstate);
 
 struct mux_control *mux_control_get(struct device *dev, const char *mux_name);
+struct mux_control *mux_control_get_optional(struct device *dev, const char *mux_name);
 void mux_control_put(struct mux_control *mux);
 
-struct mux_control *devm_mux_control_get(struct device *dev,
-					 const char *mux_name);
-struct mux_state *devm_mux_state_get(struct device *dev,
-				     const char *mux_name);
+struct mux_control *devm_mux_control_get(struct device *dev, const char *mux_name);
+struct mux_state *devm_mux_state_get(struct device *dev, const char *mux_name);
+struct mux_state *devm_mux_state_get_optional(struct device *dev, const char *mux_name);
+struct mux_state *devm_mux_state_get_selected(struct device *dev, const char *mux_name);
+struct mux_state *devm_mux_state_get_optional_selected(struct device *dev, const char *mux_name);
+
+#else
+
+static inline unsigned int mux_control_states(struct mux_control *mux)
+{
+	return 0;
+}
+static inline int __must_check mux_control_select_delay(struct mux_control *mux,
+							unsigned int state, unsigned int delay_us)
+{
+	return -EOPNOTSUPP;
+}
+static inline int __must_check mux_state_select_delay(struct mux_state *mstate,
+						      unsigned int delay_us)
+{
+	return -EOPNOTSUPP;
+}
+static inline int __must_check mux_control_try_select_delay(struct mux_control *mux,
+							    unsigned int state,
+							    unsigned int delay_us)
+{
+	return -EOPNOTSUPP;
+}
+static inline int __must_check mux_state_try_select_delay(struct mux_state *mstate,
+							  unsigned int delay_us)
+{
+	return -EOPNOTSUPP;
+}
+
+static inline int __must_check mux_control_select(struct mux_control *mux,
+						  unsigned int state)
+{
+	return -EOPNOTSUPP;
+}
+
+static inline int __must_check mux_state_select(struct mux_state *mstate)
+{
+	return -EOPNOTSUPP;
+}
+
+static inline int __must_check mux_control_try_select(struct mux_control *mux,
+						      unsigned int state)
+{
+	return -EOPNOTSUPP;
+}
+
+static inline int __must_check mux_state_try_select(struct mux_state *mstate)
+{
+	return -EOPNOTSUPP;
+}
+
+static inline int mux_control_deselect(struct mux_control *mux)
+{
+	return -EOPNOTSUPP;
+}
+static inline int mux_state_deselect(struct mux_state *mstate)
+{
+	return -EOPNOTSUPP;
+}
+
+static inline struct mux_control *mux_control_get(struct device *dev, const char *mux_name)
+{
+	return ERR_PTR(-EOPNOTSUPP);
+}
+static inline struct mux_control *mux_control_get_optional(struct device *dev,
+							   const char *mux_name)
+{
+	return NULL;
+}
+static inline void mux_control_put(struct mux_control *mux) {}
+
+static inline struct mux_control *devm_mux_control_get(struct device *dev, const char *mux_name)
+{
+	return ERR_PTR(-EOPNOTSUPP);
+}
+static inline struct mux_state *devm_mux_state_get(struct device *dev, const char *mux_name)
+{
+	return ERR_PTR(-EOPNOTSUPP);
+}
+static inline struct mux_state *devm_mux_state_get_optional(struct device *dev,
+							    const char *mux_name)
+{
+	return NULL;
+}
+static inline struct mux_state *devm_mux_state_get_selected(struct device *dev,
+							    const char *mux_name)
+{
+	return ERR_PTR(-EOPNOTSUPP);
+}
+static inline struct mux_state *devm_mux_state_get_optional_selected(struct device *dev,
+								     const char *mux_name)
+{
+	return NULL;
+}
+
+#endif /* CONFIG_MULTIPLEXER */
 
 #endif /* _LINUX_MUX_CONSUMER_H */

-- 
2.43.0

Hi Josua,

kernel test robot noticed the following build warnings:

url:    https://github.com/intel-lab-lkp/linux/commits/Josua-Mayer/phy-can-transceiver-rename-temporary-helper-function-to-avoid-conflict/20260121-173607
base:   8f0b4cce4481fb22653697cced8d0d04027cb1e8
patch link:    https://lore.kernel.org/r/20260121-rz-sdio-mux-v6-2-38aa39527928%40solid-run.com
patch subject: [PATCH v6 2/7] mux: Add helper functions for getting optional and selected mux-state
config: arm64-randconfig-r073-20260121 (https://download.01.org/0day-ci/archive/20260122/202601221036.J0kR78Uw-lkp@intel.com/config)
compiler: aarch64-linux-gcc (GCC) 8.5.0
smatch version: v0.5.0-8985-g2614ff1a

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
| Closes: https://lore.kernel.org/r/202601221036.J0kR78Uw-lkp@intel.com/

New smatch warnings:
drivers/mux/core.c:776 __devm_mux_state_get() warn: passing zero to 'PTR_ERR'
drivers/mux/core.c:796 __devm_mux_state_get() warn: passing zero to 'ERR_PTR'

Old smatch warnings:
drivers/mux/core.c:722 mux_state_get() warn: passing zero to 'ERR_CAST'

vim +/PTR_ERR +776 drivers/mux/core.c

0610cc3159b786 Josua Mayer       2026-01-21  761  static struct mux_state *__devm_mux_state_get(struct device *dev, const char *mux_name,
0610cc3159b786 Josua Mayer       2026-01-21  762  					      bool optional,
0610cc3159b786 Josua Mayer       2026-01-21  763  					      int (*init)(struct mux_state *mstate),
0610cc3159b786 Josua Mayer       2026-01-21  764  					      int (*exit)(struct mux_state *mstate))
0610cc3159b786 Josua Mayer       2026-01-21  765  {
0610cc3159b786 Josua Mayer       2026-01-21  766  	struct devm_mux_state_state *devm_state;
0610cc3159b786 Josua Mayer       2026-01-21  767  	struct mux_state *mstate;
0610cc3159b786 Josua Mayer       2026-01-21  768  	int ret;
84564481bc4520 Aswath Govindraju 2022-01-07  769  
0610cc3159b786 Josua Mayer       2026-01-21  770  	devm_state = devres_alloc(devm_mux_state_release, sizeof(*devm_state), GFP_KERNEL);
0610cc3159b786 Josua Mayer       2026-01-21  771  	if (!devm_state)
0610cc3159b786 Josua Mayer       2026-01-21  772  		return ERR_PTR(-ENOMEM);
0610cc3159b786 Josua Mayer       2026-01-21  773  
0610cc3159b786 Josua Mayer       2026-01-21  774  	mstate = mux_state_get(dev, mux_name, optional);
0610cc3159b786 Josua Mayer       2026-01-21  775  	if (IS_ERR_OR_NULL(mstate)) {

Please, change this to if (IS_ERR(mstate)) {.  The mux_state_get() never
returns NULL.  I have written a blog about functions which return both
error pointers and NULL.

https://staticthinking.wordpress.com/2022/08/01/mixing-error-pointers-and-null/

0610cc3159b786 Josua Mayer       2026-01-21 @776  		ret = PTR_ERR(mstate);
0610cc3159b786 Josua Mayer       2026-01-21  777  		goto err_mux_state_get;
0610cc3159b786 Josua Mayer       2026-01-21  778  	}
0610cc3159b786 Josua Mayer       2026-01-21  779  
0610cc3159b786 Josua Mayer       2026-01-21  780  	if (init) {
0610cc3159b786 Josua Mayer       2026-01-21  781  		ret = init(mstate);
0610cc3159b786 Josua Mayer       2026-01-21  782  		if (ret)
0610cc3159b786 Josua Mayer       2026-01-21  783  			goto err_mux_state_init;
0610cc3159b786 Josua Mayer       2026-01-21  784  	}
0610cc3159b786 Josua Mayer       2026-01-21  785  
0610cc3159b786 Josua Mayer       2026-01-21  786  	devm_state->mstate = mstate;
0610cc3159b786 Josua Mayer       2026-01-21  787  	devm_state->exit = exit;
0610cc3159b786 Josua Mayer       2026-01-21  788  	devres_add(dev, devm_state);
0610cc3159b786 Josua Mayer       2026-01-21  789  
0610cc3159b786 Josua Mayer       2026-01-21  790  	return mstate;
0610cc3159b786 Josua Mayer       2026-01-21  791  
0610cc3159b786 Josua Mayer       2026-01-21  792  err_mux_state_init:
84564481bc4520 Aswath Govindraju 2022-01-07  793  	mux_state_put(mstate);
0610cc3159b786 Josua Mayer       2026-01-21  794  err_mux_state_get:
0610cc3159b786 Josua Mayer       2026-01-21  795  	devres_free(devm_state);
0610cc3159b786 Josua Mayer       2026-01-21 @796  	return ERR_PTR(ret);
84564481bc4520 Aswath Govindraju 2022-01-07  797  }

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Hi Josua,

kernel test robot noticed the following build warnings:

[auto build test WARNING on 8f0b4cce4481fb22653697cced8d0d04027cb1e8]

url:    https://github.com/intel-lab-lkp/linux/commits/Josua-Mayer/phy-can-transceiver-rename-temporary-helper-function-to-avoid-conflict/20260121-173607
base:   8f0b4cce4481fb22653697cced8d0d04027cb1e8
patch link:    https://lore.kernel.org/r/20260121-rz-sdio-mux-v6-2-38aa39527928%40solid-run.com
patch subject: [PATCH v6 2/7] mux: Add helper functions for getting optional and selected mux-state
config: i386-randconfig-001-20260121 (https://download.01.org/0day-ci/archive/20260121/202601212215.qNGA0wVh-lkp@intel.com/config)
compiler: gcc-14 (Debian 14.2.0-19) 14.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260121/202601212215.qNGA0wVh-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202601212215.qNGA0wVh-lkp@intel.com/

All warnings (new ones prefixed by >>):

>> Warning: drivers/mux/core.c:56 struct member 'mstate' not described in 'devm_mux_state_state'
>> Warning: drivers/mux/core.c:56 struct member 'mstate' not described in 'devm_mux_state_state'

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