linux-kernel - Re: [PATCH 0/7] gnss: add new GNSS subsystem

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Date:   Sat, 5 May 2018 19:28:47 +0200
From:   Sebastian Reichel <sebastian.reichel@...labora.co.uk>
To:     Pavel Machek <pavel@....cz>
Cc:     Johan Hovold <johan@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Andreas Kemnade <andreas@...nade.info>,
        Arnd Bergmann <arnd@...db.de>,
        "H . Nikolaus Schaller" <hns@...delico.com>,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH 0/7] gnss: add new GNSS subsystem

Hi,

On Fri, May 04, 2018 at 10:03:15PM +0200, Pavel Machek wrote:
> > > Finally, note that documentation (including kerneldoc) remains to be
> > > written, but hopefully this will not hinder review given that the
> > > current interfaces are fairly self-describing.
> > 
> > Great work. I like your design decisions. I have quite a few devices
> > with have non-serial based GPS peripherals using binary protocols (*).
> > As far as I can see it should be possible to add support for those.
> > 
> > I have one concern, though. While providing raw data by
> > default is fine generally, it is a problem with device
> > auto-discovery. I think there should be some IOCTL from
> > the start, that can be used to inform userspace about
> > the raw protocol being used (i.e. "NMEA"). I fear, that
> > userspace may start to just assume raw = NMEA without
> > having this (especially since all initial drivers provide
> > NMEA).
> 
> Yep, that would be nice.
> 
> > (*) I have those in mind:
> > 
> > Nokia N900: The phone has GPS integrated into the modem and uses
> > ISI encapsulated data. The protocol has been reverse engineered
> > and it should be possible to write a kernel driver for handling
> > the GPS packets and dumping the raw data to /dev/gnss0. I don't
> > think this is particularly useful without a non-raw interface,
> > though. It would still require a custom userspace implementation.
> 
> Actually... in this case it would be nice to do the protocol
> processing in kernel and just present reasonable interface to
> userland... or maybe NMEA.

Converting a binary protocol to NMEA, which needs to be string
parsed is not very nice. I think first step would be to write a
simple driver, which exposes the binary location protocol without
ISI header. Then in a second step we can think about a reasonable
interface, that should be supported by all GNSS devices.

> > Droid 4: GPS is similar to N900, but different protocol and QMI
> > encapsulated. This one also has known protocol with userspace
> > implementation. I did not yet have a detailed look, if its possible
> > to (un)wrap this in the kernel.
> 
> So, this is actually NMEA over QMI. I do have patches libqmi that
> provides NMEA on stdout.

Ok. So raw data is NMEA for this one. Should be reasonably easy to
write a driver exposing this via /dev/gnss device.

> But there seems to be another possibile interface (yes, that modem is
> crazy, and you can talk to it over few different interfaces), and
> that's NMEA over GSM07.10. That one should be feasible to decode in
> kernel and just provide NMEA to userland.

I think both should be feasible. I suggest to wait a bit more
until you and Tony figured out some more details. You've got
the libqmi patch as workaround for now and we want a stable API
later.

-- Sebastian

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