linux-kernel - Re: [PATCH v2 1/3] hwmon: (lm90) Add power control

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Date:	Thu, 08 Aug 2013 04:25:41 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Mark Brown <broonie@...nel.org>
CC:	Wei Ni <wni@...dia.com>, khali@...ux-fr.org, swarren@...dotorg.org,
	linux-kernel@...r.kernel.org, lm-sensors@...sensors.org,
	MLongnecker@...dia.com, linux-tegra@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/3] hwmon: (lm90) Add power control

On 08/08/2013 04:01 AM, Mark Brown wrote:
> On Thu, Aug 08, 2013 at 02:56:29PM +0800, Wei Ni wrote:
>
>> +	mutex_lock(&data->update_lock);
>> +
>> +	if (is_enable)
>> +		ret = regulator_enable(data->lm90_reg);
>> +	else
>> +		ret = regulator_disable(data->lm90_reg);
>> +
>> +	if (ret < 0)
>> +		dev_err(&client->dev,
>> +			"Error in %s rail vdd, error %d\n",
>> +			(is_enable) ? "enabling" : "disabling", ret);
>> +	else
>> +		dev_info(&client->dev, "success in %s rail vdd\n",
>> +			 (is_enable) ? "enabling" : "disabling");
>> +
>> +	mutex_unlock(&data->update_lock);
>
> Two things here.  One is that it's not clear what this lokc is
> protecting since the only thing in the locked region is the regulator
> operation and that is thread safe.  The other thing is that I'm not
> seeing anthing that ensures that enables and disables are matched -
> regulators are reference counted so two enables need two disables.
>
>> +	data->lm90_reg = regulator_get(&client->dev, "vdd");
>> +	if (IS_ERR_OR_NULL(data->lm90_reg)) {
>
> NULL is a valid regulator, use IS_ERR().
>
>> +		if (PTR_ERR(data->lm90_reg) == -ENODEV)
>> +			dev_info(&client->dev,
>> +				 "No regulator found for vdd. Assuming vdd is always powered.");
>> +		else
>> +			dev_warn(&client->dev,
>> +				 "Error [%ld] in getting the regulator handle for vdd.\n",
>> +				 PTR_ERR(data->lm90_reg));
>
> You shouldn't just be ignoring errors here, though there are deployment
> difficulties with making sure a stub regulator is provided.  These
> should be getting easier after the next merge window, the stubs will be
> being tweaked slightly to have an "assume it's there" option even when
> regulators are used.  Especially in cases with device tree you should be
> paying attention to -EPROBE_DEFER, that will accurately reflect if a
> regulator is present but not loaded yet.
>
> That said if you *are* going to do this you should request the
> regulator using devm_regulator_get_optional(), this is intended to
> support things that don't need regulators (though that's not the case
> here).
>
The lm90 driver works perfectly fine without regulator.

Guenter

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