linux-kernel - Re: [RFC PATCH 0/3] bluetooth/gnss: GNSS support for TiWi chips

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Message-ID: <ZXNDd57ImXjelBf4@hovoldconsulting.com>
Date:   Fri, 8 Dec 2023 17:25:27 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Andreas Kemnade <andreas@...nade.info>
Cc:     Tony Lindgren <tony@...mide.com>, marcel@...tmann.org,
        johan.hedberg@...il.com, luiz.dentz@...il.com, arnd@...db.de,
        gregkh@...uxfoundation.org, linux-bluetooth@...r.kernel.org,
        linux-kernel@...r.kernel.org, tomi.valkeinen@...asonboard.com,
        Péter Ujfalusi <peter.ujfalusi@...il.com>,
        robh@...nel.org
Subject: Re: [RFC PATCH 0/3] bluetooth/gnss: GNSS support for TiWi chips

On Mon, Nov 27, 2023 at 09:51:08PM +0100, Andreas Kemnade wrote:
> On Mon, 27 Nov 2023 15:54:24 +0200
> Tony Lindgren <tony@...mide.com> wrote:

> > > - Output at /dev/gnssX:
> > >   AI2 vs. NMEA
> > >   The chip can be configured into sending AI2-encapsulated NMEA,
> > >   or proving data in a binary format.
> > >   Some research has to be done yet for the details.
> > >   A pile of logs is waiting for further analysis...

Can you say something more about what the protocol looks like? Is there
some common framing that can/should be stripped by the driver in both
modes?

> > > 
> > >   Arguments for/against NMEA:
> > >   + Userspace is prepared to handle it
> > >   + Power management can be easily done by the kernel
> > >   - Less functionality can be used.  
> > 
> > I'd go with NMEA format as the default setting :)
> > 
> yes, that would also be my preference.
> 
> > >   Arguments for/against AI2:
> > >   + Full functionality can be accessed from userspace (incl. A-GPS,
> > >     maybe raw satellite data)
> > >   - Userspace has to behave to have proper power management
> > >   - No freely (not even as in beer) tool available to fully use AI2,
> > >     so there will be only a real advantage after long "French Cafe"
> > >     sessions.  
> > 
> > Seems AI2 could be optionally enabled as needed with some writes
> > to /dev/gnss0 to change the mode?
> 
> Hmm, we have
> /sys/class/gnss/gnss0/type to get the mode, maybe we add some file
> to change the mode? Or having it hidden behing a module parameter
> and implement something better accessible if any need arrives?

The 'type' attribute is intended to reveal the GNSS receiver type
(class) as a hint to user space to avoid having to detect it at runtime
using heuristics.

It does not reflect which mode is currently active for receivers that
provide both a vendor specific protocol and NMEA (e.g. u-blox
receivers).

User space can currently switch modes at will by writing to /dev/gnss0
as Tony mentioned.

It may or may not make sense to make sure a particular mode is set
during probe, for example, if there's no real use for the proprietary
protocol and everyone would just switch away from it immediately.

Johan