linux-kernel - Re: [PATCH 4/7] dt-bindings: gnss: add u-blox binding

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Date:   Fri, 4 May 2018 07:16:00 +0200
From:   "H. Nikolaus Schaller" <hns@...delico.com>
To:     Andreas Kemnade <andreas@...nade.info>
Cc:     Johan Hovold <johan@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Arnd Bergmann <arnd@...db.de>, Pavel Machek <pavel@....cz>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH 4/7] dt-bindings: gnss: add u-blox binding

Hi Andreas,

> Am 03.05.2018 um 20:50 schrieb Andreas Kemnade <andreas@...nade.info>:
> 
> On Thu, 3 May 2018 11:35:21 +0200
> H. Nikolaus Schaller <hns@...delico.com> wrote:
> 
> 
>> I have realized that the w2sg0004 is an exception (although a Sirf chip)
>> that it does not provide a WAKEUP signal. And another significant
>> difference is that we have to keep the serdev UART enabled even if there
>> is no user-space client. Otherwise we are not able to detect unexpected
>> activity. So we unfortunately can't move serdev open/close into the .open
>> and .close ops but need to open it in probe.
>> 
> how much power does it use to keep the uart enabled? Or should it
> better be reprogrammed as gpio?

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 other
UARTs).

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-line
is already connected to the UART-rx).

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 this
way.

BR and thanks,
Nikolaus



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