[PATCH next] doc: develop: Add a general section on gdb usage

Alexander Dahl ada at thorsis.com
Wed Jun 26 12:47:18 CEST 2024


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 at denx.de>
Signed-off-by: Alexander Dahl <ada at thorsis.com>
---
 doc/develop/gdb.rst   | 154 ++++++++++++++++++++++++++++++++++++++++++
 doc/develop/index.rst |   1 +
 2 files changed, 155 insertions(+)
 create mode 100644 doc/develop/gdb.rst

diff --git a/doc/develop/gdb.rst b/doc/develop/gdb.rst
new file mode 100644
index 00000000000..18d97857ce1
--- /dev/null
+++ b/doc/develop/gdb.rst
@@ -0,0 +1,154 @@
+.. 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 gdbserver, connect gdb to gdbserver, and use gdb as usual.
+
+Preparing build
+---------------
+
+Building U-Boot with debug symbols and without optimizations is
+recommended for easier debugging::
+
+    CONFIG_CC_OPTIMIZE_FOR_DEBUG=y
+
+Otherwise build, install, and run U-Boot as usual.
+
+Using OpenOCD as GDB server
+---------------------------
+
+[OpenOCD]_ is a FOSS tool supporting hardware debug probes, and
+providing a GDB interface.
+It is readily available in major Linux distributions or you can build it
+from source.
+Example for starting OpenOCD from Debian with a J-Link adapter and a
+board using 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 the port 3333 the OpenOCD server is listening on for gdb
+connections.
+
+Running a gdb session
+----------------------
+
+You need a gdb suited for your target.
+This can be gdb coming with your toolchain or *gdb-multiarch* available
+in your Linux distribution.
+
+.. code-block:: console
+
+    % gdb-multiarch u-boot
+    GNU gdb (Debian 13.1-3) 13.1
+    Copyright (C) 2023 Free Software Foundation, Inc.
+    License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
+    This is free software: you are free to change and redistribute it.
+    There is NO WARRANTY, to the extent permitted by law.
+    Type "show copying" and "show warranty" for details.
+    This GDB was configured as "x86_64-linux-gnu".
+    Type "show configuration" for configuration details.
+    For bug reporting instructions, please see:
+    <https://www.gnu.org/software/gdb/bugs/>.
+    Find the GDB manual and other documentation resources online at:
+        <http://www.gnu.org/software/gdb/documentation/>.
+
+    For help, type "help".
+    Type "apropos word" to search for commands related to "word"...
+    Reading symbols from u-boot...
+    (gdb)
+
+Notice in the above command-line *u-boot* is the binary file from your
+build folder, for which you might need to adapt the path when calling
+gdb.
+
+Connect to the gdb server then:
+
+.. code-block:: console
+
+    (gdb) target extended-remote :3333
+    Remote debugging using :3333
+    0x27fa9ac6 in ?? ()
+    (gdb)
+
+This is fine for running before U-Boot relocates itself.
+For debugging U-Boot after relocation you need to tell gdb.
+You can get the relocation address from U-Boot shell with the command
+*bdinfo* like this:
+
+.. 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 at 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.
+In gdb shell discard the previously loaded symbol file and add it once
+again with that 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.
+
+.. [OpenOCD] https://openocd.org/
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index f82e148b101..f9c4bf839ee 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -60,6 +60,7 @@ Debugging
    :maxdepth: 1
 
    crash_dumps
+   gdb
    trace
 
 Packaging

base-commit: a7eada24327a40f7ef6c55c220e119839c9d4227
-- 
2.39.2



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