[PATCH linux dev-6.12 v1 5/6] i2c: core: Add mux root adapter operations

Ninad Palsule
2025-09-02T18:33:13Z

Some I2C clients need the ability to control the root I2C bus even if the
endpoint device is behind a mux. For example, a driver for a chip that
can't handle any I2C traffic on the bus while coming out of reset
(including an I2C-driven mux switching channels) may need to lock the root
bus with the mux selection fixed for the entire time the device is in
reset.
For this purpose, add a new structure containing two function pointers to
the adapter structure. These functions pointers should be defined for
every adapter. The lock_select operation, for a mux adapter, locks the
parent adpaters up to the root and selects the adapter's channel. The
unlock_deselect operation deselects the mux channel and unlocks all the
adapters. For a non-mux adapter, the operations lock and unlock the
adapters up to the root. This scheme should work with multiple levels of
muxes and regular adapters in between.

OpenBMC-Staging-Count: 4
Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20220906202829.1921114-2-eajames@linux.ibm.com
Signed-off-by: Joel Stanley <joel@jms.id.au>
---
drivers/i2c/i2c-core-base.c | 38 ++++++++++++++++++++++++++++
drivers/i2c/i2c-mux.c | 50 +++++++++++++++++++++++++++++++++++++
include/linux/i2c.h | 42 +++++++++++++++++++++++++++++++
3 files changed, 130 insertions(+)

diff --git a/drivers/i2c/i2c-core-base.c b/drivers/i2c/i2c-core-base.c
index 75d30861ffe21..0e60f24521220 100644
--- a/drivers/i2c/i2c-core-base.c
+++ b/drivers/i2c/i2c-core-base.c
@@ -1430,6 +1430,41 @@ static const struct i2c_lock_operations i2c_adapter_lock_ops = {
.unlock_bus = i2c_adapter_unlock_bus,
};

+/*
+ * For a non-mux adapter, the lock_select operation locks the chain of
+ * adapters upwards, returning the root. If there's a mux above this adapter
+ * somehow, it should also get locked and the desired channel selected.
+ */
+static struct i2c_adapter *i2c_adapter_lock_select(struct i2c_adapter *adapter)
+{
+ struct i2c_adapter *ret = adapter;
+ struct i2c_adapter *parent = i2c_parent_is_i2c_adapter(adapter);
+
+ if (parent) {
+ ret = parent->mux_root_ops->lock_select(parent);
+ if (IS_ERR(ret))
+ return ret;
+ }
+
+ adapter->lock_ops->lock_bus(adapter, I2C_LOCK_ROOT_ADAPTER);
+ return ret;
+}
+
+static void i2c_adapter_unlock_deselect(struct i2c_adapter *adapter)
+{
+ struct i2c_adapter *parent = i2c_parent_is_i2c_adapter(adapter);
+
+ adapter->lock_ops->unlock_bus(adapter, I2C_LOCK_ROOT_ADAPTER);
+
+ if (parent)
+ parent->mux_root_ops->unlock_deselect(parent);
+}
+
+static const struct i2c_mux_root_operations i2c_adapter_mux_root_ops = {
+ .lock_select = i2c_adapter_lock_select,
+ .unlock_deselect = i2c_adapter_unlock_deselect,
+};
+
static void i2c_host_notify_irq_teardown(struct i2c_adapter *adap)
{
struct irq_domain *domain = adap->host_notify_domain;
@@ -1527,6 +1562,9 @@ static int i2c_register_adapter(struct i2c_adapter *adap)
if (!adap->lock_ops)
adap->lock_ops = &i2c_adapter_lock_ops;

+ if (!adap->mux_root_ops)
+ adap->mux_root_ops = &i2c_adapter_mux_root_ops;
+
adap->locked_flags = 0;
rt_mutex_init(&adap->bus_lock);
rt_mutex_init(&adap->mux_lock);
diff --git a/drivers/i2c/i2c-mux.c b/drivers/i2c/i2c-mux.c
index fda72e8be8850..3ba9a8beb7685 100644
--- a/drivers/i2c/i2c-mux.c
+++ b/drivers/i2c/i2c-mux.c
@@ -197,6 +197,49 @@ static void i2c_parent_unlock_bus(struct i2c_adapter *adapter,
rt_mutex_unlock(&parent->mux_lock);
}

+/*
+ * For a mux adapter, the lock_select operation first locks just like the
+ * lock_bus operation. Then it selects the channel for this adapter and
+ * returns the root adapter. If there is another mux above this one, calling
+ * the parent lock_select should ensure that the channel is correctly
+ * selected.
+ */
+static struct i2c_adapter *i2c_mux_lock_select(struct i2c_adapter *adapter)
+{
+ int ret;
+ struct i2c_mux_priv *priv = adapter->algo_data;
+ struct i2c_mux_core *muxc = priv->muxc;
+ struct i2c_adapter *parent = muxc->parent;
+
+ rt_mutex_lock_nested(&parent->mux_lock, i2c_adapter_depth(adapter));
+
+ adapter = parent->mux_root_ops->lock_select(parent);
+ if (IS_ERR(adapter))
+ return adapter;
+
+ ret = muxc->select(muxc, priv->chan_id);
+ if (ret < 0) {
+ parent->mux_root_ops->unlock_deselect(parent);
+ rt_mutex_unlock(&parent->mux_lock);
+ return ERR_PTR(ret);
+ }
+
+ return adapter;
+}
+
+static void i2c_mux_unlock_deselect(struct i2c_adapter *adapter)
+{
+ struct i2c_mux_priv *priv = adapter->algo_data;
+ struct i2c_mux_core *muxc = priv->muxc;
+ struct i2c_adapter *parent = muxc->parent;
+
+ if (muxc->deselect)
+ muxc->deselect(muxc, priv->chan_id);
+
+ parent->mux_root_ops->unlock_deselect(parent);
+ rt_mutex_unlock(&parent->mux_lock);
+}
+
struct i2c_adapter *i2c_root_adapter(struct device *dev)
{
struct device *i2c;
@@ -267,6 +310,11 @@ static const struct i2c_lock_operations i2c_parent_lock_ops = {
.unlock_bus = i2c_parent_unlock_bus,
};

+static const struct i2c_mux_root_operations i2c_mux_root_ops = {
+ .lock_select = i2c_mux_lock_select,
+ .unlock_deselect = i2c_mux_unlock_deselect,
+};
+
int i2c_mux_add_adapter(struct i2c_mux_core *muxc,
u32 force_nr, u32 chan_id)
{
@@ -326,6 +374,8 @@ int i2c_mux_add_adapter(struct i2c_mux_core *muxc,
else
priv->adap.lock_ops = &i2c_parent_lock_ops;

+ priv->adap.mux_root_ops = &i2c_mux_root_ops;
+
/*
* Try to populate the mux adapter's of_node, expands to
* nothing if !CONFIG_OF.
diff --git a/include/linux/i2c.h b/include/linux/i2c.h
index 388ce71a29a97..3081a48b9ec67 100644
--- a/include/linux/i2c.h
+++ b/include/linux/i2c.h
@@ -595,6 +595,26 @@ struct i2c_lock_operations {
void (*unlock_bus)(struct i2c_adapter *adapter, unsigned int flags);
};

+/**
+ * struct i2c_mux_root_operations - represent operations to lock and select
+ * the adapter's mux channel (if a mux is present)
+ * @lock_select: Get exclusive access to the root I2C bus adapter with the
+ * correct mux channel selected for the adapter
+ * @unlock_deslect: Release exclusive access to the root I2C bus adapter and
+ * deselect the mux channel for the adapter
+ *
+ * Some I2C clients need the ability to control the root I2C bus even if the
+ * endpoint device is behind a mux. For example, a driver for a chip that
+ * can't handle any I2C traffic on the bus while coming out of reset (including
+ * an I2C-driven mux switching channels) may need to lock the root bus with
+ * the mux selection fixed for the entire time the device is in reset.
+ * These operations are for such a purpose.
+ */
+struct i2c_mux_root_operations {
+ struct i2c_adapter *(*lock_select)(struct i2c_adapter *adapter);
+ void (*unlock_deselect)(struct i2c_adapter *adapter);
+};
+
/**
* struct i2c_timings - I2C timing information
* @bus_freq_hz: the bus frequency in Hz
@@ -737,6 +757,7 @@ struct i2c_adapter {

/* data fields that are valid for all devices */
const struct i2c_lock_operations *lock_ops;
+ const struct i2c_mux_root_operations *mux_root_ops;
struct rt_mutex bus_lock;
struct rt_mutex mux_lock;

@@ -834,6 +855,27 @@ i2c_unlock_bus(struct i2c_adapter *adapter, unsigned int flags)
adapter->lock_ops->unlock_bus(adapter, flags);
}

+/**
+ * i2c_lock_select_bus - Get exclusive access to the root I2C bus with the
+ * target's mux channel (if a mux is present) selected.
+ * @adapter: Target I2C bus
+ *
+ * Return the root I2C bus if mux selection succeeds, an ERR_PTR otherwise
+ */
+static inline struct i2c_adapter *i2c_lock_select_bus(struct i2c_adapter *adapter)
+{
+ return adapter->mux_root_ops->lock_select(adapter);
+}
+
+/**
+ * i2c_unlock_deselect_bus - Release exclusive access to the root I2C bus
+ * @adapter: Target I2C bus
+ */
+static inline void i2c_unlock_deselect_bus(struct i2c_adapter *adapter)
+{
+ adapter->mux_root_ops->unlock_deselect(adapter);
+}
+
/**
* i2c_mark_adapter_suspended - Report suspended state of the adapter to the core
* @adap: Adapter to mark as suspended
--
2.48.1