Date:   Fri, 17 Apr 2020 10:46:40 +0530
From:   rishi gupta <gupt21@...il.com>
To:     Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
Cc:     Greg KH <gregkh@...uxfoundation.org>, jslaby@...e.com,
        robh+dt@...nel.org, corbet@....net,
        andriy.shevchenko@...ux.intel.com, devicetree@...r.kernel.org,
        linux-serial@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v3 4/4] tty: documentation: document how to use ttyvs driver

Thanks Mauro and Randy, I will spin v4 with above suggestions soon.

On Thu, Apr 16, 2020 at 12:54 PM Mauro Carvalho Chehab
<mchehab+huawei@...nel.org> wrote:
>
> Em Thu, 16 Apr 2020 10:26:14 +0530
> Rishi Gupta <gupt21@...il.com> escreveu:
>
> > The commit documents how to use ttyvs driver to create/delete
> > virtual tty devices, how to emulate various serial port events
> > through this driver etc.
> >
> > Signed-off-by: Rishi Gupta <gupt21@...il.com>
> > ---
> > Changes in v3:
> > - Rebased on top of v5.7-rc1
> > - Moved virtual-tty-ttyvs.rst from Documentation/virtual to Documentation/admin-guide
> >
> > Changes in v2:
> > - Added this file from v2 only
> >
> >  Documentation/admin-guide/index.rst             |   1 +
> >  Documentation/admin-guide/virtual-tty-ttyvs.rst | 142 ++++++++++++++++++++++++
> >  2 files changed, 143 insertions(+)
> >  create mode 100644 Documentation/admin-guide/virtual-tty-ttyvs.rst
> >
> > diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst
> > index 5a6269f..9a72fb8 100644
> > --- a/Documentation/admin-guide/index.rst
> > +++ b/Documentation/admin-guide/index.rst
> > @@ -50,6 +50,7 @@ ABI will be found here.
> >     :maxdepth: 1
> >
> >     sysfs-rules
> > +   virtual-tty-ttyvs
> >
> >  The rest of this manual consists of various unordered guides on how to
> >  configure specific aspects of kernel behavior to your liking.
> > diff --git a/Documentation/admin-guide/virtual-tty-ttyvs.rst b/Documentation/admin-guide/virtual-tty-ttyvs.rst
> > new file mode 100644
> > index 0000000..c30b768
> > --- /dev/null
> > +++ b/Documentation/admin-guide/virtual-tty-ttyvs.rst
> > @@ -0,0 +1,142 @@
> > +================================================
> > +Kernel driver for virtual tty null modem devices
> > +================================================
> > +
> > +Author: Rishi Gupta <gupt21@...il.com>
> > +
> > +The ttyvs driver (drivers/tty/ttyvs.c) creates virtual tty devices
> > +that can be used with standard POSIX APIs for terminal devices.
> > +
> > +Applications can write to the sysfs file provided by this driver to
> > +emulate various serial port communication events and error conditions.
> > +
> > +This driver creates a virtual card which can have 0 to 65535 virtual
> > +tty devices.
> > +
> > +Use cases
> > +=========
> > +- Automated performance and scalability testing
> > +- Serial port redirector to any other subsystem like TCP/IP
> > +- Feeding data to GPS simulator
> > +- Segregating hardware issues from software bugs quickly
> > +- Serial port communication sniffer or test sniffer application itself
> > +- Application development when hardware is still not available
> > +- Testing user space drivers & corner case by injecting handcrafted data
> > +- Migrate binary only or legacy applications to new communication medium
> > +- Analyze and reverse-engineer serial protocols
> > +- Cases where socat utility does not meet requirements for unix-like OS
> > +- Cases where available physical serial ports don't meet requirements
> > +- Product demo where data from hardware needs to be sent to the GUI app
> > +- Stress and corner case testing of user space application
> > +
> > +How to create devices
> > +=====================
> > +There are two ways to create devices:
> > +
> > +1. Using device tree:
> > +The card is modelled as a node with zero or more child nodes each
> > +representing a virtual tty device. To create a device simply define
> > +a child node with the required device parameters. This is explained
> > +in detail in DT binding file:
> > +Documentation/devicetree/bindings/serial/ttyvs.yaml
>
> There's a typo there: "modelled" -> "modeled"
>
> -
>
> I suspect you didn't try to build it with Sphinx [1]. The html output of the
> above would be:
>
>         "1. Using device tree: The card is modelled as a node with zero or more child nodes each representing a virtual tty device. To create a device simply define a child node with the required device parameters. This is explained in detail in DT binding file: Documentation/devicetree/bindings/serial/ttyvs.yaml"
>
> (E.g. everything on a single line).
>
>
> [1] To build, just use:
>                 make htmldocs
>
>     it will likely point you some packages to install, in order to be
>     able to generate both html and pdf outputs. After installing them,
>     run "make htmldocs" again. The output should be under:
>
>         Documentation/output/admin-guide/virtual-tty-ttyvs.html
>
>
> For Sphinx, a paragraph requires an extra blank line. So, the
> above should be written, instead, as:
>
>         1. Using device tree:
>
>         The card is modeled as a node with zero or more child nodes each
>         representing a virtual tty device. To create a device simply define
>         a child node with the required device parameters. This is explained
>         in detail in DT binding file:
>
>         Documentation/devicetree/bindings/serial/ttyvs.yaml
>
> Personally, I would make it look a little nicer writing it instead as:
>
>         1. Using device tree
>         --------------------
>
>         The card is modeled as a node with zero or more child nodes each
>         representing a virtual tty device. To create a device simply define
>         a child node with the required device parameters. This is explained
>         in detail in DT binding file:
>
>         - Documentation/devicetree/bindings/serial/ttyvs.yaml
>
> > +
> > +2. Using configfs:
> > +When ttyvs driver is loaded, it will create ttyvs directory inside
> > +configfs mount point. For ex; if configfs is mounted at /config, then
> > +/config/ttyvs directory will be created. To create a device, simply
> > +create directory inside this, write values to be used as device
> > +parameters and finally write 1 to create attribute. Defining ownidx
> > +and devtype is mandatory.
>
> Same as above.
>
> > +
> > +Pin mappings are bit maps; set bit 0 to connect a pin to CTS pin,
> > +set bit 1 to connect to DCD pin, set bit 2 to connect to DSR and
> > +set bit 3 to connect to RI. Pin naming conventions are follows
> > +standard RS232 DB9 connector naming conventions.
>
> As Randy suggested, use a table here, like:
>
> Pin mappings are bit maps:
>
>         ===  =========================================
>         bit  meaning
>         ===  =========================================
>         0    connect a pin to CTS pin
> ...
>         3    connect to RI
>         ===  =========================================
>
> > +
> > +An example to create a loop-back device with device number as 0
> > +(/dev/ttyvs0), RTS and DTR pins unconnected, no need to assert DTR
> > +when device is opened would be something like this:
> > +
> > +.. code-block:: sh
> > +
> > + mkdir /config/ttyvs/devlb-0
> > + echo 0 > /config/ttyvs/devlb-0/ownidx
> > + echo lb > /config/ttyvs/devlb-0/devtype
> > + echo 0 > /config/ttyvs/devlb-0/ortsmap
> > + echo 0 > /config/ttyvs/devlb-0/odtrmap
> > + echo 0 > /config/ttyvs/devlb-0/odtratopn
> > + echo 1 > /config/ttyvs/devlb-0/create
>
> You need to add spaces (or tabs) at the lines under the code-block,
> as otherwise Sphinx will ignore it (and place everything on a single
> line).
>
> > +
> > +An example to create a standard null modem pair with device numbers
> > +0 and 1 with pin numbers as per RS232 standards will be something
> > +like this:
> > +
> > +.. code-block:: sh
>
> Hmm... the code below is not shell script. Btw, I would use, instead, "::"
> notation:
>
>         An example to create a standard null modem pair with device numbers
>         0 and 1 with pin numbers as per RS232 standards will be something
>         like this::
>
>                 /dev/ttyvs0        /dev/ttyvs1
> ...
>
>
> > +
> > + /dev/ttyvs0        /dev/ttyvs1
> > +   TX  (3)   ---->    (2) RX
> > +   RX  (2)   <----    (3) TX
> > +   RTS (7)   ---->    (8) CTS
> > +   DTR (4)   --+->    (1) DCD
> > +               +->    (6) DSR
> > +   CTS (8)   <----    (7) RTS
> > +   DCD (1)   <-+--    (4) DTR
> > +   DSR (6)   <-+
>
>
> > +
> > + mkdir /config/ttyvs/devnm-0-1
> > + echo nm > /config/ttyvs/devnm-0-1/devtype
> > + echo 0 > /config/ttyvs/devnm-0-1/ownidx
> > + echo 1 > /config/ttyvs/devnm-0-1/ortsmap
> > + echo 6 > /config/ttyvs/devnm-0-1/odtrmap
> > + echo 0 > /config/ttyvs/devnm-0-1/odtratopn
> > + echo 1 > /config/ttyvs/devnm-0-1/peeridx
> > + echo 1 > /config/ttyvs/devnm-0-1/prtsmap
> > + echo 6 > /config/ttyvs/devnm-0-1/pdtrmap
> > + echo 0 > /config/ttyvs/devnm-0-1/pdtratopn
> > + echo 1 > /config/ttyvs/devnm-0-1/create
>
> Ok, the above is a shell code block, but it is not part of the
> ASCII artwork. So, it deserves its own code block.
>
> > +
> > +Directory name devnm-0-1 can be user defined. We used this simple style
> > +as it is intuitive to understand that the device is null modem with
> > +numbers 0 and 1. Further, to use configfs based approach, kernel must
> > +be compiled with CONFIG_CONFIGFS_FS=y option.
>
> While not mandatory, I would use this for constants:
>
>         ``CONFIG_CONFIGFS_FS=y``
>
> (this changes the font to monotonic).
>
> On the documents I write myself, I use this for variables, paths, modprobe
> parameters, config options, inlined code snippets, etc.
>
> A side effect is that several text editors (emacs, pico, vim, kate, ...) that
> will highlight those in-lined code blocks with a different color, with helps
> a lot, even when looking on them as plain texts.
>
> > +
> > +How to delete devices
> > +=====================
> > +To delete a device created by configfs simply delete the directory
> > +created in /config/ttyvs directory. If the device is part of a null
> > +modem pair, peer device will also be deleted automatically.
> > +
> > +How to emulate events
> > +=====================
> > +When a virtual tty device is created, an event sysfs file will also
> > +be created by the driver (/sys/class/tty/ttyvsN/event N is device
> > +number).
> > +
> > +1. Emulating framing error: the driver inserts -7 in data buffer as
> > +the byte that got corrupted due to framing error while receiving data.
> > +To emulate this write 1 to /sys/class/tty/ttyvsN/event file.
>
> You need to properly indent lists:
>
>         1. Emulating framing error: the driver inserts -7 in data buffer as
>            the byte that got corrupted due to framing error while receiving data.
>            To emulate this write 1 to /sys/class/tty/ttyvsN/event file.
>
>
> > +
> > +2. Emulating parity error: the driver inserts -8 in data buffer as
> > +the byte that got corrupted due to parity error while receiving data.
> > +To emulate this write 2 to /sys/class/tty/ttyvsN/event file.
> > +
> > +3. Emulating overrun error: the driver reports to tty layer that an
> > +overrun has happened.To emulate this write 3 to /sys/class/tty/ttyvsN/event
> > +file.
> > +
> > +4. Emulating ring indication: to emulate as if ring indication has been
> > +observed write 4 to the event file. To emulate as if ring indication has
> > +been removed write 5 to the event file.
> > +
> > +5. Emulate break received: to emulate as if break condition has been received
> > +write 6 to the /sys/class/tty/ttyvsN/event file.
> > +
> > +6. Emulate faulty cable: to emulate as if the cable is faulty write 7
> > +to the event file. In this case data sent from sender will not be received
> > +by the receiver end. To remove this condition write 8 to the event file.
> > +
> > +How to support more devices
> > +===========================
> > +By default ttyvs driver supports upto 64 devices. This can be
>
>         upto -> up to
>
> (ok, "upto" is a sort of slang, but I would prefer to avoid slangs on
>  documents).
>
> > +changed by passing module parameter max_num_vs_devs or by defining
> > +max-num-vs-devs device tree property.
>
> Thanks,
> Mauro

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