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

[Sebastian Reichel]

Hi Johan,

On Tue, Apr 24, 2018 at 06:34:51PM +0200, Johan Hovold wrote:
> This series adds a new subsystem for GNSS receivers (e.g. GPS
> receivers).
> 
> While GNSS receivers are typically accessed using a UART interface they
> often also support other I/O interfaces such as I2C, SPI and USB, while
> yet other devices use iomem or even some form of remote-processor
> messaging (rpmsg).
>     
> The new GNSS subsystem abstracts the underlying interface and provides a
> new "gnss" class type, which exposes a character-device interface (e.g.
> /dev/gnss0) to user space. This allows GNSS receivers to have a
> representation in the Linux device model, something which is important
> not least for power management purposes and which also allows for easy
> detection and (eventually) identification of GNSS devices.
> 
> Note that the character-device interface provides raw access to whatever
> protocol the receiver is (currently) using, such as NMEA 0183, UBX or
> SiRF Binary. These protocols are expected to be continued to be handled
> by user space for the time being, even if some hybrid solutions are also
> conceivable (e.g. to have kernel drivers issue management commands).
>
> This will still allow for better platform integration by allowing GNSS
> devices and their resources (e.g. regulators and enable-gpios) to be
> described by firmware and managed by kernel drivers rather than
> platform-specific scripts and services.
>     
> While the current interface is kept minimal, it could be extended using
> IOCTLs, sysfs or uevents as needs and proper abstraction levels are
> identified and determined (e.g. for device and feature identification).
> 
> Another possible extension is to add generic 1PPS support.
> 
> I decided to go with a custom character-device interface rather than
> pretend that these abstract GNSS devices are still TTY devices (e.g.
> /dev/ttyGNSS0). Obviously, modifying line settings or reading modem
> control signals does not make any sense for a device using, say, a
> USB (not USB-serial) or iomem interface. This also means, however, that
> user space would no longer be able to set the line speed to match a new
> port configuration that can be set using the various GNSS protocols when
> the underlying interface is indeed a UART; instead this would need to be
> taken care of by the driver.
> 
> Also note that writes are always synchronous instead of requiring user
> space to call tcdrain() after every command.
> 
> This all seems to work well-enough (e.g. with gpsd), but please let me
> know if I've overlooked something which would indeed require a TTY
> interface instead.
> 
> As proof-of-concept I have implemented drivers for receivers based on
> two common GNSS chipsets (SiRFstar and u-blox), but due to lack of
> hardware these have so far only been tested using mockup devices and a
> USB-serial-based GPS device (using out-of-tree code). [ Let me know if
> you've got any evalutation kits to spare. ]
> 
> 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).

(*) 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.

Nokia N950/N9: Those phones have an I2C connected BCM4751, which
uses (proprietary, non-reverse-engineered) MEIF binary protocol.
Nokia's kernel had a driver, which provides a similar userspace
interface (char device with raw data from chip).

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.

-- Sebastian

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