linux-kernel - Re: [PATCH] hwmon: (max6650) Rename the device ids to contain the hwmon suffix

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Message-ID: <CAOMwXhPdu=uNEcffKdmjKsD0q5SKiwDx_Hnv8BTbSsGzqa4SGQ@mail.gmail.com>
Date:	Mon, 10 Feb 2014 18:27:07 +0000
From:	Laszlo Papp <lpapp@....org>
To:	Jean Delvare <jdelvare@...e.de>
Cc:	Lee Jones <lee.jones@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>, lm-sensors@...sensors.org
Subject: Re: [PATCH] hwmon: (max6650) Rename the device ids to contain the
 hwmon suffix

On Mon, Feb 10, 2014 at 5:43 PM, Jean Delvare <jdelvare@...e.de> wrote:
> Hi Lee,
>
> On Mon, 10 Feb 2014 16:58:53 +0000, Lee Jones wrote:
>> > > >  static const struct i2c_device_id max6650_id[] = {
>> > > > -       { "max6650", 1 },
>> > > > -       { "max6651", 4 },
>> > > > +       { "max6650-hwmon", 1 },
>> > > > +       { "max6651-hwmon", 4 },
>> >
>> > No, this is not acceptable, sorry. This will change the name of the
>> > hwmon device as seen from user-space, breaking any configuration file
>> > referring to it. Additionally, dashes are explicitly forbidden in hwmon
>> > device names. And lastly this will break any explicit instantiation of
>> > theses devices (which is the only way, as the driver doesn't support
>> > device auto-detection), be it in the kernel itself or from user-space.
>> >
>> > The change doesn't make sense anyway. If you move to the MFD framework,
>> > the core driver will be an I2C driver binding to the I2C device, and it
>> > will spawn the logical devices, presumably in the form of platform
>> > devices. That's what the current max6650 driver would have to bind to.
>> > Just renaming the device won't work, you also need to change the type.
>> >
>> > If you want to turn this into an MFD driver, I believe you must first
>> > convert the hwmon part to register using
>> > devm_hwmon_device_register_with_groups(). This will dissociate the i2c
>> > device name from the hwmon device name and create a clean name-space
>> > for each function. Guenter, maybe you had a plan to do so already
>> > anyway?
>> >
>> > That being said, going with MFD in this case seems quite overkill to
>> > me. MFD makes a lot of sense when each function has its own resources.
>> > As this isn't the case here, a single driver registering both an hwmon
>> > interface and a pinctrl interface would seem sufficient to me. But I
>> > think Guenter already discussed this in the past so I'll let him
>> > continue and decide.
>>
>> I'll get you guys decide if you want to go the MFD route or not.
>>
>> Either is okay with me, but if you do decide in favour, a name change
>> with the device type appended would be preferred. Else the core device
>> would have the same name as all of its children which would be quite
>> unworkable.
>
> No problem with that. The main (I2C) device should be named max6650 (or
> max6651), the subdevices can be named whatever you want, as long as the
> hwmon (class) device's name attribute is also "max6650" (or "max6651".)
> As stated above, this is required to preserve compatibility with
> existing users.
>
>> > > Might be worth taking the opportunity to swap out these magic numbers
>> > > now.
>> >
>> > There's nothing magic about them, they tell the driver how many fans
>> > each device supports. If you don't pass them as driver_data you'll have
>> > to derive them from the device name in the probe function.
>>
>> They're magic in that they're not easily identifiable. In the few
>> moments that I looked at the patch I assumed they were device
>> IDs. They should be clearly defined.
>
> They could have been device IDs, some drivers do that, and that would
> have been equally fine. driver_data can be anything. Best thing to do
> is to document it right above the device id array if you really find it
> confusing (I don't.) I don't know what else exactly you had in mind,
> but #defining FOUR_FANS as 4 and ONE_FAN as 1 and using that doesn't
> strike me as the best coding practice.

Err... no. 1/4 fan is not the only difference between max6650 and
max6651 ... (might be worth looking up the datasheet).
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