linux-kernel - Re: [PATCH v2 2/2] IIO: st_accel_i2c.c: Use probe

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Date:   Wed, 4 Jul 2018 12:49:29 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Nikolaus Voss <nikolaus.voss@...wensteinmedical.de>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Lorenzo Bianconi <lorenzo.bianconi83@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Xiongfeng Wang <xiongfeng.wang@...aro.org>,
        linux-iio@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        nv@...n.de
Subject: Re: [PATCH v2 2/2] IIO: st_accel_i2c.c: Use probe_new() instead of
 probe()

Hi Andy,

On 07/04/2018 12:19 PM, Andy Shevchenko wrote:
> Summon Javier to the discussion.
> For my opinion he is an expert in this topic.

I wouldn't call myself an expert in anything to be honest :)

[snip]

> 
>> The table is not used by the driver, but is necessary to
>>
>> a) bind an i2c device declared via i2c_board_info with type field set
>>    to one of the names of the i2c_device_id table to this driver
>> b) bind an i2c device declared via DT or ACPI but with no match in of_id/
>>    acpi_id table but an i2c_device_id table match to this driver (fallback
>>    matching)
>> c) create the right modaliases at compile time for this driver to make
>>    module auto-loading work in case of a) and b)
>

I think Nikolaus is correct, assuming that the driver can be used on legacy
platforms that register the I2C devices using board files / platform data.
In that case you still need a I2C device ID table for (a) and (c) as he said.

I don't buy on (b) though, that's a bug in my opinion. If you register an I2C
device via DT then you must have a OF device ID entry that matches the device
and the same for ACPI. You can't rely on the I2C device table to do the match.

I would also remove the struct i2c_device_id .driver_data fields from the I2C
device ID table, since are not used and just makes reading the code confusing
(only the struct i2c_device_id .name is used as far as I can see).

> Javier, just a summary of the above. Nikolaus switched one driver to
> use ->probe_new() hook and left i2c ID table at the same time.
> My understanding that this table is not anymore in use.
> 
> But I have to admit I didn't see entire picture of this. Can you shed a light?
> 

So to shed some light, in the past even {OF,ACPI}-only drivers needed an I2C ID
table because: 1) the .probe callback had a struct i2c_device_id * parameter
and 2) the I2C core always reported a modalias of the form i2c:<foo> even for
devices registered via OF.

The .probe_new callbacks solves (1) and the I2C core now reports of:N*T*Cfoo*
solving (2). So the I2C device table isn't required anymore for {OF,ACPI}-only
drivers, but it's still required for drivers that support legacy board files
that calls i2c_register_board_info() directly. Same for the old .probe callback,
it's needed if struct i2c_device_id .driver_data is used in the probe function.

Best regards,
-- 
Javier Martinez Canillas
Software Engineer - Desktop Hardware Enablement
Red Hat