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

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

Hi Laurent,

On Fri, 20 Jan 2023 11:55:21 +0200
Laurent Pinchart <laurent.pinchart@ideasonboard.com> wrote:

> Hello,
> 
> On Thu, Jan 19, 2023 at 02:00:56PM +0100, Luca Ceresoli wrote:
> > On Thu, 19 Jan 2023 14:22:26 +0200 Tomi Valkeinen wrote:  
> > > On 19/01/2023 13:35, Luca Ceresoli wrote:  
> > > > On Thu, 19 Jan 2023 12:09:57 +0200 Tomi Valkeinen 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".  
> 
> Have you considered allowing drivers to embed i2c_atr in a larger
> structure, instead of forcing allocation through i2c_atr_new() ? Drivers
> could then use container_of() instead of the get/set driver/device data
> accessors.

Off the top of my head I don't see a good reason to not do it, and it
would be nice to have indeed.

For the sake of historical discussion, I guess I didn't do initially
just because my starting point was i2c-mux where allocation is dynamic.
But i2c_mux_alloc() also takes a 'int sizeof_priv' parameter to
allocate some extra space for private driver data. I don't love that
approach but it probably makes sense for mux devices which tend to be
very simple, not for the ATR where chips are definitely complex. Indeed
embedded i2c_atr in the larger driver-specific struct seems the best
option.

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