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

Tomi Valkeinen
2023-01-19T12:23:04Z

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?

That said, I'm also fine with i2c_atr_set_drvdata if that's the popular
opinion (between the three of us, so far ;).

Tomi