Re: [PATCH v6 2/5] i2c: mux: add support for per channel bus frequency

[Peter Rosin]

Hi!

2026-02-16 at 19:50, Marcus Folkesson wrote:
> Hi Peter!
> 
> On Mon, Feb 16, 2026 at 05:40:37PM +0100, Peter Rosin wrote:
>> Hi!
>>
>>> +static struct i2c_mux_core *i2c_mux_first_mux_locked(struct i2c_adapter *adap)
>>> +{
>>> +	struct i2c_adapter *parent;
>>> +
>>> +	while ((parent = i2c_parent_is_i2c_adapter(adap)) != NULL) {
>>> +		struct i2c_mux_priv *priv = adap->algo_data;
>>
>> This assumption does not hold, making the cast pretty wild indeed. There
>> are other i2c_adapters with a parent besides muxes. See e.g. i2c_atr.c
> 
> I see. Hrm, not sure how to decide if it is a mux or not. The best I
> could come up with is to look at the i2c_adapter.lock_ops. E.g.
> 
> 
> 	while ((parent = i2c_parent_is_i2c_adapter(adap)) != NULL) {
> 		/*
> 		 * Check if this adapter is a mux channel by verifying its
> 		 * lock_ops. Only mux channels use these specific lock operations.
> 		 */
> 		if (adap->lock_ops == &i2c_mux_lock_ops ||
> 		    adap->lock_ops == &i2c_parent_lock_ops) {
> 			struct i2c_mux_priv *priv = adap->algo_data;
> 
> 			if (priv->muxc->mux_locked)
> 				return priv->muxc;
> 		}
> 		adap = parent;
> 	}
> 
> Or do you have a better idea?

That looks fragile. My recommendation would be to avoid trying to
guess how a potentially diverse adapter tree should be handled
locally in the mux code. To me, it would feel better to introduce
locking/recursion in i2c_adapter_set_clk_freq() for muxes (and
address translators), i.e. take inspiration from i2c_transfer()
and i2c_smbus_xfer().

I guess an unlocked __i2c_adapter_set_clk_freq() is needed.

Cheers,
Peter