Embedded Linux Kernel debugging using GDB and JTAG debugger

In previous posts we learned way to debug user layer application, u-boot bootloader on target hardware. Now it is time to get Linux Kernel debuggable. Here described steps to make Linux Kernel debugging on remote target hardware (IMX6ULL CPU) using JLink debugger via JTAG interface.


After getting source code and compiling it:

$ git clone https://github.com/wireless-road/imx6ull-openwrt.git
$ cd imx6ull-openwrt
$ ./compile.sh flexcan_ethernet

Kernel source code might be found at:

Let’s try to recompile kernel without optimization. Type:
$ cd ./build_dir/target-arm_cortex-a7+neon-vfpv4_musl_eabi/linux-imx6ull_cortexa7/linux-4.14.199/
$ make menuconfig
linux kernel debugging
linux kernel debugging
linux kernel debugging
surprisingly there are only two optimization options:
  • optimize for perfomance
  • optimize for size

No options like No optimization or Optimize for debugging. If you will try to walk through the image compiled with one of two options provided above you will face lot of optimized out variable values and randomly jumping Program Counter on trying to move step by step to the next instruction.
Another one an unpleasant surprise is that if you will try to edit Makefile manually to compile image with -O0/-Og you will get failed build process with lot of error messages. It turns out that you can’t just get and compile Linux Kernel without optimization. Here explained why. In short optimization used to remove dead code. But fortunately, search in google gave us explanation how to compile Linux Kernel with -Og flag. Awesome patch not approved for usage in kernel for some reasons.

Compile Linux Kernel with -Og flag

So you have to modify source code manually to build kernel optimized for debugging purpoces. Third build option to be added to Makefile:

KBUILD_CFLAGS += $(call cc-disable-warning,maybe-uninitialized,)

and ignore few compile time errors during the build process by using:
#if !defined(__CHECKER__) && !defined(CONFIG_CC_OPTIMIZE_FOR_DEBUGGING)
and add that build option to config/Config-kernel.in also in a case of OpenWrt. Here related commits.
Next thing you have to check is that JTAG pins on your target hardware not reconfigured for alternative function usage. Just check dts used by you and make sure you don’t see something like that:
 pinctrl_sai2: sai2grp {
fsl,pins = <
If you have it — just remove it but keep in mind that related peripheral will not be initialized so you will get reduced functionality.
And the last one thing:
add following configuration option to your config file:
Now everything ready to recompile image by:
./compile.sh flexcan_ethernet
or just linux kernel itself:
make target/linux/compile

IDE setup

Similar to U-boot (described here in IDE setup chapter) with only difference you have to import Linux Kernel sources (./build_dir/target-arm_cortex-a7+neon-vfpv4_musl_eabi/linux-imx6ull_cortexa7/linux-4.14.199/) instead of U-boot sources. Recompile project from Eclipse IDE so resulted image will be appeared inside the linux kernel folder.
At this moment we didn’t find way to make debugging process from Eclipse in GUI mode (only in command line mode described below) so at this moment we are using Eclipse for navigation only.

Debugging in command line mode

Let’s try to debug.

Power up board and type anything in COM port console before U-boot starts load kernel:
u-boot console

Start GDB server:

$ JLinkGDBServer -device MCIMX6Y2 -if JTAG -speed 1000

Start GDB session:

$ cd ./build_dir/target-arm_cortex-a7+neon-vfpv4_musl_eabi/linux-imx6ull_cortexa7/
$ gdb-multiarch vmlinux.debug —nx

then in gdb console:

(gdb) target remote localhost:2331
(gdb) restore flexcan_ethernet-uImage binary 0x82000000
(gdb) restore image-flexcan_ethernet.dtb binary 0x83000000
(gdb) b __hyp_stub_install
Breakpoint 1 at 0x80110b20: file arch/arm/kernel/hyp-stub.S, line 89.
(gdb) c

then in U-boot console start linux Kernel boot process:

=> bootm 82000000 — 83000000

then switch back to gdb console. You should see something like that:

Breakpoint 1, __hyp_stub_install () at arch/arm/kernel/hyp-stub.S:89
89 store_primary_cpu_mode r4, r5, r6

try to walk through the code by typing s and print registers/variables value by using p (print) command:
kernel gdb

Then set breakpoint at start_kernel function and run there:

(gdb) b start_kernel
(gdb) c
Then walk through the code using s and n commands:

kernel gdb

You may use eclipse to navigate through the code. start_kernel function that implements most of initialization things might be found at ./build_dir/target-arm_cortex-a7+neon-vfpv4_musl_eabi/linux-imx6ull_cortexa7/linux-4.14.199/init/main.c file:
start kernel

In next posts we will explain docker usage to simplify toolchain and ide setup, will try to load U-boot and kernel both using JTAG interface, will look how U-boot starts Kernel and other interesting things.