Re: [PATCH v1 4/4] i2c-mux: i2c_add_mux_adapter() should use -1 for auto bus num

[Jean Delvare <khali@linux-fr.org>]

On Wed, 13 Feb 2013 14:09:08 -0700, Stephen Warren wrote:
> On 02/13/2013 11:02 AM, Doug Anderson wrote:
> > The force_nr parameter to i2c_add_mux_adapter() uses 0 to signify that
> > we don't want to force the bus number of the adapter.  This is
> > non-ideal because:
> > * 0 is actually a valid bus number to request
> > * i2c_add_numbered_adapter() (which i2c_add_mux_adapter() calls) uses
> >   -1 to mean the same thing.  That means extra logic in
> >   i2c_add_mux_adapter().
> > 
> > Fix i2c_add_mux_adapter() to use -1 and update all mux drivers
> > accordingly.
> > 
> > Signed-off-by: Doug Anderson <dianders@chromium.org>
> > ---
> > Notes:
> > - If there's a good reason that force_nr uses 0 for auto then feel
> >   free to drop this patch.  I've place it at the end of the series to
> >   make it easy to just drop it.
> 
> IIRC (and I only vaguely do...) it's because:
> 
> > diff --git a/drivers/i2c/muxes/i2c-mux-gpio.c b/drivers/i2c/muxes/i2c-mux-gpio.c
> > index 9f50ef0..301ed0b 100644
> > --- a/drivers/i2c/muxes/i2c-mux-gpio.c
> > +++ b/drivers/i2c/muxes/i2c-mux-gpio.c
> > @@ -208,7 +208,7 @@ static int i2c_mux_gpio_probe(struct platform_device *pdev)
> >  	}
> >  
> >  	for (i = 0; i < mux->data.n_values; i++) {
> > -		u32 nr = mux->data.base_nr ? (mux->data.base_nr + i) : 0;
> > +		int nr = mux->data.base_nr ? (mux->data.base_nr + i) : -1;
> 
> Here, mux->data.base_nr is platform data (or copied directly from it),
> and any field in a platform data struct stored in a global variable not
> explicitly initialized would be 0, hence 0 would typically mean "no
> explicit bus number desired". Since a mux can't exist without a parent
> I2C bus, it's unlikely anyone would want a mux to be I2C bus 0, but
> rather the parent to have that number.

Yes, as I recall this is exactly the reason why the current code is the
way it is.

-- 
Jean Delvare