Kernel freeze down while booting

[Christian Stalp]

Hello, I have the issue that my kernel freezes down during the boot phase. It is most probably in the later states, when the inbuild drivers are executed and initialized, shortly before the filesystem mount. I enabled ftrace by putting "ftrace_dump_on_oops" and ftrace=function or ftrace=wakeup into the kernel commandline. But ftrace=function led to a system, that was so slowly that the issue never hit. Ftrace=wakeup was answered with a message "ftrace bootup tracer 'wakeup' not registered. But not always. I have several devices with the same kernel and image but sometimes this message shows up.
But the main problem here is, that the kernel freezes sometimes down and doesn't has a chance to throw an oops. And it's different where it freeze each time. Which is most probably due the problem that printk doesn't write its buffer to the console each time it's invoked. So printk isn't a utility to pinpoint exactly where the kernel freezes. And ftrace neither, because it's within the kernel and freezes down with the whole kernel. At least in the way I using it. Is there any mean to narrow down, where a kernel freezes which is not invasive and doesn't slow down the kernel so much? Besides, I had one kernel oops but then I got the message: Dumping ftrace buffer (ftrace buffer empty) but in that case I didn't choose a tracer (no ftrace=......)
The system is a i.MX6 ARMv7 with kernel 5.4.256. It's a yocto build.


Thank you in advance

BR Chris
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