linux-kernel - Re: [PATCH 1/2] hwmon: (lm90) Add power control

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Date:	Wed, 07 Aug 2013 00:50:04 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Wei Ni <wni@...dia.com>
CC:	Alexander Shiyan <shc_work@...l.ru>,
	"khali@...ux-fr.org" <khali@...ux-fr.org>,
	"swarren@...dotorg.org" <swarren@...dotorg.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lm-sensors@...sensors.org" <lm-sensors@...sensors.org>,
	Matthew Longnecker <MLongnecker@...dia.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/2] hwmon: (lm90) Add power control

On 08/07/2013 12:32 AM, Wei Ni wrote:
> On 08/07/2013 03:27 PM, Alexander Shiyan wrote:
>>> The device lm90 can be controlled by the vdd rail.
>>> Adding the power control support to power on/off the vdd rail.
>>> And make sure that power is enabled before accessing the device.
>>>
>>> Signed-off-by: Wei Ni <wni@...dia.com>
>>> ---
>>>   drivers/hwmon/lm90.c |   52 ++++++++++++++++++++++++++++++++++++++++++++++++++
>> [...]
>>> +	if (!data->lm90_reg) {
>>> +		data->lm90_reg = regulator_get(&client->dev, "vdd");
>>> +		if (IS_ERR_OR_NULL(data->lm90_reg)) {
>>> +			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));
>>> +			data->lm90_reg = NULL;
>>> +			mutex_unlock(&data->update_lock);
>>> +			return -ENODEV;
>>> +		}
>>> +	}
>>> +	if (is_enable) {
>>> +		ret = regulator_enable(data->lm90_reg);
>>> +		msleep(POWER_ON_DELAY);
>>
>> Can this delay be handled directly from regulator?
>
> I think it should be handled in the device driver.
> Because there have different delay time to wait devices stable.
>

Then why does no other caller of regulator_enable() need this ?
I don't think lm90 is so much different to other users of regulator
functionality.

Besides that, your delay is unconditional, even for static regulators
which are always on. Other callers of regulator_enable() don't need
all that complexity, which I take as sign that it is not needed here either.

Guenter

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