linux-kernel - Re: [RFC PATCH] hwmon: (jc42) Add I2C_CLASS

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Date:	Mon, 4 Jul 2016 16:27:07 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Alison Schofield <amsfield22@...il.com>
Cc:	Jean Delvare <jdelvare@...e.com>,
	"open list:JC42.4 TEMPERATURE SENSOR DRIVER" 
	<linux-hwmon@...r.kernel.org>, linux-kernel@...r.kernel.org,
	daniel.baluta@...il.com
Subject: Re: [RFC PATCH] hwmon: (jc42) Add I2C_CLASS_HWMON to detection class

On 07/04/2016 04:04 PM, Alison Schofield wrote:
> On Mon, Jul 04, 2016 at 02:04:34PM -0700, Guenter Roeck wrote:
>> On 07/04/2016 12:19 PM, Alison Schofield wrote:
>>> In 2011, commit 774466add7c810fd7e4c8bcf41995b6799608880 changed
>>> the detection class of these chips to I2C_CLASS_SPD based on this
>>> premise: "makes more sense because these chips always live on
>>> memory modules"
>>>
>>> Today these chips have applications beyond memory modules.
>>>
>>
>> Do you have a specific example ?
> The mcp9808 is popular among hobbyists and makers in projects that
> monitor everything from their fish pond to their wine cellar. Beyond
> that I'm just referring to Microchips datasheet and marketing words
> saying that they target it for a wide range of apps beyond memory
> devices.  I guess the chance of one of them trying to use it, with a
> Linux driver, and caring about auto detect are pretty slim.
>

A better example would be the MCP9085, which is officially JC42.4 compliant
but also suggested for use as general purpose temperature sensor in its
datasheet.

I'll add your patch to -next.

Thanks,
Guenter

> As you've guessed, I changed the Diolan for my purposes. I just found
> it odd that jc42 was the only driver in hwmon, without I2C_CLASS_HWMON,
> so I thought it was worth a look and rethink.
>
> So, no, nothing broken, nothing suspected incompatible.
>
> alisons
>
>
>>
>>> Add I2C_CLASS_HWMON as an additional detection class to allow
>>> detection by hwmon class i2c adapters.
>>>
>>
>> Practical impact should be limited, though. Most adapters have both
>> I2C_CLASS_HWMON and I2C_CLASS_SPD flags set. Besides the Diolan adapters,
>> which are experimental in nature anyway (and where it actually might make
>> sense to add I2C_CLASS_SPD), do you have an example where a JC-42 compatible
>> chip is used with an adapter which does not have I2C_CLASS_SPD set in
>> its flags ?
>>
>>> Alternative is to replace the SPD w HWMON class, but that carries
>>> risk for existing usage.
>>>
>> Yes, the driver would stop working on adapters which only have I2C_CLASS_SPD
>> set. There are only two of those, but those two presumably _do_ have memory
>> modules connected.
>>
>> Guenter
>>
>>> Signed-off-by: Alison Schofield <amsfield22@...il.com>
>>> Cc: Daniel Baluta <daniel.baluta@...il.com>
>>> ---
>>>   drivers/hwmon/jc42.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/hwmon/jc42.c b/drivers/hwmon/jc42.c
>>> index 9887d32..1537ba0 100644
>>> --- a/drivers/hwmon/jc42.c
>>> +++ b/drivers/hwmon/jc42.c
>>> @@ -538,7 +538,7 @@ static const struct i2c_device_id jc42_id[] = {
>>>   MODULE_DEVICE_TABLE(i2c, jc42_id);
>>>
>>>   static struct i2c_driver jc42_driver = {
>>> -	.class		= I2C_CLASS_SPD,
>>> +	.class		= I2C_CLASS_SPD | I2C_CLASS_HWMON,
>>>   	.driver = {
>>>   		.name	= "jc42",
>>>   		.pm = JC42_DEV_PM_OPS,
>>>
>>
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