Re: [PATCH v4 2/5] i2c: mux: add support for per channel bus frequency

[Marcus Folkesson]

Hi Peter,

On Thu, Feb 12, 2026 at 10:47:36PM +0100, Marcus Folkesson wrote:
> Hi Peter!
> 
> Thank you so much for your thoughs!
> 
> On Thu, Feb 12, 2026 at 04:02:16PM +0100, Peter Rosin wrote:
> > Hi!
> > 
> 
> [...]
> 
> > > +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
> > > +		 * adapter before changing bus clock.
> > > +		 */
> > 
> > The assumptions made for the "otherwise" case is wrong, I think.
> 
> I think you are right.
> 
> > Consider e.g. the case where we are parent-locket, our parent is
> > another parent-locked mux and our grand-parent is the root adapter.
> > In that case we also have all the locks we need. Trying to grab
> > them again will be a deadlock.
> > 
> > The more correct approach is to do the parent walk in search of
> > the first ancestor mux that is not parent-locked and then call
> 
> Hrm, does it not apply for all mutex-locked ancestors?
> 
> Consider the following chain:
> Root - P1 - M1 - M2 - P2 - D1
> 
> P - Parent locked
> M - Mux locked
> D - Device
> 
> In this case we need to lock both M1 and M2, not just M2 ?
> I'm not completely sure though, I need to refresh myself on the code
> base.

Okay, after a good night's sleep, I think it should lock not the first
but topmost not parent-locked mux only, IOW, M1 in the example above.
Then the locking would be propageted up to the root adapter.

I will send out a v5 for this soon.

Best regards,
Marcus Folkesson