== pstore ==

see also [https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/admin-guide/ramoops.rst]

do debug the boot process the pstore driver is used to create a persitant storage area that survives boot.

The principle idea is:
- boot the boot image that you want to debug and write the boot log to pstore
- generate a kernel panic
- boot a recovery boot image that also has pstore enabled to analyze the log

In the TWRP recovery image for the chiron pstore is enabled and pstore is automatically mounted to /sys/fs/pstore/

=== Kernel ===
The following options have to be enabled in the kernel config (in both boot images)

{{{
CONFIG_PSTORE=y
CONFIG_PSTORE_CONSOLE=y
CONFIG_PSTORE_PMSG=y
CONFIG_PSTORE_RAM=y
}}}

=== Mounting pstore ===
To read the info in pstore it has to be mounted (in the boot image that is used to analyze the pstore) with something like
{{{
mount -t pstore pstore /sys/fs/pstore
}}}

=== Writing to pstore ===
When CONFIG_PSTORE_PMSG is enabled it defines a device /dev/pmsg0 to which one can write messages
{{{
echo "Hello Guhl" > /dev/pmsg0
}}}
After the kernel panic and reboot the result should be in /sys/fs/pstore/pmsg-ramoops-X

=== Kernel panic ===
For what ever reason the standard way to generate a kernel panic 
{{{
echo c > /proc/sysrq-trigger
}}}
did not work for me so I created a kernel module that I can modprobe to generate a panic.

I am using the attached kernel module to create a kernel panic (modprobe the module)
See also [https://github.com/guhl/android_kernel_xiaomi_msm8998-1/commit/50c8d7f1ba19002b6743b96b6ce043470e4bbb87]
The files are placed in a directory "panic" in the kernel directory and enabled in kernel config with 
{{{
CONFIG_GUHL_PANIC=m
}}}
In addition the following line has to be added to the Makefile in the kernel directory.
{{{
obj-y += panic/
}}}
and the following line has to be added to the to the Kconfig in the root of the kernel directory
{{{
source "kernel/panic/Kconfig"
}}}