Date:	Mon, 09 Sep 2013 22:54:38 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Wei Ni <wni@...dia.com>
CC:	Mark Brown <broonie@...nel.org>,
	"khali@...ux-fr.org" <khali@...ux-fr.org>,
	"swarren@...dotorg.org" <swarren@...dotorg.org>,
	"lm-sensors@...sensors.org" <lm-sensors@...sensors.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/2] hwmon: (lm90) Add power control

On 09/09/2013 10:39 PM, Wei Ni wrote:
> On 09/10/2013 12:50 PM, Guenter Roeck wrote:
>> On 09/09/2013 09:05 PM, Wei Ni wrote:
>>> On 09/10/2013 04:39 AM, Mark Brown wrote:
>>>> * PGP Signed by an unknown key
>>>>
>>>> On Mon, Sep 09, 2013 at 09:17:35AM -0700, Guenter Roeck wrote:
>>>>> On Mon, Sep 09, 2013 at 05:02:37PM +0100, Mark Brown wrote:
>>>>
>>>>>> It does, though it gets complicated trying to use it for a case like
>>>>>> this since you can't really tell if the regulator was powered on
>>>>>> immediately before the device got probed by another device on the bus.
>>>>
>>>>> Why not ? Just keep a timestamp.
>>>>
>>>> The support is a callback on state changes; we could keep a timestamp
>>>> but there's still going to be race conditions around bootloaders.  It's
>>>> doable though.
>>>>
>>>>>>> On a higher level, I wonder if such functionality should be added in the i2c
>>>>>>> subsystem and not in i2c client drivers. Has anyone thought about this ?
>>>>
>>>>>> I'm not sure what the subsystem would do for such delays?  It's fairly
>>>>>> common for things that need this to also want to do things like
>>>>>> manipulate GPIOs as part of the power on sequence so the applicability
>>>>>> is relatively limited, plus it's not even I2C specific, the same applies
>>>>>> to other buses so it ought to be a driver core thing.
>>>>
>>>>> Possibly. I just thought about i2c since it also takes care of basic
>>>>> devicetree bindings. Something along the line of
>>>>> 	if devicetree bindings for this device declare one or more
>>>>> 	regulators, enable those regulators before calling the driver
>>>>> 	probe function.
>>>>
>>>> That's definitely a driver core thing, not I2C - there's nothing
>>>> specific to I2C in there at all, needing power is pretty generic.  I
>>>> have considered this before, something along the lines of what we have
>>>> for pinctrl, but unfortunately the generic case isn't quite generic
>>>> enough to make it easy.  It'd need to be an explicit list of regulators
>>>> (partly just to make it opt in and avoid breaking things) and you'd want
>>>> to have a way of handling the different suspend/resume behaviour that
>>>> devices want.  There's a few patterns there.
>>>>
>>>> It's definitely something I think about from time to time and it would
>>>> be useful to factor things out, the issue is getting a good enough model
>>>> of what's going on.
>>>>
>>>>>> There was some work on a generic helper for power on sequences but it
>>>>>> stalled since it wasn't accepted for the original purpose (LCD panel
>>>>>> power ons IIRC).
>>>>
>>>>> Too bad. I think it could be kept quite simple, though, by handling it
>>>>> through the regulator subsystem as suggested above. A generic binding
>>>>> for a per-regulator and per-device poweron delay should solve that
>>>>> and possibly even make it transparent to the actual driver code.
>>>>
>>>> Lots of things have a GPIO for reset too, and some want clocks too.  For
>>>> maximum usefulness this should be cross subsystem.  I suspect the reset
>>>> controller API may be able to handle some of it.
>>>>
>>>> The regulator power on delays are already handled transparently, by the
>>>> time regulator_enable() returns the ramp should be finished.
>>>
>>> I think the regulator should encoded its own startup delay. Each
>>> individual device should handle its own requirements for delay after
>>> power is stable.
>>> The regulator_enable() will handle the delays for the regulator device.
>>> And adding the msleep(25) is for lm90 device. If without delay,
>>> sometimes the device can't work properly. If read lm90 register
>>> immediately after enabling regulator, the reading may be failed.
>>> I'm not sure if 25ms is the right value, I read the LM90 SPEC, the max
>>> of "SMBus Clock Low Time" is 25ms, so I supposed that it may need about
>>> 25ms to stable after power on.
>>>
>>
>> Problem is that you are always waiting, even if the same regulator was
>> turned on already, and even if it is a dummy regulator.
>>
>> Imagine every driver doing that. Booting would take forever, just because of
>> unnecessary delays all over the place. There has to be a better solution
>> which does not include a mandatory and potentially unnecessary wait time
>> in the driver. At a previous company we had a design with literally dozens
>> of those chip. You really want to force such a boot delay on every user ?
>>
>> But essentially you don't even know if it is needed; you are just guessing.
>> That is not an acceptable reason to add such a delay, mandatory or not.
> I think the device need time to wait stable after power on, but it's
> difficult to get an exact delay value, and this delay may also relate
> with platform design, so how about to add a optional property in the DT
> node, such as "power-on-delay-ms" ?
>

Possibly, but that still doesn't solve the problem that you are going
to wait even if the regulator was already turned on. Simple example:
A system with two sensors, both of which share the same regulator.
Each of them will require a delay after turning on power,
but only if it was just turned on and not if it was already active.

Guenter

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