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

[Luca Ceresoli]

Hello Andy,

On Wed, 18 Jan 2023 16:23:53 +0200
Andy Shevchenko <andriy.shevchenko@xxxxxxxxx> wrote:

> On Wed, Jan 18, 2023 at 02:40:25PM +0200, Tomi Valkeinen wrote:
> > From: Luca Ceresoli <luca@xxxxxxxxxxxxxxxx>
> > 
> > An ATR is a device that looks similar to an i2c-mux: it has an I2C
> > slave "upstream" port and N master "downstream" ports, and forwards
> > transactions from upstream to the appropriate downstream port. But is
> > is different in that the forwarded transaction has a different slave  
> 
> is is ?
> 
> > address. The address used on the upstream bus is called the "alias"
> > and is (potentially) different from the physical slave address of the
> > downstream chip.
> > 
> > Add a helper file (just like i2c-mux.c for a mux or switch) to allow
> > implementing ATR features in a device driver. The helper takes care or
> > adapter creation/destruction and translates addresses at each transaction.  
> 
> ...
> 
> > +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.

...

> > +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().

[v3]
https://lore.kernel.org/all/20220206115939.3091265-3-luca@xxxxxxxxxxxxxxxx/

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com