Start OpenOCD on USB hotplug
Working on Embedded
I work on embedded devices with few tools and avoid GUIs and IDEs like the plague. I feel that GUIs and IDEs leave me too disconnected from what’s actually going on. While nice when walking a straight and narrow path set by others, they quickly become a huge headache when I leave the path. I like to tip-toe off the path and then blaze my own trails.
My daily tools include neovim + cscope + cgdb + embedded gcc + gdb + OpenOCD. OpenOCD talks to my SWD JTAG debugger of choice, these days it’s anything that implements CMSIS-DAP like my FRDM-K64F. The arm-none-eabi-gdb tool then talks to OpenOCD and provides me an interface to program and debug everything from Cortex-M0s to Cortex-M4s. Good enough and when it doesn’t support the latest chip, I can patch OpenOCD and charge on.
The Problem
The problem is that I don’t want to run OpenOCD manually. I’m lazy. The IDEs do all this stuff behind the scenes while you wait 30 seconds to launch each debug session. It slowly launches openocd, then arm-none-eabi-gdb then connects, then programs the target and somewhere along the line it wasted a few seconds changing it’s view/perspective.
Why can’t a lot of this happen in parallel or only happen once?
Read on my friend.
Solution
The solution is simple, as all good solutions are:
- Launch
openocdin the background whenever the debugger is plugged in via USB. Kill it when it is disconnected. Simple. - Make it easy for
gdbto reconnect toopenocd - Add a simple
gdbfunction to reprogram the chip. - Regain 10’s of minutes from each day or more!
Enter systemd
I love systemd. Most of the time it does what I want (but not always). I use systemd as a process manager to manage the lifetime of the openocd process. When udev tells systemd a certain device is plugged in, start the openocd process. When the device is removed, kill the process. Oh yeah, handle graceful initialization of the chip when gdb connects.
Setup udev
Tell udev to watch for a certain USB product identification string matching *CMSIS-DAP*and add a device alias to it. This device alias will be something systemd can act on the addition and removal of.
# Copy this file to /etc/udev/rules.d/
ACTION!="add|change", GOTO="cmsis_dap_rules_end"
SUBSYSTEM!="usb|tty|hidraw", GOTO="cmsis_dap_rules_end"
# CMSIS-DAP compatible adapters
ATTRS{product}=="*CMSIS-DAP*", MODE="664", GROUP="uucp", TAG+="systemd", ENV{SYSTEMD_ALIAS}="/sys/devices/swd/cmsis/dap/%E{ID_SERIAL_SHORT}"
LABEL="cmsis_dap_rules_end"
For reference, this file is on Github.
Place the file at /etc/udev/rules.d/98-cmsis-dap.rules and reload udev:
sudo udevadm control --reload-rules
To test if this worked, plug and unplug a FRDM-K64F with OpenSDA with udevadm monitor running:
$ udevadm monitor -up
UDEV [17366.055607] add /devices/pci0000:00/0000:00:1a.0/usb1/1-1/1-1.3/1-1.3.3/1-1.3.3:1.1/tty/ttyACM0 (tty)
ACTION=add
DEVLINKS=/dev/serial/by-path/pci-0000:00:1a.0-usb-0:1.3.3:1.1 /dev/serial/by-id/usb-MBED_MBED_CMSIS-DAP_02400226c3123e5e0000000000000000000000003ecbc3e6-if01
ID_MODEL=MBED_CMSIS-DAP
ID_MODEL_ENC=MBED\x20CMSIS-DAP
ID_MODEL_FROM_DATABASE=LPC1768
ID_SERIAL=MBED_MBED_CMSIS-DAP_02400226c3123e5e0000000000000000000000003ecbc3e6
ID_SERIAL_SHORT=02400226c3123e5e0000000000000000000000003ecbc3e6
ID_TYPE=generic
ID_USB_CLASS_FROM_DATABASE=Communications
ID_USB_DRIVER=cdc_acm
ID_USB_INTERFACES=:080650:030000:020201:0a0000:
ID_USB_INTERFACE_NUM=01
ID_VENDOR=MBED
ID_VENDOR_ENC=MBED
ID_VENDOR_FROM_DATABASE=NXP
ID_VENDOR_ID=0d28
SYSTEMD_ALIAS=/sys/devices/swd/cmsis/dap/02400226c3123e5e0000000000000000000000003ecbc3e6
TAGS=:systemd:
...
Note the SYSTEMD_ALIAS property. Udev is fulfilling it’s task of creating a device alias.
Setup systemd service
Next step is to configure systemd to launch a service while the device is attached. To do that, we need to create a service file:
# Author: Kyle Manna <kyle[at]kylemanna[d0t]com>
[Unit]
Description=OpenOCD Daemon
After=sys-devices-swd-cmsis-dap.device
BindsTo=sys-devices-swd-cmsis-dap.device
[Service]
ExecStart=/bin/sh -c 'sleep 1; exec openocd -f interface/cmsis-dap.cfg -f target/kx.cfg -c "kx.cpu configure -event gdb-attach { reset init }"'
[Install]
WantedBy=sys-devices-swd-cmsis-dap.device
Also available on Github.
Tweak the service file as needed for your target device, mine works on a NXP Kinetis FRDM-K64F dev board as well as some custom KV11 and K22 boards.
Place the service file in a place where the user’s systemd daemon (not system!) will pick it up: $HOME/.config/systemd/user/openocd-cmsis-dap.service and reload systemd:
systemctl --user daemon-reload
Next enable the service so that the binding starts it:
systemctl --user enable openocd-cmsis-dap.service
If all goes well, unplug (if currently plugged) and re-plug the CMSIS-DAP’s USB cable should launch openocd. Ensure there is an embedded target attached to the SWD interface for openocd to talk to. Verify with:
$ systemctl --user status openocd-cmsis-dap.service
● openocd-cmsis-dap.service - OpenOCD Daemon
Loaded: loaded ($HOME/.config/systemd/user/openocd-cmsis-dap.service; enabled; vendor preset: enabled)
Active: active (running) since Tue 2016-05-24 13:53:11 PDT; 3s ago
Main PID: 20333 (openocd)
CGroup: /user.slice/user-1000.slice/[email protected]/openocd-cmsis-dap.service
└─20333 /usr/bin/openocd -f interface/cmsis-dap.cfg -f target/kx.cfg -c kx.cpu configure -event gdb-attach { reset init }
May 24 13:53:12 core.hq sh[20333]: cortex_m reset_config sysresetreq
May 24 13:53:12 core.hq sh[20333]: Info : CMSIS-DAP: SWD Supported
May 24 13:53:12 core.hq sh[20333]: Info : CMSIS-DAP: Interface Initialised (SWD)
May 24 13:53:12 core.hq sh[20333]: Info : CMSIS-DAP: FW Version = 1.0
May 24 13:53:12 core.hq sh[20333]: Info : SWCLK/TCK = 0 SWDIO/TMS = 1 TDI = 0 TDO = 0 nTRST = 0 nRESET = 1
May 24 13:53:12 core.hq sh[20333]: Info : CMSIS-DAP: Interface ready
May 24 13:53:12 core.hq sh[20333]: Info : clock speed 1000 kHz
May 24 13:53:12 core.hq sh[20333]: Info : SWD IDCODE 0x2ba01477
May 24 13:53:12 core.hq sh[20333]: Info : kx.cpu: hardware has 6 breakpoints, 4 watchpoints
May 24 13:53:12 core.hq sh[20333]: Info : MDM: Chip is unsecured. Continuing.
Unplugging the CMSIS-DAP debugger will seamlessly stop it:
$ systemctl --user status openocd-cmsis-dap.service
● openocd-cmsis-dap.service - OpenOCD Daemon
Loaded: loaded ($HOME/.config/systemd/user/openocd-cmsis-dap.service; enabled; vendor preset: enabled)
Active: inactive (dead)
May 24 13:55:39 core.hq sh[20333]: Error: error writing data: (null)
May 24 13:55:39 core.hq sh[20333]: Error: error writing data: (null)
May 24 13:55:39 core.hq sh[20333]: Error: CMSIS-DAP command CMD_DISCONNECT failed.
May 24 13:55:39 core.hq sh[20333]: Error: error writing data: (null)
May 24 13:55:39 core.hq sh[20333]: Error: CMSIS-DAP command CMD_CONNECT failed.
May 24 13:55:39 core.hq sh[20333]: Error: error writing data: (null)
May 24 13:55:39 core.hq sh[20333]: Error: Could not initialize the debug port
May 24 13:55:39 core.hq sh[20333]: Examination failed, GDB will be halted. Polling again in 100ms
May 24 13:55:39 core.hq systemd[7275]: Stopping OpenOCD Daemon...
May 24 13:55:39 core.hq systemd[7275]: Stopped OpenOCD Daemon.
Tweak GDB a Little
I have two handy user-defined commands I use to simplify using gdb (via cgdb of course) and they are located in my ~/.gdbinit file:
define reconnect
target remote :3333
end
define reload
mon reset halt
make
load
mon reset init
continue
end
The reconnect command reduces the amount of typing to connect gdb to openocd. I’m lazy, and tab-completion helps even more.
The reload command runs make in my current working directory to rebuild my .elf file and then flashes it to the target. Handles explicitly halting the remote target and resuming execution to avoid surprises.
Conclusion
This systemd magic makes it so that openocd is always running before I know I need it and the gdb commands allow me to easily reflash my target for debugging.
Run this all in tmux with minicom and vim on different windows or panes and you’ll be considerably more productive.
Cheers.
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