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

From: Matthijs van Duin <matthijsvanduin@gmail.com>
To: Nishanth Menon <nm@ti.com>
Cc: "H. Nikolaus Schaller" <hns@goldelico.com>,
	"Tony Lindgren" <tony@atomide.com>,
	"Grygorii Strashko" <grygorii.strashko@ti.com>,
	"Laxman Dewangan" <ldewangan@nvidia.com>,
	"Benoît Cousson" <bcousson@baylibre.com>,
	"Rob Herring" <robh+dt@kernel.org>,
	"Pawel Moll" <pawel.moll@arm.com>,
	"Mark Rutland" <mark.rutland@arm.com>,
	"Ian Campbell" <ijc+devicetree@hellion.org.uk>,
	"Kumar Gala" <galak@codeaurora.org>,
	"Russell King" <linux@arm.linux.org.uk>,
	linux-omap <linux-omap@vger.kernel.org>,
	devicetree <devicetree@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"Marek Belisko" <marek@goldelico.com>,
	"Gražvydas Ignotas" <notasas@gmail.com>,
	Keerthy <j-keerthy@ti.com>
Subject: Re: [PATCH 1/3] ARM: dts: omap5-board-common: enable rtc and charging of backup battery
Date: Thu, 25 Aug 2016 04:43:40 +0200	[thread overview]
Message-ID: <CAALWOA_JStjvXv3u46bq91Dz0sR6HwAoL8Y6t2MzPVMw5zz=Tw@mail.gmail.com> (raw)
In-Reply-To: <5696A002.6050402@ti.com>

On 13 January 2016 at 19:18, Nishanth Menon <nm@ti.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@ti.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