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doc: develop: Add a general section on gdb usage
Mashed up from different sources linked below, including the now gone Wiki and doc/README.arm-relocation file. Tested on a custom board with AT91 SAMA5D2 SoC and Segger J-Link Base adapter. This is only generic advice here, the usage is not board specific. Some board docs have more specific instructions on using gdb with a particular board. Link: https://www.slideshare.net/slideshow/embedded-recipes-2019-introduction-to-jtag-debugging/177511981 Link: https://boundarydevices.com/debugging-using-segger-j-link-jtag/ Link: https://web.archive.org/web/20141224200032/http://www.denx.de/wiki/view/DULG/DebuggingUBoot Link: https://web.archive.org/web/20141206064148/http://www.denx.de/wiki/view/DULG/GDBScripts1 Suggested-by: Marek Vasut <marex@denx.de> Signed-off-by: Alexander Dahl <ada@thorsis.com> Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
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Alexander Dahl
Heinrich Schuchardt
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doc/develop/gdb.rst
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171
doc/develop/gdb.rst
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.. SPDX-License-Identifier: GPL-2.0+
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.. Copyright (c) 2024 Alexander Dahl
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Debugging U-Boot with GDB
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=========================
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Using a JTAG adapter it is possible to debug a running U-Boot with GDB.
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A common way is to connect a debug adapter to the JTAG connector of your
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board, run a GDB server, connect GDB to the GDB server, and use GDB as usual.
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Similarly QEMU can provide a GDB server.
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Preparing build
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---------------
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Building U-Boot with with reduced optimization (-Og) and without link time
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optimization is recommended for easier debugging::
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CONFIG_CC_OPTIMIZE_FOR_DEBUG=y
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CONFIG_LTO=n
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Otherwise build, install, and run U-Boot as usual.
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Using OpenOCD as GDB server
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---------------------------
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`OpenOCD <https://openocd.org/>`_ is an open source tool supporting hardware
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debug probes, and providing a GDB server. It is readily available in major Linux
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distributions or you can build it from source.
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Here is example of starting OpenOCD on Debian using a J-Link adapter and a
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board with an AT91 SAMA5D2 SoC:
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.. code-block:: console
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$ openocd -f interface/jlink.cfg -f target/at91sama5d2.cfg -c 'adapter speed 4000'
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Open On-Chip Debugger 0.12.0
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Licensed under GNU GPL v2
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For bug reports, read
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http://openocd.org/doc/doxygen/bugs.html
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Info : auto-selecting first available session transport "jtag". To override use 'transport select <transport>'.
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adapter speed: 4000 kHz
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Info : Listening on port 6666 for tcl connections
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Info : Listening on port 4444 for telnet connections
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Info : J-Link V10 compiled Jan 30 2023 11:28:07
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Info : Hardware version: 10.10
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Info : VTarget = 3.244 V
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Info : clock speed 4000 kHz
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Info : JTAG tap: at91sama5d2.cpu tap/device found: 0x5ba00477 (mfg: 0x23b (ARM Ltd), part: 0xba00, ver: 0x5)
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Info : at91sama5d2.cpu_a5.0: hardware has 3 breakpoints, 2 watchpoints
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Info : at91sama5d2.cpu_a5.0: MPIDR level2 0, cluster 0, core 0, mono core, no SMT
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Info : starting gdb server for at91sama5d2.cpu_a5.0 on 3333
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Info : Listening on port 3333 for gdb connections
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Notice that OpenOCD is listening on port 3333 for GDB connections.
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Using QEMU as GDB server
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------------------------
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When running U-Boot on QEMU you can used the '-gdb' parameter to provide a
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GDB server:
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qemu-system-riscv64 -M virt -nographic -gdb tcp::3333 -kernel u-boot
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Running a GDB session
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----------------------
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You need a GDB suited for your target. This can be the GDB coming with your
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toolchain or *gdb-multiarch* available in your Linux distribution.
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.. prompt:: bash $
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gdb-multiarch u-boot
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In the above command-line *u-boot* is the U-boot binary in your build
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directory. You may need to adjust the path when calling GDB.
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Connect to the GDB server like this:
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.. code-block:: console
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(gdb) target extended-remote :3333
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Remote debugging using :3333
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0x27fa9ac6 in ?? ()
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(gdb)
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This is fine for debugging before U-Boot relocates itself.
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For debugging U-Boot after relocation you need to indicate the relocation
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address to GDB. You can retrieve the relocation address from the U-Boot shell
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with the command *bdinfo*:
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.. code-block:: console
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U-Boot> bdinfo
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boot_params = 0x20000100
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DRAM bank = 0x00000000
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-> start = 0x20000000
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-> size = 0x08000000
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flashstart = 0x00000000
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flashsize = 0x00000000
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flashoffset = 0x00000000
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baudrate = 115200 bps
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relocaddr = 0x27f7a000
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reloc off = 0x0607a000
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Build = 32-bit
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current eth = ethernet@f8008000
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ethaddr = 00:50:c2:31:58:d4
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IP addr = <NULL>
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fdt_blob = 0x27b36060
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new_fdt = 0x27b36060
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fdt_size = 0x00003e40
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lmb_dump_all:
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memory.cnt = 0x1 / max = 0x10
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memory[0] [0x20000000-0x27ffffff], 0x08000000 bytes flags: 0
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reserved.cnt = 0x1 / max = 0x10
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reserved[0] [0x27b31d00-0x27ffffff], 0x004ce300 bytes flags: 0
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devicetree = separate
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arch_number = 0x00000000
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TLB addr = 0x27ff0000
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irq_sp = 0x27b36050
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sp start = 0x27b36040
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Early malloc usage: cd8 / 2000
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Look out for the line starting with *relocaddr* which has the address
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you need, ``0x27f7a000`` in this case.
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On most architectures (not sandbox, x86, Xtensa) the global data pointer is
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stored in a fixed register:
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============ ========
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Architecture Register
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============ ========
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arc r25
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arm r9
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arm64 x18
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m68k d7
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microblaze r31
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mips k0
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nios2 gp
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powerpc r2
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riscv gp
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sh r13
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============ ========
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On these architecture the relocation address cat be determined by
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dereferencing the global data pointer stored in register, *r9* in the example:
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.. code-block:: console
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(gdb) p/x (*(struct global_data*)$r9)->relocaddr
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$1 = 0x27f7a000
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In the GDB shell discard the previously loaded symbol file and add it once
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again with the relocation address like this:
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.. code-block:: console
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(gdb) symbol-file
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Discard symbol table from `/home/adahl/build/u-boot/v2024.04.x/u-boot'? (y or n) y
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No symbol file now.
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(gdb) add-symbol-file u-boot 0x27f7a000
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add symbol table from file "u-boot" at
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.text_addr = 0x27f7a000
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(y or n) y
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Reading symbols from u-boot...
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(gdb)
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You can now use GDB as usual, setting breakpoints, printing backtraces,
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inspecting variables, stepping through the code, etc.
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@ -60,6 +60,7 @@ Debugging
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:maxdepth: 1
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crash_dumps
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gdb
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trace
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Packaging
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