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

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Message-ID: <Y8gu4mlXUlyiFKZD@smile.fi.intel.com>
Date:   Wed, 18 Jan 2023 19:39:46 +0200
From:   Andy Shevchenko <andriy.shevchenko@...el.com>
To:     Luca Ceresoli <luca.ceresoli@...tlin.com>
Cc:     Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
        linux-media@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Wolfram Sang <wsa@...nel.org>,
        Matti Vaittinen <Matti.Vaittinen@...rohmeurope.com>,
        Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Peter Rosin <peda@...ntia.se>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Michael Tretter <m.tretter@...gutronix.de>,
        Shawn Tu <shawnx.tu@...el.com>,
        Hans Verkuil <hverkuil@...all.nl>,
        Mike Pagano <mpagano@...too.org>,
        Krzysztof Hałasa <khalasa@...p.pl>,
        Marek Vasut <marex@...x.de>,
        Luca Ceresoli <luca@...aceresoli.net>
Subject: Re: [PATCH v7 1/7] i2c: add I2C Address Translator (ATR) support

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@...el.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