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

[Wei Ni]

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@xxxxxxxxxx>
>>>> ---
>>>>   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.

Thanks.
Wei.

> 
> 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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