Re: [PATCH v5 2/5] i2c: mux: add support for per channel bus frequency

[Andy Shevchenko <andriy.shevchenko@intel.com>]

On Fri, Feb 13, 2026 at 12:06:51PM +0100, Marcus Folkesson wrote:
> There may 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.

...

> +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_mux_core *mux_locked_ancestor = NULL;
> +	struct i2c_adapter *root;
> +	int ret;
> +
> +	if (priv->adap.clock_hz && priv->adap.clock_hz != parent->clock_hz) {
> +		mux_locked_ancestor = i2c_mux_topmost_mux_locked(adap);
> +		root = i2c_root_adapter(&adap->dev);
> +
> +		/*
> +		 * If there's a mux-locked mux in our ancestry, lock the parent
> +		 * of the topmost one. Mux-locked muxes don't propagate locking
> +		 * to their parents, so we must explicitly acquire the lock above
> +		 * the highest mux-locked ancestor to reach the root adapter.
> +		 */
> +		if (mux_locked_ancestor)
> +			i2c_lock_bus(mux_locked_ancestor->parent, I2C_LOCK_ROOT_ADAPTER);
> +
> +		ret = i2c_adapter_set_clk_freq(root, priv->adap.clock_hz);
> +
> +		if (mux_locked_ancestor)
> +			i2c_unlock_bus(mux_locked_ancestor->parent, I2C_LOCK_ROOT_ADAPTER);

> +		if (ret < 0) {

Would it (ever) have any positive returned values?
Ditto for other similar cases.

> +			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);
> +}

...

> @@ -223,6 +317,7 @@ struct i2c_adapter *i2c_root_adapter(struct device *dev)
>  }
>  EXPORT_SYMBOL_GPL(i2c_root_adapter);
>  
> +
>  struct i2c_mux_core *i2c_mux_alloc(struct i2c_adapter *parent,

Stray and unneeded change.

>  				   struct device *dev, int max_adapters,
>  				   int sizeof_priv, u32 flags,

...

> +		of_property_read_u32(child, "clock-frequency", &priv->adap.clock_hz);

Why OF-centric APIs? Muxes may and do appear on other systems as well.
Okay, this function seems fully OF-centric :-(

-- 
With Best Regards,
Andy Shevchenko