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The kernel, like any software, has bugs. It's a large, complex project and it can be difficult to troubleshoot problems. This document covers some basic troubleshooting techniques to help narrow down the root cause of an issue.
Boot failures
Sometimes the kernel fails to boot. Depending on where the problem is in the boot process, there may or may not be any output. Some good first steps are:
Remove `quiet` (enable more log messages) and `rhgb` (disable graphical boot) from the boot flags. If the text output is too fast to read, add `boot_delay=1000` (the number of milliseconds to delay in between printk during boot). You can use a camera to take pictures of the output.
Booting with vga=791 (or even just vga=1 if the video card won't support 791) will put the framebuffer into high resolution mode to get more lines of text on screen, allowing more context for bug analysis.
Add `initcall_debug` parameter, which traces the initcalls as they are executed.
If you get no output at all from the kernel, booting with `earlyprintk=vga` can sometimes yield something of interest.
Hangs and freezes
Checking whether or not the CapsLock key (or NumLock or ScrollLock) causes
the light on the keyboard to change state can be used as an indication of whether or not the kernel has hung completely, or if there is something else going on.
The SysRq magic keys may still work. You may need to add `sysrq_always_enabled=1` to the kernel boot command line. See https://fedoraproject.org/wiki/QA/Sysrq[the wiki article on SysRq on usage details].
Setting `nmi_watchdog=1` on the kernel command line will cause a panic when an NMI watchdog timeout occurs.
Logs to collect
When reporting an issue with the kernel you should always attach the kernel logs, usually collected with the `dmesg` command. For some types of issues, you may need to collect more logs.
Input issues (touchpad etc.)
Sound issues
`alsa-info.sh` provides information about both kernel and userspace components. If you have a working and non-working kernel, you should provide `alsa-info.sh` for both cases.
Bisecting the kernel
If the problem you've encountered isn't present in older versions of the kernel, it is very helpful to use `git-bisect` to find the commit that introduced the problem. For a general overview of `git-bisect`, see its https://git-scm.com/docs/git-bisect[documentation]. An outline on how to bisect the kernel is included in the https://www.kernel.org/doc/html/latest/admin-guide/bug-bisect.html[kernel documentation]. This guide contains Fedora-specific details.
Bisecting is a time-consuming task, but it's very straightforward and is often the best way to find the cause of a problem. If you're really interested in getting the problem you're seeing fixed, bisecting will speed up the process considerably in most cases.