linux-kernel - Re: [PATCH 1/2] regulator: DT: Add support to scale ramp delay based on platform behavior

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Date:	Tue, 1 Mar 2016 09:18:46 +0530
From:	Laxman Dewangan <ldewangan@...dia.com>
To:	Mark Brown <broonie@...nel.org>,
	Bjorn Andersson <bjorn.andersson@...aro.org>
CC:	<robh+dt@...nel.org>, <pawel.moll@....com>, <mark.rutland@....com>,
	<ijc+devicetree@...lion.org.uk>, <lgirdwood@...il.com>,
	<bjorn.andersson@...ymobile.com>, <swarren@...dotorg.org>,
	<devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] regulator: DT: Add support to scale ramp delay based
 on platform behavior


On Tuesday 01 March 2016 07:53 AM, Mark Brown wrote:
> * PGP Signed by an unknown key
>
> On Mon, Feb 29, 2016 at 09:47:51AM -0800, Bjorn Andersson wrote:
>> On Mon 29 Feb 06:40 PST 2016, Laxman Dewangan wrote:
>>> It is observed that voltage change in given rail affected by the load
>>> and the capacitor in the rail. This may cause the slow ramp in voltage
>>> against what PMIC has programmed.
>> The regulator-ramp-delay is a variable you can tweak on a board basis,
>> so I'm not sure what benefit it gives to be able to add a scaling
>> factor to this.
>> In my experience your HW engineer will say "you have to wait X ms", not
>> "you have to wait 125% of X ms".
>> Can you please elaborate on why the original knob isn't sufficient?
> Right, this definitely feels like the wrong thing is being specified
> here (and also like the PMIC might be going out of spec, possibly as a
> result of being overloaded) and that the existing board specific
> controls should be used.  It just doesn't correspond to the way people
> usually talk about specs for PMICs.
>

Most of PMICs offer to configure the slew rate (ramp time). For 
discussion, I am considering MAX77620 LDOs provides option to 27mV/us 
and 100mV/us.

HW team characterize the board and its rail and come up with the 
following data:
- Configure PMIC to 27mV/us for ramp time.
- With this measured value of ramp on board is 10mV/us and it is safe 
to  assume 5mv/us to consider the board variations.

So we have now two input from HW team:
1. What should be configure in PMIC.
2. And for calculation, how much ramp need to be consider.

For (1), it is 25mV/us and for (2) which 540% (27 *100/5).

Currently, we can provide the 27mv/us as ramp-delay but do not have 
option for scaling it.

My patch add for knob for (2).