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

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Date:	Wed, 07 Aug 2013 12:45:05 +0400
From:	Alexander Shiyan <shc_work@...l.ru>
To:	Wei Ni <wni@...dia.com>
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 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.

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