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