linux-kernel - Re: [PATCH 1/3] ARM: dts: omap5-board-common: enable rtc and charging of backup battery

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Date:	Wed, 13 Jan 2016 19:44:55 +0100
From:	"H. Nikolaus Schaller" <hns@...delico.com>
To:	Nishanth Menon <nm@...com>
Cc:	Tony Lindgren <tony@...mide.com>,
	Grygorii Strashko <grygorii.strashko@...com>,
	Laxman Dewangan <ldewangan@...dia.com>,
	Benoît Cousson <bcousson@...libre.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Russell King <linux@....linux.org.uk>,
	linux-omap <linux-omap@...r.kernel.org>,
	devicetree@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Marek Belisko <marek@...delico.com>,
	Gražvydas Ignotas <notasas@...il.com>,
	Keerthy <j-keerthy@...com>
Subject: Re: [PATCH 1/3] ARM: dts: omap5-board-common: enable rtc and charging of backup battery


Am 13.01.2016 um 19:31 schrieb Nishanth Menon <nm@...com>:

> On 01/13/2016 12:08 PM, H. Nikolaus Schaller wrote:
> [...]
> 
>>> OK. So are we sure the TWL driver will never have to toggle this pin?
>> 
>> After studying the Palmas TRM it appears that this pin just should be "high"
>> to be able to write to RTC and some scratchpad register. If the Palmas OTP
>> is programmed to use gpio7 as msecure input.
> 
> Thanks for digging it up. we dont use the scratchpad, but in some cases
> where SoC cold reset is involved, those registers may store additional
> information.

I remember a similar thing from omap3-twl4030 where the boot source is stored
so that a warm reboot searches there. But I don#t know if the OMPAP5 Boot ROM
is using that.

> 
>> 
>> Since the scratchpad is not used we can permanently enable msecure. Which
>> means that we must somehow get the driving output to be "1".
>> 
>> This can be either done by
>> * a gpio with pull-up - switched to input mode as I proposed, or
> 
> I think you intended to suggest to do a mux to gpio with just pinmux
> pull?

Yes.

> The internal pull on padconf is very weak
> - for typical needs like
> these, it is rather suggested to stick with real GPIO drive to prevent
> conditions like noise interference(for example).


well, on OMAP5 pull up/down are astonishingly strong :)
100-250µA. Which translated roughly to 7 .. 18 kOhm @ 1.8V logic.
So a noise source must be coupled by an impedance in the 1 kOhm range.
This is quite rare. So I would not worry about that.

But if there is MUX_MODE1 for this purpose that works equally well, we
should use it instead of a gpin+pullup.

BR,
Nikolaus