PStore command

Design

Linux PStore and Ramoops modules (Linux config options PSTORE and PSTORE_RAM) allow to use memory to pass data from the dying breath of a crashing kernel to its successor. This command allows to read those records from U-Boot command line.

Ramoops is an oops/panic logger that writes its logs to RAM before the system crashes. It works by logging oopses and panics in a circular buffer. Ramoops needs a system with persistent RAM so that the content of that area can survive after a restart.

Ramoops uses a predefined memory area to store the dump.

Ramoops parameters can be passed as kernel parameters or through Device Tree, i.e.:

ramoops.mem_address=0x30000000 ramoops.mem_size=0x100000 ramoops.record_size=0x2000 ramoops.console_size=0x2000 memmap=0x100000$0x30000000

The same values should be set in U-Boot to be able to retrieve the records. This values can be set at build time in U-Boot configuration file, or at runtime. U-Boot automatically patches the Device Tree to pass the Ramoops parameters to the kernel.

The PStore configuration parameters are:

Name Default
CMD_PSTORE_MEM_ADDR  
CMD_PSTORE_MEM_SIZE 0x10000
CMD_PSTORE_RECORD_SIZE 0x1000
CMD_PSTORE_CONSOLE_SIZE 0x1000
CMD_PSTORE_FTRACE_SIZE 0x1000
CMD_PSTORE_PMSG_SIZE 0x1000
CMD_PSTORE_ECC_SIZE 0

Records sizes should be a power of 2. The memory size and the record/console size must be non-zero.

Multiple ‘dump’ records can be stored in the memory reserved for PStore. The memory size has to be larger than the sum of the record sizes, i.e.:

MEM_SIZE >= RECORD_SIZE * n + CONSOLE_SIZE + FTRACE_SIZE + PMSG_SIZE

Usage

Generate kernel crash

For test purpose, you can generate a kernel crash by setting reboot timeout to 10 seconds and trigger a panic:

$ sudo sh -c "echo 1 > /proc/sys/kernel/sysrq"
$ sudo sh -c "echo 10 > /proc/sys/kernel/panic"
$ sudo sh -c "echo c > /proc/sysrq-trigger"

Retrieve logs in U-Boot

First of all, unless PStore parameters as been set during U-Boot configuration and match kernel ramoops parameters, it needs to be set using ‘pstore set’, e.g.:

=> pstore set 0x30000000 0x100000 0x2000 0x2000

Then all available dumps can be displayed using:

=> pstore display

Or saved to an existing directory in an Ext2 or Ext4 partition, e.g. on root directory of 1st partition of the 2nd MMC:

=> pstore save mmc 1:1 /