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Date: Fri, 8 Dec 2023 17:25:27 +0100
From: Johan Hovold <johan@kernel.org>
To: Andreas Kemnade <andreas@kemnade.info>
Cc: Tony Lindgren <tony@atomide.com>, marcel@holtmann.org,
	johan.hedberg@gmail.com, luiz.dentz@gmail.com, arnd@arndb.de,
	gregkh@linuxfoundation.org, linux-bluetooth@vger.kernel.org,
	linux-kernel@vger.kernel.org, tomi.valkeinen@ideasonboard.com,
	=?utf-8?B?UMOpdGVy?= Ujfalusi <peter.ujfalusi@gmail.com>,
	robh@kernel.org
Subject: Re: [RFC PATCH 0/3] bluetooth/gnss: GNSS support for TiWi chips
Message-ID: <ZXNDd57ImXjelBf4@hovoldconsulting.com>
References: <20231126191840.110564-1-andreas@kemnade.info>
 <20231127135424.GO5169@atomide.com>
 <20231127215108.6e985819@aktux>
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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@atomide.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