linux-kernel - Re: [PATCH V2] regulator: core: add support for configuring turn-on time through constraints

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Message-ID: <5230AF94.60007@wwwdotorg.org>
Date:	Wed, 11 Sep 2013 11:59:48 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Laxman Dewangan <ldewangan@...dia.com>
CC:	"broonie@...nel.org" <broonie@...nel.org>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"mark.rutland@....com" <mark.rutland@....com>,
	"rob@...dley.net" <rob@...dley.net>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lgirdwood@...il.com" <lgirdwood@...il.com>
Subject: Re: [PATCH V2] regulator: core: add support for configuring turn-on
 time through constraints

On 09/11/2013 12:09 PM, Laxman Dewangan wrote:
> On Wednesday 11 September 2013 11:16 PM, Stephen Warren wrote:
>> On 09/11/2013 11:46 AM, Laxman Dewangan wrote:
>>> On Wednesday 11 September 2013 10:47 PM, Stephen Warren wrote:
>>>> - regulator-enable-ramp-delay: The time taken, in uSec, for the supply
>>>> rail to reach the target voltage, plus/minus whatever tolerance the
>>>> board design requires, once the regulator output itself has ramped up.
>>>> This value is in addition to whatever built-in ramp time is inherent in
>>>> the regulator's own internal design or configuration. This property
>>>> describes the additional ramp time required due to board design issues
>>>> such as trace capacitance and load on the supply.
>>>>
>>>> That's text repeats "additional" a bit, but I think describes the
>>>> situation correctly?
>>> I wanted to provide the absolute delay rather than additional delay on
>>> top of inherit delay from device.
>> I suppose that either is fine from a DT perspective. But, the regulator
>> drivers already know their internal delay, so presumably driver code
>> will have to take the value from DT, and subtract out whatever delay the
>> driver already embodies, in order to calculate the extra delay required?
>> Or, if this property is set, does the driver-specified delay just get
>> ignored?
> 
> Yes, if property is available then driver-specified delay get ignored.
> Delay will be used from dt provided value.

I'm not sure whether I prefer one option or the other. Perhaps Mark can
decide?

That said, what if the internal ramp rate of the regulator itself is
configurable, and can change at run-time. Won't that affect the total
time? If so, having this property represent the additional time might
better allow the total time to be recalculated based on internal
regulator ramp delay changes. Perhaps the additional time varies if the
internal ramp rate varies though, so perhaps it's not worth thinking
about this situation.

I'd suggest the following text if this property represents the total time:

- regulator-enable-ramp-delay: The time taken, in uSec, for the supply
rail to reach the target voltage, plus/minus whatever tolerance the
board design requires. This property describes the total system ramp
time required due to the combination of internal ramping of the
regulator itself, and board design issues such as trace capacitance and
load on the supply.
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