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Document kernel boot command line options

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Ciro Santilli
2018-02-15 13:01:09 +00:00
parent bae2138b73
commit 5d60fd0903
1 changed files with 59 additions and 27 deletions
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README.adoc
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@@ -101,13 +101,35 @@ Linux and QEMU rebuilds are so common that we have dedicated shortcut flags for
./build -l -q
....
Sometimes, if you change the version of the submodules a lot, builds fail. We should try to understand why and report bugs, but if you are lazy, first try something like:
==== Clean the build
You did something crazy, and nothing seems to work anymore?
All builds are stored under `buildroot/`,
The most coarse thing you can do is:
....
cd buildroot
git checkout -- .
git clean -xdf .
....
To only nuke one architecture, do:
....
rm -rf buildroot/output.x86_64~
....
Only nuke one one package:
....
rm -rf buildroot/output.x86_64~/build/host-qemu-custom
./build -q
....
This is sometimes necessary when changing the version of the submodules, and then builds fail. We should try to understand why and report bugs.
==== Filesystem persistency
The root filesystem is persistent across:
@@ -290,6 +312,42 @@ or on host:
cat buildroot/output.*~/build/linux-custom/.config
....
==== Kernel boot command line arguments
Bootloaders can pass a string as input to the Linux kernel when it is booting to control its behaviour, much like the `execve` system call does to userland processes.
This allows us to control the behaviour of the kernel without rebuilding anything.
With QEMU, QEMU itself acts as the bootloader, and provides the `-append` option and we expose it through `./run -e`, e.g.:
....
./run -e 'foo bar'
....
Then inside the host, you can check which options were given with:
....
cat /proc/cmdline
....
They are also printed at the beginning of the boot message:
....
dmesg | grep "Command line"
....
See also:
* https://unix.stackexchange.com/questions/48601/how-to-display-the-linux-kernel-command-line-parameters-given-for-the-current-bo
* https://askubuntu.com/questions/32654/how-do-i-find-the-boot-parameters-used-by-the-running-kernel
The arguments are documented in the kernel documentation: https://www.kernel.org/doc/html/v4.14/admin-guide/kernel-parameters.html
When dealing with real boards, extra command line options are provided on some magic bootloader configuration file, e.g.:
* GRUB configuration files: https://askubuntu.com/questions/19486/how-do-i-add-a-kernel-boot-parameter
* Raspberry pi `/boot/cmdline.txt` on a magic partition: https://raspberrypi.stackexchange.com/questions/14839/how-to-change-the-kernel-commandline-for-archlinuxarm-on-raspberry-pi-effectly
==== QEMU GUI is unresponsive
Sometimes in Ubuntu 14.04, after the QEMU SDL GUI starts, it does not get updated after keyboard strokes, and there are artifacts like disappearing text.
@@ -333,32 +391,6 @@ Just make sure that you never click inside the QEMU window when doing that, othe
You can still send key presses to QEMU however even without the mouse capture, just either click on the title bar, or alt tab to give it focus.
==== Clean the build
You did something crazy, and nothing seems to work anymore?
All builds are stored under `buildroot/`,
The most coarse thing you can do is:
....
cd buildroot
git checkout -- .
git clean -xdf .
....
To only nuke one architecture, do:
....
rm -rf buildroot/output.x86_64~
....
Only nuke one one package:
....
rm -rf buildroot/output.x86_64~/build/<package>
....
[[gdb]]
=== GDB step debugging