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

From: Luca Ceresoli <luca.ceresoli@bootlin.com>
To: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Cc: "Andy Shevchenko" <andriy.shevchenko@intel.com>,
	linux-media@vger.kernel.org, devicetree@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-i2c@vger.kernel.org,
	"Rob Herring" <robh+dt@kernel.org>,
	"Krzysztof Kozlowski" <krzysztof.kozlowski+dt@linaro.org>,
	"Wolfram Sang" <wsa@kernel.org>,
	"Matti Vaittinen" <Matti.Vaittinen@fi.rohmeurope.com>,
	"Laurent Pinchart" <laurent.pinchart+renesas@ideasonboard.com>,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
	"Peter Rosin" <peda@axentia.se>,
	"Liam Girdwood" <lgirdwood@gmail.com>,
	"Mark Brown" <broonie@kernel.org>,
	"Sakari Ailus" <sakari.ailus@linux.intel.com>,
	"Michael Tretter" <m.tretter@pengutronix.de>,
	"Shawn Tu" <shawnx.tu@intel.com>,
	"Hans Verkuil" <hverkuil@xs4all.nl>,
	"Mike Pagano" <mpagano@gentoo.org>,
	"Krzysztof Hałasa" <khalasa@piap.pl>,
	"Marek Vasut" <marex@denx.de>,
	"Luca Ceresoli" <luca@lucaceresoli.net>
Subject: Re: [PATCH v7 1/7] i2c: add I2C Address Translator (ATR) support
Date: Thu, 19 Jan 2023 14:00:56 +0100	[thread overview]
Message-ID: <20230119140056.686c0dea@booty> (raw)
In-Reply-To: <79331f60-0849-9d5a-822a-987df01a4b96@ideasonboard.com>

Hi Tomi, Andy,

On Thu, 19 Jan 2023 14:22:26 +0200
Tomi Valkeinen <tomi.valkeinen@ideasonboard.com> wrote:

> On 19/01/2023 13:35, Luca Ceresoli wrote:
> > Hi Tomi, Andy,
> > 
> > On Thu, 19 Jan 2023 12:09:57 +0200
> > Tomi Valkeinen <tomi.valkeinen@ideasonboard.com> wrote:
> >   
> >> On 19/01/2023 10:21, Luca Ceresoli wrote:
> >>
> >> <snip>
> >>  
> >>>>>>> +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() ?  
> >>>
> >>> Not sure I'm following you here. The i2c_atr_set_clientdata() name was
> >>> given for similarity with i2c_set_clientdata(). The latter wraps
> >>> dev_set_drvdata(), which sets `struct device`->driver_data. There is
> >>> one driver_data per each `struct device` instance, not per each driver.
> >>> The same goes for i2c_atr_set_driver_data(): there is one priv pointer
> >>> per each `struct i2c_atr` instance.  
> >>
> >> I'm a bit confused. What is "driver data" and what is "device instance
> >> data"?
> >>
> >> This deals with the driver's private data, where the "driver" is the
> >> owner/creator of the i2c-atr. The i2c-atr itself doesn't have a device
> >> (it's kind of part of the owner's device), and there's no driver in
> >> i2c-atr.c
> >>
> >> I don't like "client" here, as it reminds me of i2c_client (especially
> >> as we're in i2c context).
> >>
> >> What about i2c_atr_set_user_data()? Or "owner_data"?  
> > 
> > Ah, only now I got the point Andy made initially about "client" not
> > being an appropriate word.
> > 
> > In i2c we have:
> > 
> >    i2c_set_clientdata(struct i2c_client *client, void *data)
> >            ^^^^^^~~~~            ^^^^^^                ~~~~
> > 
> > so "client" clearly makes sense there, now here.  
> 
> Isn't that also used by the i2c_client? A driver which handles an i2c 
> device is the "i2c client", in a sense?
> 
> > The same logic applied here would lead to:
> > 
> >    i2c_atr_set_atrdata(struct i2c_atr *atr, void *data)
> >                ^^^~~~~            ^^^             ~~~~
> > 
> > which makes sense but it is a ugly IMO.  
> 
> Here, I think, there's a bit of a difference to the i2c_client case, as 
> we have a separate component for the i2c-atr. Although I guess one can 
> argue that the top level driver is the ATR driver, as it handles the HW, 
> and i2c-atr.c is just a set of helpers, so... I don't know =).
> 
> > So I think i2c_atr_get_driver_data() in this v7 makes sense, it's to
> > set the data that the ATR driver instance needs.
> > 
> > This is coherent with logic in spi/spi.h:
> > 
> >    spi_set_drvdata(struct spi_device *spi, void *data)
> > 
> > except for the abbreviation ("_drvdata" vs "_driver_data").
> > 
> > Andy, Tomi, would i2c_atr_set_drvdata() be OK for you, just like SPI
> > does?  
> 
> Well, I'm good with the current i2c_atr_set_driver_data(). If all agrees 
> that it's "driver data", I'd rather keep it like that. I find this 
> "drvdata" style very odd. Why no underscore between drv and data? Why 
> abbreviate drv, it doesn't really help anything here?

Agreed, I'm OK with either form of "driver data".

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