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

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Date:   Thu, 19 Jan 2023 14:00:56 +0100
From:   Luca Ceresoli <luca.ceresoli@...tlin.com>
To:     Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...el.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

Hi Tomi, Andy,

On Thu, 19 Jan 2023 14:22:26 +0200
Tomi Valkeinen <tomi.valkeinen@...asonboard.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@...asonboard.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