Re: [PATCH v7 1/7] i2c: add I2C Address Translator (ATR) support

Andy Shevchenko
2023-01-18T17:40:15Z

On Wed, Jan 18, 2023 at 06:17:53PM +0100, Luca Ceresoli wrote:
> On Wed, 18 Jan 2023 16:23:53 +0200
> Andy Shevchenko <andriy.shevchenko@intel.com> wrote:

...

> > > +A typical example follows.
> > > +
> > > +Topology::
> > > +
> > > + Slave X @ 0x10
> > > + .-----. |
> > > + .-----. | |---+---- B
> > > + | CPU |--A--| ATR |
> > > + `-----' | |---+---- C
> > > + `-----' |
> > > + Slave Y @ 0x10
> > > +
> > > +Alias table:
> > > +
> > > +.. table::
> > > +
> > > + ====== =====
> > > + Client Alias
> > > + ====== =====
> > > + X 0x20
> > > + Y 0x30
> > > + ====== =====
> > > +
> > > +Transaction:
> > > +
> > > + - Slave X driver sends a transaction (on adapter B), slave address 0x10
> > > + - ATR driver rewrites messages with address 0x20, forwards to adapter A
> > > + - Physical I2C transaction on bus A, slave address 0x20
> > > + - ATR chip propagates transaction on bus B with address translated to 0x10
> > > + - Slave X chip replies on bus B
> > > + - ATR chip forwards reply on bus A
> > > + - ATR driver rewrites messages with address 0x10
> > > + - Slave X driver gets back the msgs[], with reply and address 0x10
> >
> > I'm not sure I got the real / virtual status of the adapters. Are the B and C
> > virtual ones, while A is the real?
>
> Let me reply, as I wrote these docs back at the times and thus I feel
> guilty in case that's unclear. :)
>
> I don't like the word "virtual" in this situation. A, B and C are all
> physical busses, made of copper and run by electrons on PCBs. B and C
> are the "remote" or "downstream" busses (w.r.t. the CPU), where the i2c
> devices are and where transactions happen using the address that the
> chip responds to. A is the "local" or "upstream" bus that is driven
> directly by the CPU (*) and where address aliases are used. Using
> aliases there is necessary because using address 0x10 would be
> ambiguous as there are two 0x10 chips out there.
>
> (*) There could be more layers of course, but still A is "closer to the
> CPU than B and C", for the sake of completeness.

Can the diagram and/or text be updated to elaborate this?

...

> > > +void i2c_atr_set_driver_data(struct i2c_atr *atr, void *data)
> > > +{
> > > + atr->priv = data;
> > > +}
> > > +EXPORT_SYMBOL_NS_GPL(i2c_atr_set_driver_data, I2C_ATR);
> > > +
> > > +void *i2c_atr_get_driver_data(struct i2c_atr *atr)
> > > +{
> > > + return atr->priv;
> > > +}
> > > +EXPORT_SYMBOL_NS_GPL(i2c_atr_get_driver_data, I2C_ATR);
> >
> > Just to be sure: Is it really _driver_ data and not _device instance_ data?
>
> It is device instance data indeed. I don't remember why this got
> changed, but in v3 it was i2c_atr_set_clientdata().

It's me who was and is against calling it clientdata due to possible
confusion with i2c_set/get_clientdata() that is about *driver data*.
I missed that time the fact that this is about device instance data.
I dunno which name would be better in this case, i2c_atr_set/get_client_priv() ?

--
With Best Regards,
Andy Shevchenko