Re: Problem with multiple i2c multiplexers on one bus, and mux bus naming

[Jean Delvare <khali-PUYAD+kWke1g9hUCZPvPmw@public.gmane.org>]

Hi Guenter,

On Sun, 17 Nov 2013 13:13:43 -0800, Guenter Roeck wrote:
> On 11/17/2013 12:41 PM, Jean Delvare wrote:
> > On Sun, 17 Nov 2013 10:56:24 -0800, Guenter Roeck wrote:
> >> This retains the old mux name if the mux is not an i2c device, and adds
> >> the i2c device address if it is. This solves the problem for me.
> >>
> >> Comments ?
> >
> > This "for me" worries me a bit. You basically restored the uniqueness
> > for the multiple I2C-based multiplexer case. But for multiple
> > GPIO-based multiplexers, for example, the name confusion still exists.
> 
> Only if some genius connects multiple separate gpio based multiplexers
> to the same bus.

This is totally possible, for a variety of reasons, including physical
location of the GPIO pins and bus segment ends, but also economical
(using the same chips on several boards.)

> In practice, though, that can be modeled as single
> gpio mux with more pins, so I didn't think that is that much of a problem.

I admit I had not considered this. But would it work if the different
GPIO pins are on different chips (say south bridge and Super-I/O for
example)? The i2c-mux-gpio driver assumes a single chip, at least when
passed by name.

> > If we are going to address this problem, I believe we should do so in a
> > way which doesn't need revisiting in a couple months. You are changing
> > adapter names people or tools may be relying on, we really don't want
> > to do this too often.
>
> Good point. One possible solution might be to add the mux type into the name.
> 	i2c-2-mux-i2c-76 (chan_id 0)
> 	i2c-2-mux-gpio-77 (chan_id 0)

Yes, that's pretty much what I had in mind.

> We could add an additional parameter to the API, such as
> 	const char *qualifier
> to provide the "i2c-76" or "gpio-77" string. If the qualifier is NULL,
> the name would revert to the old naming scheme. Would that make sense ?

Yes, although your first draft gave me some hope that i2c-mux could do
some magic to guess the type and identifier automatically. But maybe
it's not that clean to have to touch i2c-mux for every new i2c-mux-*
driver. Not that I expect too many of them... so it's mostly a matter
of principle. An explicit qualifier as you suggest above might be
cleaner.

-- 
Jean Delvare