Kernel won't boot under debugger

almaz1c · 10-20-2021, 01:52 PM

Hello. Having Linux (OpenWrt) running hardware working like a charm.
CPU - is NXP's IMX6ULL.
This CPU has JTAG interface and supported by J-Link debugger.
Got U-boot booting well under JTAG debugger but having troubles with linux kernel.
Using

Code:

vmlinux

compiled with

Code:

CONFIG_DEBUG_INFO=y

and

Code:

CONFIG_DEBUG_KERNEL=y

.
Here is entry point of kernel starting screenshoot
I am able to walk through the code step by step until

Code:

__enable_mmu

function.
Here are four steps before execution stucks: 1, 2, 3 and 4.

e.g. on execution

Code:

mcr	p15, 0, r4, c2, c0, 0		@ load page table pointer

line of code it stucks. Registers state on each step shown on the right side of screenshot. Don't know anything about assembler so can't understand what is wrong.

Because of facing such issue under debugging I am pretty sure that something not initialized properly. Some things to be initialized by u-boot during production flow, e.g. on booting kernel after U-boot initialized and passes execution to the kernel.
Trying to find what is page table pointer, trying to understand what register should store what value without luck. Could someone please give me a hint how can I handle that issue? Probably how to initialize page table pointer properly?

DarrenDrapkin · 10-24-2021, 10:02 AM

I am not a big-league kernel hacker, but I having seen your screenshot, I can suggest the following. The function you are commencing with has a comment over it, that says that it will not work with MMU enabled. You will have to step over the function enabling MMU in order to continue debugging.
As to why this is, can someone else comment?

almaz1c · 10-26-2021, 05:07 AM

Quote:

Originally Posted by DarrenDrapkin

I am not a big-league kernel hacker, but I having seen your screenshot, I can suggest the following. The function you are commencing with has a comment over it, that says that it will not work with MMU enabled. You will have to step over the function enabling MMU in order to continue debugging.
As to why this is, can someone else comment?

Thank you for reply.
Got kernel partially debuggable. The trick was in adding u-boot image in memory too, set PC counter there and let u-boot pass initialization procedure. Then it loads kernel and here I am catching breakpoint at first kernel instructions:

Code:

target remote localhost:2331
monitor reset
monitor halt
monitor sleep 200
load
display /x $pc
b __hyp_stub_install
b start_kernel
add-symbol-file u-boot 0x87800000
set $pc = 0x87800000
display /x $pc
c

Now I am fighting with

Code:

-O2

optimization level. Seems Kernel won't compile with

Code:

-O0

so trying to compile it with

Code:

-Og

. Without luck, unfortunately.