There could be several reasons why you may need to use a certain speed
on an I2C bus. E.g.

- When several devices are attached to the bus, the speed must be
  selected according to the slowest device.

- Electrical conditions may limit the usuable speed on the bus for
  different reasons.

With an I2C multiplexer, it is possible to group the attached devices
after their preferred speed by e.g. put all "slow" devices on a separate
channel on the multiplexer.

Consider the following topology:

                      .----------. 100kHz .--------.
    .--------. 400kHz |          |--------| dev D1 |
    |  root  |--+-----| I2C MUX  |        '--------'
    '--------'  |     |          |--. 400kHz .--------.
                |     '----------'  '-------| dev D2 |
                |  .--------.               '--------'
                '--| dev D3 |
                   '--------'

One requirement with this design is that a multiplexer may only use the
same or lower bus speed as its parent.
Otherwise, if the multiplexer would have to increase the bus frequency,
then all siblings (D3 in this case) would run into a clock speed it may
not support.

The bus frequency for each channel is set in the devicetree. As the
i2c-mux bindings import the i2c-controller schema, the clock-frequency
property is already allowed.
If no clock-frequency property is set, the channel inherit their parent
bus speed.

The following example uses dt bindings to illustrate the topology above:

    i2c {
	clock-frequency = <400000>;

        i2c-mux {
            i2c@0 {
                clock-frequency = <100000>;

                D1 {
                    ...
                };
            };

            i2c@1 {
                D2 {
                  ...
                };
            };
        };

        D3 {
            ...
        }
    };

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
---
 drivers/i2c/i2c-mux.c | 126 +++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 114 insertions(+), 12 deletions(-)

diff --git a/drivers/i2c/i2c-mux.c b/drivers/i2c/i2c-mux.c
index 5caa927c0caae512af029f0d1ae9b7f845ba3f6f..22b404597ff91eddb965c48112fdc63250d71e43 100644
--- a/drivers/i2c/i2c-mux.c
+++ b/drivers/i2c/i2c-mux.c
@@ -36,6 +36,72 @@ struct i2c_mux_priv {
 	u32 chan_id;
 };
 
+static int i2c_mux_select_chan(struct i2c_adapter *adap, u32 chan_id)
+{
+	struct i2c_mux_priv *priv = adap->algo_data;
+	struct i2c_mux_core *muxc = priv->muxc;
+	struct i2c_adapter *parent = muxc->parent;
+	struct i2c_adapter *root;
+	int ret;
+
+	if (priv->adap.clock_hz && priv->adap.clock_hz != parent->clock_hz) {
+		root = i2c_root_adapter(&adap->dev);
+
+		/* if we are parent-locked and the root adapter is our parent,
+		 * we already have the lock we need. Otherwise take the bus lock for the root
+		 * adaper before changing bus clock.
+		 */
+		if ((root != parent && !muxc->mux_locked) || muxc->mux_locked)
+			i2c_lock_bus(parent, I2C_LOCK_ROOT_ADAPTER);
+
+		ret = i2c_adapter_set_clk_freq(root, priv->adap.clock_hz);
+
+		if ((root != parent && !muxc->mux_locked) || muxc->mux_locked)
+			i2c_unlock_bus(parent, I2C_LOCK_ROOT_ADAPTER);
+
+		if (ret < 0) {
+			dev_err(&adap->dev,
+				"Failed to set clock frequency %dHz on root adapter %s: %d\n",
+				priv->adap.clock_hz, root->name, ret);
+
+			return ret;
+		}
+	}
+
+	return muxc->select(muxc, priv->chan_id);
+}
+
+static void i2c_mux_deselect_chan(struct i2c_adapter *adap, u32 chan_id)
+{
+	struct i2c_mux_priv *priv = adap->algo_data;
+	struct i2c_mux_core *muxc = priv->muxc;
+	struct i2c_adapter *parent = muxc->parent;
+	struct i2c_adapter *root;
+	int ret;
+
+	if (parent->clock_hz && parent->clock_hz != priv->adap.clock_hz) {
+		root = i2c_root_adapter(&parent->dev);
+
+		/* if we are parent-locked and the root adapter is our parent,
+		 * we already have the lock we need. Otherwise take the bus lock for the root
+		 * adaper before changing bus clock.
+		 */
+		if ((root != parent && !muxc->mux_locked) || muxc->mux_locked)
+			i2c_lock_bus(parent, I2C_LOCK_ROOT_ADAPTER);
+
+		ret = i2c_adapter_set_clk_freq(root, parent->clock_hz);
+
+		if ((root != parent && !muxc->mux_locked) || muxc->mux_locked)
+			i2c_unlock_bus(parent, I2C_LOCK_ROOT_ADAPTER);
+
+		if (ret < 0)
+			return;
+	}
+
+	if (muxc->deselect)
+		muxc->deselect(muxc, priv->chan_id);
+}
+
 static int __i2c_mux_master_xfer(struct i2c_adapter *adap,
 				 struct i2c_msg msgs[], int num)
 {
@@ -46,11 +112,11 @@ static int __i2c_mux_master_xfer(struct i2c_adapter *adap,
 
 	/* Switch to the right mux port and perform the transfer. */
 
-	ret = muxc->select(muxc, priv->chan_id);
+	ret = i2c_mux_select_chan(adap, priv->chan_id);
 	if (ret >= 0)
 		ret = __i2c_transfer(parent, msgs, num);
-	if (muxc->deselect)
-		muxc->deselect(muxc, priv->chan_id);
+
+	i2c_mux_deselect_chan(adap, priv->chan_id);
 
 	return ret;
 }
@@ -65,11 +131,11 @@ static int i2c_mux_master_xfer(struct i2c_adapter *adap,
 
 	/* Switch to the right mux port and perform the transfer. */
 
-	ret = muxc->select(muxc, priv->chan_id);
+	ret = i2c_mux_select_chan(adap, priv->chan_id);
 	if (ret >= 0)
 		ret = i2c_transfer(parent, msgs, num);
-	if (muxc->deselect)
-		muxc->deselect(muxc, priv->chan_id);
+
+	i2c_mux_deselect_chan(adap, priv->chan_id);
 
 	return ret;
 }
@@ -86,12 +152,12 @@ static int __i2c_mux_smbus_xfer(struct i2c_adapter *adap,
 
 	/* Select the right mux port and perform the transfer. */
 
-	ret = muxc->select(muxc, priv->chan_id);
+	ret = i2c_mux_select_chan(adap, priv->chan_id);
 	if (ret >= 0)
 		ret = __i2c_smbus_xfer(parent, addr, flags,
 				       read_write, command, size, data);
-	if (muxc->deselect)
-		muxc->deselect(muxc, priv->chan_id);
+
+	i2c_mux_deselect_chan(adap, priv->chan_id);
 
 	return ret;
 }
@@ -108,12 +174,12 @@ static int i2c_mux_smbus_xfer(struct i2c_adapter *adap,
 
 	/* Select the right mux port and perform the transfer. */
 
-	ret = muxc->select(muxc, priv->chan_id);
+	ret = i2c_mux_select_chan(adap, priv->chan_id);
 	if (ret >= 0)
 		ret = i2c_smbus_xfer(parent, addr, flags,
 				     read_write, command, size, data);
-	if (muxc->deselect)
-		muxc->deselect(muxc, priv->chan_id);
+
+	i2c_mux_deselect_chan(adap, priv->chan_id);
 
 	return ret;
 }
@@ -366,6 +432,42 @@ int i2c_mux_add_adapter(struct i2c_mux_core *muxc,
 			}
 		}
 
+		of_property_read_u32(child, "clock-frequency", &priv->adap.clock_hz);
+
+		/*
+		 * Warn if the mux adapter does not disconnect channels or
+		 * if a low-speed channel is seleced during idle.
+		 */
+		if (priv->adap.clock_hz < parent->clock_hz)
+			if (muxc->idle_state != MUX_IDLE_DISCONNECT ||
+			    muxc->idle_state == chan_id)
+				dev_warn(muxc->dev,
+					 "channel %u has improper idle state for this configuration\n",
+					 chan_id);
+
+		/*
+		 * Warn if the mux adapter is not parent-locked as
+		 * this may cause issues for some hardware topologies.
+		 */
+		if ((priv->adap.clock_hz < parent->clock_hz) && muxc->mux_locked)
+			dev_warn(muxc->dev,
+				 "channel %u is slower than parent on a non parent-locked mux\n",
+				 chan_id);
+
+		/* If the mux adapter has no clock-frequency property, inherit from parent */
+		if (!priv->adap.clock_hz)
+			priv->adap.clock_hz = parent->clock_hz;
+
+		/* We don't support mux adapters faster than their parent */
+		if (priv->adap.clock_hz > parent->clock_hz) {
+			dev_err(muxc->dev,
+				"channel (%u) is faster than parent (%u)\n",
+				chan_id, priv->adap.clock_hz, parent->clock_hz);
+
+			of_node_put(mux_node);
+			goto err_free_priv;
+		}
+
 		priv->adap.dev.of_node = child;
 		of_node_put(mux_node);
 	}

-- 
2.50.1