netdev - Re: [RFC PATCH 4/6] net: wwan: add NMEA port support

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Message-ID: <CAFEp6-1tHbVgAG8LZHyzB=5c0n9D-F7d-VFe7K+LC5gYMq0Thw@mail.gmail.com>
Date: Wed, 16 Apr 2025 22:04:13 +0200
From: Loic Poulain <loic.poulain@....qualcomm.com>
To: Sergey Ryazanov <ryazanov.s.a@...il.com>
Cc: Johannes Berg <johannes@...solutions.net>,
        Andrew Lunn <andrew+netdev@...n.ch>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
        netdev@...r.kernel.org, Slark Xiao <slark_xiao@....com>,
        Muhammad Nuzaihan <zaihan@...ealasia.net>,
        Qiang Yu <quic_qianyu@...cinc.com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Johan Hovold <johan@...nel.org>
Subject: Re: [RFC PATCH 4/6] net: wwan: add NMEA port support

On Wed, Apr 9, 2025 at 1:31 AM Sergey Ryazanov <ryazanov.s.a@...il.com> wrote:
>
> Many WWAN modems come with embedded GNSS receiver inside and have a
> dedicated port to output geopositioning data. On the one hand, the
> GNSS receiver has little in common with WWAN modem and just shares a
> host interface and should be exported using the GNSS subsystem. On the
> other hand, GNSS receiver is not automatically activated and needs a
> generic WWAN control port (AT, MBIM, etc.) to be turned on. And a user
> space software needs extra information to find the control port.
>
> Introduce the new type of WWAN port - NMEA. When driver asks to register
> a NMEA port, the core allocates common parent WWAN device as usual, but
> exports the NMEA port via the GNSS subsystem and acts as a proxy between
> the device driver and the GNSS subsystem.
>
> From the WWAN device driver perspective, a NMEA port is registered as a
> regular WWAN port without any difference. And the driver interacts only
> with the WWAN core. From the user space perspective, the NMEA port is a
> GNSS device which parent can be used to enumerate and select the proper
> control port for the GNSS receiver management.
>
> CC: Slark Xiao <slark_xiao@....com>
> CC: Muhammad Nuzaihan <zaihan@...ealasia.net>
> CC: Qiang Yu <quic_qianyu@...cinc.com>
> CC: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> CC: Johan Hovold <johan@...nel.org>
> Suggested-by: Loic Poulain <loic.poulain@....qualcomm.com>
> Signed-off-by: Sergey Ryazanov <ryazanov.s.a@...il.com>
> ---
>  drivers/net/wwan/Kconfig     |   1 +
>  drivers/net/wwan/wwan_core.c | 157 ++++++++++++++++++++++++++++++++++-
>  include/linux/wwan.h         |   2 +
>  3 files changed, 156 insertions(+), 4 deletions(-)
>
[...]
> +static void wwan_port_unregister_gnss(struct wwan_port *port)
> +{
> +       struct wwan_device *wwandev = to_wwan_dev(port->dev.parent);
> +       struct gnss_device *gdev = port->gnss;
> +
> +       dev_info(&wwandev->dev, "port %s disconnected\n", dev_name(&gdev->dev));
> +
> +       gnss_deregister_device(gdev);
> +       gnss_put_device(gdev);
> +
> +       __wwan_port_destroy(port);
> +}
> +#else
> +static inline int wwan_port_register_gnss(struct wwan_port *port)
> +{
> +       __wwan_port_destroy(port);
> +       return -EOPNOTSUPP;
> +}

I don't think the wwan core should return an error in case GNSS_CONFIG
is not enabled, a wwan driver may consider aborting the full
probing/registration if one of the port registrations is failing.
Maybe we should silently ignore such ports, and/or simply print a
warning.

> +
> +static inline void wwan_port_unregister_gnss(struct wwan_port *port)
> +{
> +       WARN_ON(1);     /* This handler cannot be called */
> +}
> +#endif