linux-kernel - Re: [RFC 0/3] misc: new serdev based drivers for w2sg00x4 GPS module and w2cbw003 wifi/bluetooth

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Date:   Mon, 22 May 2017 21:26:39 -0500
From:   Rob Herring <robh+dt@...nel.org>
To:     "H. Nikolaus Schaller" <hns@...delico.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Benoît Cousson <bcousson@...libre.com>,
        Tony Lindgren <tony@...mide.com>,
        Russell King <linux@...linux.org.uk>,
        Thierry Reding <treding@...dia.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-omap <linux-omap@...r.kernel.org>,
        Discussions about the Letux Kernel 
        <letux-kernel@...nphoenux.org>, kernel@...a-handheld.com
Subject: Re: [RFC 0/3] misc: new serdev based drivers for w2sg00x4 GPS module
 and w2cbw003 wifi/bluetooth

On Sun, May 21, 2017 at 5:44 AM, H. Nikolaus Schaller <hns@...delico.com> wrote:
> Since our proposed API was not acceptable and the new serdev API has arrived in 4.11 kernels,
> we finally took the challenge to update the w2sg and w2cbw drivers to use the serdev API.
>
> The approach is to write a "man in the middle" driver which is on one side a serdev client
> which directly controls the UART where the device is connected to and on the other side
> presents a new tty port so that user-space software can talk to the chips as if they would
> directly talk to the UART of the SoC (e.g. ttyO1). This is similar to connecting to a remote
> serial device e.g. through USB (ttyACM) or Bluetooth UART profiles.
>
> For example gpsd or hciattach expect a /dev/tty they can control (flow control, baud rate
> etc.).

I understand from the prior discussion why you want to pass the data
thru for gps, but why do you need to do that for BT?

> Here is the result of our first hack which is working as a demo on GTA04 devices (and the
> w2cbw driver can also be used to control a GTA04 variant with WL1837).
>
> Since it is just a demo hack, the code is not yet cleaned up, nor does it completely pass
> check-patch, nor follows 100% the coding styles. And certainly has some bugs.
>
> The most significant issue is that calling tty_port_register_device() inside of the
> serdev probe() function makes the serdev probe() function to be entered a second
> time. This does not lead to big problems since we currently have minor = 0
> and this makes the second call assume the device is not available.
>
> But we have no idea why this happens and how it can be prevented.

Johan's fixes may help there, but it is intended to be temporary to
have a separate API for registering tty ports with or without serdev.

Rob