Re: [PATCH v9 2/5] i2c: mux: add support for per channel bus frequency

[Andy Shevchenko]

On Tue, Mar 24, 2026 at 02:54:16PM +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 usable 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. putting 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 inherits their parent
> bus speed.

...

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

Wondering if we may use existing API to get i2c timings from fw description?
i2c_parse_fw_timings() (it might be that it needs to be split for pure fwnode
one and device wrapped).

> +
> +		/* If the mux adapter has no clock-frequency property, inherit from parent */
> +		if (!priv->adap.clock_hz)
> +			priv->adap.clock_hz = parent->clock_hz;
> +
> +		/*
> +		 * 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);
> +
> +		/* 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 (%u) than parent (%u)\n",
> +				chan_id, priv->adap.clock_hz, parent->clock_hz);
> +
> +			of_node_put(mux_node);
> +			ret = -EINVAL;
> +			goto err_free_priv;
> +		}

-- 
With Best Regards,
Andy Shevchenko