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Date:   Fri, 4 May 2018 15:37:59 +0200
From:   Sebastian Reichel <sre@kernel.org>
To:     "H. Nikolaus Schaller" <hns@goldelico.com>
Cc:     Andreas Kemnade <andreas@kemnade.info>,
        Johan Hovold <johan@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Arnd Bergmann <arnd@arndb.de>, Pavel Machek <pavel@ucw.cz>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Rob Herring <robh+dt@kernel.org>,
        Tony Lindgren <tony@atomide.com>
Subject: Re: [PATCH 4/7] dt-bindings: gnss: add u-blox binding
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Hi,

On Fri, May 04, 2018 at 02:04:15PM +0200, H. Nikolaus Schaller wrote:
> > Am 04.05.2018 um 13:42 schrieb Sebastian Reichel <sre@kernel.org>:
> >> I think it does not need much more (if at all) than a gpio controller =
on
> >> the OMAP3 chip (I think the clocks are active anyways for use by the o=
ther
> >> UARTs).
> >>=20
> >> We had proposed years ago to reprogram the UART RX pin by pinmux-states
> >> into an interrupt gpio but that was rejected because it was not general
> >> enough and ugly in the device tree (an rx-gpios record where the rx-li=
ne
> >> is already connected to the UART-rx).
> >>=20
> >> Then we did experiment with tapping the UART driver and finally the
> >> serdev API was developed to solve this problem. Hence we use it now th=
is
> >> way.
> >=20
> > Having any UART active on OMAP results in the SoC not entering
> > idle/off wasting energy. For normal (i.e. not connected to a peripheral)
> > TTYs you can enable runtime autosuspend and configure the RX pin as
> > wakeup interrupt. This will wakeup the TTY on incoming traffic, but you
> > will lose the first few characters (since the serial device needs some
> > time to wakeup). This is for example supported by the N900 uart3
> > (debug uart):
> >=20
> > $ grep -A4 "&uart3 {" arch/arm/boot/dts/omap3-n900.dts=20
> > &uart3 {
> > 	interrupts-extended =3D <&intc 74 &omap3_pmx_core OMAP3_UART3_RX>;
> > 	pinctrl-names =3D "default";
> > 	pinctrl-0 =3D <&uart3_pins>;
> > };
> >=20
> > To get it working, you also need to enable autosuspend for the tty
> > in userspace (echo 3000 /sys/class/tty/ttyS2/device/power/autosuspend_d=
elay_ms).
> > This is not enabled by default due to the character loss characteristic
> > during wakeup.
> >=20
> > Having said all of this, serdev does not yet support runtime PM (at
> > all). Tony is currently looking into it. Fortunately serdev allows
> > us to enable runtime PM by default (once implemented), since we know
> > the remote side and can (hopefully) avoid losing characters (i.e.
> > with sideband wakeup gpios).
>=20
> thanks for explaining this! We originally had similar thoughts when
> proposing a w2sg0004 driver for the first time (years ago), but we can
> not accept loosing characters...
>=20
> Now, I could imagine a potential solution. The key situation why we keep
> the serdev open and listening is if the driver did try to turn the module
> off, but in fact did turn it on (because it was not in sync).
>
> It should be possible to cover this by a timer that is started
> in this case. If there is serdev data received after assuming the module
> is turned off, the driver has detected the wrong case - and can safely
> close the serdev until we want to have it powered on again.
>=20
> If there is no response after turing off, the module power state is alrea=
dy
> in sync and we can close the serdev as well - after the timeout (let's say
> 30 seconds). Then, the serdev UART can idle. We should open the serdev
> and start this timer also in the probe function to catch an initially wro=
ng
> state.

That sounds like a good plan.

> But I think we should focus on the basics of this driver first. Then it c=
an
> be optimized later on.

Definitely.

-- Sebastian.

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