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Esp8266 GDBSTUB for Sming

Background

This is a rewrite of gdbstub based on the esp8266 Arduino project.

To use the GNU Debugger (GDB) with Sming requires your application to include some code (gdbstub) which communicates via the serial port. On the ESP8266 only UART0 may be used for this as UART1 is transmit-only.

The gdbstub code will only be built if you specify :envvar:`ENABLE_GDB` =1 when compiling your application. At startup, before your init() function is called, it will claim UART0 so your application will be unable to use it directly. Therefore, the default port for Serial is changed to UART2.

UART2 is a ‘virtual’ serial port to enable serial communications to work correctly when GDB-enabled. Read/write calls and serial callbacks are handled via gdbstub. Baud rate changes affect UART0 directly.

Note that target refers to the application being debugged, and host the development system running the GDB program.

Refer to the official GDB documention for further details.

GDB

This is the application which runs on your development system and talks to gdbstub.

  • Linux: A version of this should be available in $ESP_HOME/xtensa-lx106-elf/bin/xtensa-lx106-elf-gdb
  • Windows: At time of writing, UDK doesn’t provide a GDB application - Download and run the executable installer at SysProgs
    • Copy the C:\SysGCC\esp8266\opt\xtensa-lx106-elf\bin\xtensa-lx106-elf-gdb.exe to a suitable location.
  • Mac: ?

Usage

  • Configure gdbstub by editing gdbstub-cfg.h as required. You can also configure the options by setting ::envvar:`USER_CFLAGS` in your project’s component.mk file. e.g USER_CFLAGS=-DGDBSTUB_BREAK_ON_INIT=0.
  • Optional: Add gdb_do_break() statements to your application.
  • Run make clean, then make ENABLE_GDB=1 flash to build and flash the application with debugging enabled
  • Run gdb, depending on your configuration immediately after resetting the board or after it has run into an exception. The easiest way to do it is to use the provided script: make gdb.

To run manually, see the following variables which you can inspect using make list-config.

.. envvar:: GDB_CMDLINE

   Command line used to run GDB.



.. envvar:: GDBSTUB_DIR

   Location of the GDB stub component, and the ``gdbcmds`` file.



.. envvar:: COM_PORT_GDB

   Defaults to :envvar:`COM_PORT`, but if necessary you can change it to a different value.



.. envvar:: COM_SPEED_GDB

   Same as :envvar:`COM_SPEED_SERIAL`, which is the value compiled into the gdbstub code.



.. envvar:: GDB_UART_SWAP

   If you need to use alternate serial pins, enable this option ``GDB_UART_SWAP=1``



.. envvar:: GDB

   Path to the GDB executable being used.

Useful GDB commands

c Continue execution

q Quit and detach

where Display current stopped location

bt Show stack backtrace

disass Disassemble, disass/m to mix with source code

print expr Display a variable or other value

print func() Call a function, display result, or call func() to discard result

tui enable Provides a windowed interface within the console (only seems to work in Linux)

x/32xw $sp Display contents of stack

info reg Display register values

info break Display details of currently set breakpoints

delete Delete all breakpoints

br Set a breakpoint at the given address or function name

hbr Set a hardware breakpoint

watch Set a hardware watchpoint to detect when the value of a variable changes

These commands require GDBSTUB_ENABLE_HOSTIO to be enabled:

remote get targetfile hostfile Read a file from SPIFFS (on the target)

remote put hostfile targetfile Write a file to SPIFFS

remote delete targetfile Delete a file from SPIFFS

Eclipse

Windows:

  • Ensure Use external console for inferior is checked.
  • In connection settings, specify COM port like with leading /, e.g. /COM4

Problems connecting?

  • Switch to the debug perspective before connecting
  • Ensure serial baud rate matches your application
  • Remove or disable all breakpoints before attaching. Eclipse will attempt to set these on connection, and if any are invalid it will hang and timeout.
  • Check connectivity using command-line GDB

GDB System Calls

Applications may interact with GDB directly using system calls, for example reading input from the GDB command prompt. See the :sample:`LiveDebug` sample for a demonstration.

Note that system calls are disabled in the default configuration, so set :c:macro:`GDBSTUB_ENABLE_SYSCALL` =1 to use this feature with your application.

Known Issues and Limitations

  • Unable to set requested break/watch points
    • Cause: Due to hardware limitations, only one hardware breakpount and one hardware watchpoint are available
    • Solution: None (hardware limitation)
  • System crashes if debugger is paused for too long
    • Cause: The WiFi hardware is designed to be serviced by software periodically. It has some buffers so it will behave OK when some data comes in while the processor is busy, but these buffers are not infinite. If the WiFi hardware receives lots of data while the debugger has stopped the CPU, it is bound to crash. This will happen mostly when working with UDP and/or ICMP; TCP-connections in general will not send much more data when the other side doesn’t send any ACKs.
    • Solution: In such situations avoid pausing the debugger for extended periods
  • Software breakpoints/watchpoints (‘break’ and ‘watch’) don’t work on flash code
    • Cause: GDB handles these by replacing code with a debugging instruction, therefore the code must be in RAM.
    • Solution: Use hardware breakpoint (‘hbreak’) or use :c:macro:`GDB_IRAM_ATTR` for code which requires testing
  • If hardware breakpoint is set, single-stepping won’t work unless code is in RAM.
    • Cause: GDB reverts to software breakpoints if no hardware breakpoints are available
    • Solution: Delete hardware breakpoint before single-stepping
  • Crash occurs when setting breakpoint in HardwareTimer callback routine
    • Cause: By default, HardwareTimer uses Non-maskable Interrupts (NMI) which keep running when the debugger is paused
    • Solution: Use the timer in non-maskable mode, or enable :c:macro:`GDBSTUB_PAUSE_HARDWARE_TIMER` option
  • If gdbstub isn’t initialised then UART2 won’t work, though initialisation will succeed
    • Cause: By design, uart callbacks can be registered for UART2 at any time, before or after initialisation
    • Solution: Not really an issue, just something to be aware of
  • Error reported, “packet reply is too long”
  • Whilst GDB is attached, input cannot be passed to application
    • Cause: GDB buffers keystrokes and replays them only when the target is interrupted (e.g. via ctrl+C), rather than passing them via serial connection.
    • Solution: Application may use gdb_syscall interface to communicate with debugger. See $(SMING_HOME)/system/gdb_syscall.h, and :sample:`LiveDebug` sample.
  • No apparent way to have second ‘console’ (windows terminology) separate from GDB interface
    • Cause: Unknown
    • Solution: Is this possible with remote targets?
  • GDB (in Windows) doesn’t respond at all to Ctrl+C
    • Cause: Unknown
    • Solution: Press Ctrl+Break to ‘hard kill’ GDB. You'll probably need to do the next step as well to get it back
  • When GDB is running under windows, appears to hang when target reset or restarted
    • Cause: Unknown, may not happen on all devboards but presents with NodeMCU
    • Solution
      • quit GDB quit
      • Start terminal make terminal
      • reset board
      • quit terminal
      • run GDB again make gdb
  • Debug messages don’t appear in Eclipse
    • Cause: Unknown
    • Solution: Use command-line GDB, or a better visual debugger

Configuration

.. doxygenfile:: gdbstub-cfg.h