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

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Date:   Thu, 25 Aug 2016 04:43:40 +0200
From:   Matthijs van Duin <matthijsvanduin@...il.com>
To:     Nishanth Menon <nm@...com>
Cc:     "H. Nikolaus Schaller" <hns@...delico.com>,
        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 <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

On 13 January 2016 at 19:18, Nishanth Menon <nm@...com> wrote:
> As you already see it is ridiculously round about way of protecting RTC
> time.. but anyways, for what ever reason, that was mandatory function to
> support on certain product lines.

Having secure date/time is probably necessary for some digital rights
management schemes; e.g. you rent a movie for limited time, but it may
not always be acceptable to require working internet connectivity to
be able to hit the play button hence the need to rely on a secure RTC.

There wouldn't even be an SMC for setting the RTC, probably it would
synchronize when the secure-world shizzle contacts the Big Server
O'Secrety Bits. :P

Protecting pinmux through the L4 firewall sounds hilarious: the whole
ctrl_core module (0x4a002000 - 0x4a002fff) is a single firewall
region. All permitted access to it by linux would have to be
redirected to an SMC or similar.

On 13 January 2016 at 20:05, Nishanth Menon <nm@...com> wrote:
> An internal feedback I got some time back on AM57 (not OMAP5) - context
> was that we were discussing if an external pull up resistor was needed
> for a GPIO button:
> "Internal pull-ups are relatively weak (ranging to 100kOhm or higher)

Unlike the OMAP5, AM57xx uses 1.8V/3.3V drivers for its generic IOs,
which have to do magic to not get fried by such high voltages.
Crappier specs result, especially for pulling up to 3.3V:
1.8V mode, pull-down current while pin is held high: 50-210 uA
3.3V mode, pull-down current while pin is held high: 40-200 uA
1.8V mode, pull-up current while pin is held low: 60-200 uA
3.3V mode, pull-up current while pin is held low: 10-290 uA

Note the worst-case equivalent pull-up resistance in 3.3V mode is 330
kOhm, eleven times higher than in 1.8V mode.

Matthijs