Re: [RFCv3 2/6] i2c: add I2C Address Translator (ATR) support

Hi Matti,

On 16/03/22 15:11, Vaittinen, Matti wrote:

Hi dee Ho peeps!

On 2/6/22 13:59, Luca Ceresoli wrote:

quoted

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
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.

snip

quoted

diff --git a/drivers/i2c/Kconfig b/drivers/i2c/Kconfig
index 438905e2a1d0..c6d1a345ea6d 100644
--- a/drivers/i2c/Kconfig
+++ b/drivers/i2c/Kconfig

@@ -71,6 +71,15 @@ config I2C_MUX
  
  source "drivers/i2c/muxes/Kconfig"
  
+config I2C_ATR
+	tristate "I2C Address Translator (ATR) support"
+	help
+	  Enable support for I2C Address Translator (ATR) chips.
+
+	  An ATR allows accessing multiple I2C busses from a single
+	  physical bus via address translation instead of bus selection as
+	  i2c-muxes do.
+

I continued playing with the ROHM (de-)serializer and ended up having 
.config where the I2C_ATR was ='m', while my ATR driver was ='y' even 
though it selects the I2C_ATR.

Yep, most probably my error somewhere.

Anyways, this made me think that most of the I2C_ATR users are likely to 
just silently select the I2C_ATR, right? The I2C_ATR has no much reason 
to be compiled in w/o users, right? So perhaps the menu entry for 
selecting the I2C_ATR could be dropped(?) Do we really need this entry 
in already long list of configs to be manually picked?

Maybe we could make it a blind option, sure. The only reason it could be
useful that it's visible is that one might implement a user driver could
be written out of tree. I don't care very much about that, but it is
possible. Maybe it's the reason for I2C_MUX to be a visible option too.
Peter?

quoted

+struct i2c_atr *i2c_atr_new(struct i2c_adapter *parent, struct device *dev,
+			    const struct i2c_atr_ops *ops, int max_adapters)
+{
+	struct i2c_atr *atr;
+
+	if (!ops || !ops->attach_client || !ops->detach_client)
+		return ERR_PTR(-EINVAL);
+

I believe that most of the attach_client implementations will have 
similar approach of allocating and populating an address-pool and 
searching for first unused address. As a 'further dev' it'd be great to 
see a common helper implementation for attach/detach - perhaps so that 
the atr drivers would only need to specify the slave-address 
configuration register(s) / mask and the use a 'generic' attach/detach 
helpers. Well, just thinking how to reduce the code from actual IC 
drivers but this is really not something that is required during this 
initial series :)

Also, devm-variants would be great - although that falls to the same 
category of things that do not need to be done immediately - but would 
perhaps be worth considering in the future.

Both of your proposals make sense, however I did deliberately not
generalize too much because I knew only one chipset. I don't like trying
to generalize for an unpredictable future use case, it generally leads
(me) to generalizing in the wrong direction. That means you'd be very
welcome to propose helpers and/or devm variants, possibly in the same
patchset as the first Rohm serdes driver. ;)

Reviewed-by: Matti Vaittinen <redacted>

Thanks for your review!

-- 
Luca