Re: RFI: I2c muxes I2C_MUX_LOCKED and interrupt support

[Phil Reid <preid@electromag.com.au>]

On 21/07/2016 14:52, Peter Rosin wrote:
> 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...
>
G'day Peter

Thanks for the explanation.

However I've thought about this a bit more as I've started implementation.
The irq status reading probably doesn't need to got thru the lock work around
as they won't be getting called in the mux select / release functions.

Data read will occur on a threaded interrupt request. Which would be a similar
context to the drivers resume function which directly calls i2c_smbus_write_byte.

Is my thinking right here?


-- 
Regards
Phil Reid