Date:   Tue, 21 Aug 2018 13:01:33 -0500
From:   Rob Herring <robh@...nel.org>
To:     mailinglists@...z-im-inter.net
Cc:     Andreas Färber <afaerber@...e.de>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Linux USB List <linux-usb@...r.kernel.org>,
        Linux-MIPS <linux-mips@...ux-mips.org>,
        Xue Liu <liuxuenetmail@...il.com>,
        Ben Whitten <ben.whitten@...rdtech.com>,
        devicetree@...r.kernel.org, netdev <netdev@...r.kernel.org>,
        oneukum@...e.com, Alexander Graf <agraf@...e.de>,
        LoRa_Community_Support@...tech.com,
        潘建宏 <starnight@...cu.edu.tw>,
        rehm@...omico.ch,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: serdev: How to attach serdev devices to USB based tty devices?

On Tue, Aug 21, 2018 at 11:33 AM Frank Kunz
<mailinglists@...z-im-inter.net> wrote:
>
> Am 14.08.2018 um 04:28 schrieb Andreas Färber:
> > Hi Rob et al.,
> >
> > For my LoRa network driver project [1] I have found your serdev
> > framework to be a valuable help for dealing with hardware modules
> > exposing some textual or binary UART interface.
> >
> > In particular on arm(64) and mips this allows to define an unlimited
> > number of serdev drivers [2] that are associated via their Device Tree
> > compatible string and can optionally be configured via DT properties.
> >
> > And in theory it seems serdev has also grown support for ACPI.
> >
> > Now, a growing number of vendors are placing such modules on a USB stick
> > for easy evaluation on x86_64 PC hardware, or are designing mPCIe or M.2
> > cards using their USB pins. While I do not yet have access to such a
> > device myself, it is my understanding that devices with USB-UART bridge
> > chipsets (e.g., FTDI) will show up as /dev/ttyUSBx and devices with an
> > MCU implementing the CDC USB protocol (e.g., Pico-cell gateway = picoGW)
> > will show up as /dev/ttyACMx.
> > On the Raspberry Pi I've seen that Device Tree nodes can be used to pass
> > information to on-board devices such as MAC address to Ethernet chipset,
> > but that does not seem all that useful for passing a serdev child node
> > to hot-plugged devices at unpredictable hub/port location (where it
> > should not interfere with regular USB-UART cables for debugging), nor
> > would it help ACPI based platforms such as x86_64.
> >
> > My idea then was that if we had some unique criteria like vendor and
> > product IDs (or whatever is supported in usb_device_id), we could write
> > a usb_driver with suitable USB_DEVICE*() macro. In its probe function we
> > could call into the existing tty driver's probe function and afterwards
> > try creating and attaching the appropriate serdev device, i.e. a fixed
> > USB-to-serdev driver mapping. Problem is that most devices don't seem to
> > implement any unique identifier I could make this depend on - either by
> > using a standard FT232/FT2232/CH340G chip or by using STMicroelectronics
> > virtual com port identifiers in CDC firmware and only differing in the
> > textual description [3] the usb_device_id does not seem to match on.
> >
> > The obvious solution would of course be if hardware vendors could revise
> > their designs to configure FTDI/etc. chips uniquely. I hear that that
> > may involve exchanging the chipset, increasing costs, and may impact
> > existing drivers. Wouldn't help for devices out there today either.
>
> They need to put an extra eeprom (cents) into their design and program it.
>
> >
> > For the picoGW CDC firmware, Semtech does appear to own a USB vendor ID,
> > so it would seem possible to allocate their own product IDs for SX1301
> > and SX1308 respectively to replace the generic STMicroelectronics IDs,
> > which the various vendors could offer as firmware updates.
> >
> > All outside my control though.
> >
> > Oliver therefore suggested to not mess with USB drivers and instead use
> > a line discipline (ldisc). It seems that for example the userspace tool
> > slattach takes a tty device and performs an ioctl to switch the generic
> > tty device into a special N_SLIP protocol mode, implemented in [4].
> >
> > However, the existing number of such ldisc modes appears to be below 30,
> > with hardly any vendor-specific implementation, so polluting its number
> > space seems undesirable? And in some cases I would like to use the same
> > protocol implementation over direct UART and over USB, so would like to
> > avoid duplicate serdev_device_driver and tty_ldisc_ops implementations.
> >
> > Long story short, has there been any thinking about a userspace
> > interface to attach a given serdev driver to a tty device?
> >
> > Or is there, on OF_DYNAMIC platforms, a way from userspace to associate
> > a DT fragment (!= DT Overlay) with a given USB device dynamically, to
> > attach a serdev node with sub-nodes?
> >
> > Any other ideas how to cleanly solve this?
> >
> > In some cases we're talking about a "simple" AT-like command interface;
> > the picoGW implements a semi-generic USB-SPI bridge that may host a
> > choice of 2+ chipsets, which in turn has two further sub-devices with 3+
> > chipset choices (theoretically clk output and rx/tx options etc.) each.
> > (For the latter I'm thinking we'll need a serdev driver exposing a
> > regmap_bus and then implement regmap_bus based versions of the SPI
> > drivers like Ben and I refactored SX1257 in [2] last weekend.)>
>
> There is a mPCIe module (RAK833) available by RAK wireless that uses a
> FT2232 as USB-SPI bridge, not uart. I have one here for experiments. It
> is detected as generic FT2232 device on usb. As far as I understood so
> far the serdev does only support uart based communication, is there a
> chance to get USB-SPI bridged modules also working?

That should be somewhat easier than a UART because there's not the
interactions with the tty layer to deal with. You still have the issue
of what is the DT root for the FTDI device.

Rob

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