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@xxxxxxxxx> --- Changes in v2: - Added this file from v2 only Documentation/virtual/tty-ttyvs.rst | 142 ++++++++++++++++++++++++++++++++++++ 1 file changed, 142 insertions(+) create mode 100644 Documentation/virtual/tty-ttyvs.rst diff --git a/Documentation/virtual/tty-ttyvs.rst b/Documentation/virtual/tty-ttyvs.rst new file mode 100644 index 0000000..4321e05 --- /dev/null +++ b/Documentation/virtual/tty-ttyvs.rst @@ -0,0 +1,142 @@ +================================================ +Kernel driver for virtual tty null modem devices +================================================ + +Author: Rishi Gupta <gupt21@xxxxxxxxx> + +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 + +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. + +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. + +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 + +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 + + /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 + +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. + +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. + +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 +changed by passing module parameter max_num_vs_devs or by defining +max-num-vs-devs device tree property. -- 2.7.4
