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

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Message-ID: <52027086.1090608@wwwdotorg.org>
Date:	Wed, 07 Aug 2013 10:06:30 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Wei Ni <wni@...dia.com>
CC:	Alexander Shiyan <shc_work@...l.ru>,
	Guenter Roeck <linux@...ck-us.net>,
	"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 03:35 AM, Wei Ni wrote:
> 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.

Yes, the regulator should encoded its own startup delay. Each individual
device should handle its own requirements for delay after power is stable.

> In this patch, msleep(POWER_ON_DELAY) was used to wait the lm90 stable,
> but it seems it's unnecessary now :)

No, the driver needs to handle this properly. If the datasheet says a
delay is needed, it is.

It's probably working because in your tests the supply just happens to
be on already.
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