Message-ID: <56ED0F58.7060005@nvidia.com>
Date:	Sat, 19 Mar 2016 14:05:36 +0530
From:	Laxman Dewangan <ldewangan@...dia.com>
To:	Bjorn Andersson <bjorn@...o.se>
CC:	Mark Brown <broonie@...nel.org>,
	Bjorn Andersson <bjorn.andersson@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Liam Girdwood <lgirdwood@...il.com>,
	"Bjorn Andersson" <bjorn.andersson@...ymobile.com>,
	Stephen Warren <swarren@...dotorg.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Gandhar Dighe <gdighe@...dia.com>,
	"Stuart Yates" <syates@...dia.com>
Subject: Re: [PATCH 1/2] regulator: DT: Add support to scale ramp delay based
 on platform behavior


On Saturday 19 March 2016 10:01 AM, Bjorn Andersson wrote:
> On Tue, Mar 15, 2016 at 6:41 AM, Laxman Dewangan <ldewangan@...dia.com> wrote:
>> On Wednesday 02 March 2016 10:05 AM, Mark Brown wrote:
>>> * PGP Signed by an unknown key
>>>
>>> On Wed, Mar 02, 2016 at 09:05:26AM +0530, Laxman Dewangan wrote:
>>>> On Wednesday 02 March 2016 09:08 AM, Mark Brown wrote:
>>>>> You're not trying to scale the value here, you're trying to replace the
>>>>> value because the PMIC is incapable of delivering the advertised ramp
>>>>> rate.  Trying to express this as a multiple of the advertised ramp rate
>>>>> is just adding complexity.
>>>> So should we provide absolute ramp value here for platform specific?
>>> Yes, otherwise if the PMIC vendor respecifies their ramp rates to
>>> reflect reality and the driver is updated then your DT will be broken.
>>>
>>>> Or any other suggestion to handle this situation as this is very common
>>>> and
>>>> almost all our boards have this slowness on ramp.
>>> Perhaps time to have a chat with your PMIC vendors...
>>>
>> I had discussion with our HW team to get more information about this
>> variation.
>> They said that Maxim advertise the ramp time with given condition in
>> interface i.e. capacitance etc which is very generic.
>> We did the experiment with Maxim recommendation about the rail and its
>> capacitance (2.2uF) and found that measured value is same as what they
>> advertise in datasheet.
>>
>> When chip team use this PMIC with Tegra hardware specs and did the circuit
>> simulation to ensures how our boards should be designed for signal integrity
>> they suggested that the rail capacitance should be more than what Maxim
>> recommending in general to work with our silicon. So here condition get
>> changed and hence the effective ramp time.
>>
>> So here we will need two parameters:
>> advertised-ramp-delay for PMIC configurations and
>> ramp-delay which is measured one.
>>
>> Most of time, advertised-ramp-delay is same as ramp-delay and hence one
>> value from DT will be sufficient.
>> If there is difference then both value can be provided and
>> advertised-ramp-delay  will be used for PMIC configuration and rest of
>> calculation about delay will be from ramp-delay.
>>
> Generally the device driver should describe the PMIC and the device
> tree should describe the board. So the Maxim's numbers should (if
> specified at all) go into the driver and the measures/calculated
> characteristics for your board should be specified in the dt.
>
> The ramp properties in the generic regulator binding is used to inform
> the OS about the board's ramp properties.
>
>
> If I understand you correctly the Maxim PMIC can be configured to
> drive the change at different speed, this should be configured through
> a Maxim specific property. It should not reuse the generic properties
> for ramp delays.
>

Ramp delay configurations are seen on other vendor's PMIC devices also.
Therefore, I like o me generic property rather than specific to Maxim. 
Parsing can be done in the core framework and applied during setting 
machine constraints.

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