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

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Date:	Wed, 7 Aug 2013 17:35:05 +0800
From:	Wei Ni <wni@...dia.com>
To:	Alexander Shiyan <shc_work@...l.ru>
CC:	Guenter Roeck <linux@...ck-us.net>,
	"swarren@...dotorg.org" <swarren@...dotorg.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lm-sensors@...sensors.org" <lm-sensors@...sensors.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Matthew Longnecker <MLongnecker@...dia.com>,
	"khali@...ux-fr.org" <khali@...ux-fr.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 04:45 PM, Alexander Shiyan wrote:
>> On 08/07/2013 03:50 PM, Guenter Roeck wrote:
>>> 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.
>>
>> May be I'm wrong. I noticed that in lm90 SPEC, the max of "SMBus Clock
>> Low Time" is 25ms, so I supposed that it may need about 20ms to stable
>> after power on.
>>
>> Anyway, if I remove this delay, the driver also works fine, so I will
>> remove it in my next patch.
> 
> I originally had in mind that regulator API contain own delay option.
> E.g. reg-fixed-voltage && gpio-regulator contains "startup-delay-us" property.

As I know the "startup-delay-us" is used for the regulator device, not
the consumer devices.
In this patch, msleep(POWER_ON_DELAY) was used to wait the lm90 stable,
but it seems it's unnecessary now :)

Wei.

> 
> ---
> 

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