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

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Date:   Wed, 16 Mar 2022 15:25:11 +0100
From:   Luca Ceresoli <luca@...aceresoli.net>
To:     "Vaittinen, Matti" <Matti.Vaittinen@...rohmeurope.com>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>
Cc:     "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Wolfram Sang <wsa@...-dreams.de>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Kieran Bingham <kieran.bingham@...asonboard.com>,
        Jacopo Mondi <jacopo@...ndi.org>,
        Vladimir Zapolskiy <vz@...ia.com>,
        Peter Rosin <peda@...ntia.se>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
        "Mutanen, Mikko" <Mikko.Mutanen@...rohmeurope.com>
Subject: 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:
>> 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
> 
>> 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?

>> +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 <matti.vaittinen@...rohmeurope.com>

Thanks for your review!

-- 
Luca