All devices on the same I2C bus share the same clock line and the bus
frequency has therefor be chosen so that all attached devices are able
to tolarate that clock rate. IOW, the bus speed must be set for the
slowest attached device.

With I2C multiplexers/switches on the other hand, it would be possible
to have different "domains" that runs with different speeds.

Prepare for such a feature by provide an optional callback function to
change bus frequency.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
---
 include/linux/i2c.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/linux/i2c.h b/include/linux/i2c.h
index 20fd41b51d5c..d147e388dbab 100644
--- a/include/linux/i2c.h
+++ b/include/linux/i2c.h
@@ -742,6 +742,8 @@ struct i2c_adapter {
 	struct rt_mutex mux_lock;
 
 	int timeout;			/* in jiffies */
+	int clock_hz;
+	int (*set_clk_freq)(struct i2c_adapter *adap, u32 clock_hz); /* Optional */
 	int retries;
 	struct device dev;		/* the adapter device */
 	unsigned long locked_flags;	/* owned by the I2C core */
@@ -835,6 +837,17 @@ i2c_unlock_bus(struct i2c_adapter *adapter, unsigned int flags)
 	adapter->lock_ops->unlock_bus(adapter, flags);
 }
 
+static inline int
+i2c_adapter_set_clk_freq(struct i2c_adapter *adapter, u32 clock_hz)
+{
+	int ret = -EOPNOTSUPP;
+
+	if (adapter->set_clk_freq)
+		ret = adapter->set_clk_freq(adapter, clock_hz);
+
+	return ret;
+}
+
 /**
  * i2c_mark_adapter_suspended - Report suspended state of the adapter to the core
  * @adap: Adapter to mark as suspended

-- 
2.52.0

On Fri, Feb 13, 2026 at 12:06:50PM +0100, Marcus Folkesson wrote:
> All devices on the same I2C bus share the same clock line and the bus
> frequency has therefor be chosen so that all attached devices are able
> to tolarate that clock rate. IOW, the bus speed must be set for the
> slowest attached device.
> 
> With I2C multiplexers/switches on the other hand, it would be possible
> to have different "domains" that runs with different speeds.
> 
> Prepare for such a feature by provide an optional callback function to
> change bus frequency.

...

> struct i2c_adapter {

> +	int clock_hz;

Why signed? Even inconsistent with the parameter of the below.

> +	int (*set_clk_freq)(struct i2c_adapter *adap, u32 clock_hz); /* Optional */

It's already a huge struct, can we make this compile-time chosen
(when I²C muxes are not required, for example)?

...

> +static inline int
> +i2c_adapter_set_clk_freq(struct i2c_adapter *adapter, u32 clock_hz)
> +{
> +	int ret = -EOPNOTSUPP;
> +
> +	if (adapter->set_clk_freq)
> +		ret = adapter->set_clk_freq(adapter, clock_hz);
> +
> +	return ret;

The 'ret' is redundant and can be dropped completely. This makes the function
2LoCs shorter.

> +}

-- 
With Best Regards,
Andy Shevchenko