Re: RFI: I2c muxes I2C_MUX_LOCKED and interrupt support

[Peter Rosin <peda@axentia.se>]

Hi Phil,

On 2016-07-21 05:20, Phil Reid wrote:
> G'day Peter,
> 
> I'm looking into modifying the i2c-mux-pca954x driver to add support for
> the pca_9543 interrupt mux function.
> 
> So the first thing I need to add is a reg read function.
> However based on the changes to the i2c mux code in the 4.6 series the
> locking work around shouldn't be needed now if the mux is allocated with
> I2C_MUX_LOCKED. Currently this driver is not doing this.
> Also the same with the similar i2c-mux-pca9541 driver which does implement read.
> 
> So my question is should I change the driver to use I2C_MUX_LOCKED
> or implement the read operation the same as the i2c-mux-pca9541?

Good question. I didn't dare changing the pca9541/pca954x drivers to
be mux locked. Maybe I am too conservative?

The issue is that if you have a multi-level hierarchy of muxes, the rules
are more relaxed for mux locked muxed compared to adapter locked muxes.

I.e.
             mux3
            /
        mux1
       /    \
   root      mux4
       \
        mux2

accesses to devices on e.g. mux3 and mux2 may interleave if all muxes are
mux-locked, that will never happen for adapter-locked muxes.

Building complex hierarchies feels more likely with pca954x that with the
other muxing options. But I don't know that, and maybe none exist at all?

Anyway, the safe option is to do it like in pca9541...

Cheers,
Peter