2.3. pstore filesystem with RAM backend (RAMOOPS)

Barebox supports the pstore filesystem known from the kernel. The main backend implementation is RAM. All other backends are currently not implemented by Barebox.

pstore is a filesystem to store kernel log or kernel panic messages. These messages are stored by the kernel in a specified RAM area which is never overwritten by any user. This data can be accessed after a reboot through /pstore in Barebox or the kernel. The pstore filesystem is automatically mounted at boot:

none on / type ramfs
none on /dev type devfs
none on /pstore type pstore

pstore may add additional warnings during boot due to wrong ECCs (no data written):

persistent_ram: found existing invalid buffer, size 791282217, start 1116786789
persistent_ram: uncorrectable error in header
persistent_ram: found existing invalid buffer, size 791282281, start 1133564005
persistent_ram: uncorrectable error in header
persistent_ram: found existing invalid buffer, size 791347753, start 1133564005
persistent_ram: uncorrectable error in header
persistent_ram: found existing invalid buffer, size 791347753, start 1133572197
persistent_ram: uncorrectable error in header
persistent_ram: found existing invalid buffer, size 774505001, start 1133564005
persistent_ram: uncorrectable error in header
persistent_ram: found existing invalid buffer, size 791282281, start 1133564005
persistent_ram: uncorrectable error in header
persistent_ram: found existing invalid buffer, size 791282217, start 1133564005
pstore: Registered ramoops as persistent store backend
ramoops: attached 0x200000@0x1fdf4000, ecc: 16/0

To use pstore/RAMOOPS both Barebox and Kernel have to be compiled with pstore and RAM backend support. The kernel receives the parameters describing the layout via devicetree or - as a fallback - over the kernel command line. To ensure both worlds are using the same memory layout, the required configuration data for the kernel is generated on-the-fly prior booting a kernel. For the devicetree use case Barebox adapts the kernel’s devicetree, for the kernel command line fallback the variable global.linux.bootargs.ramoops is created and its content used to build the kernel command line.

You can adapt the pstore parameters in Barebox menuconfig.

To see where the RAMOOPS area is located, you can execute the iomem command in the Barebox shell. The RAMOOPS area is listed as ‘persistent ram’:

0x10000000 - 0x1fffffff (size 0x10000000) ram0
  0x247f59c0 - 0x2fbf59bf (size 0x0b400000) malloc space
  0x2fbf59c0 - 0x2fbffffe (size 0x0000a63f) board data
  0x2fc00000 - 0x2fc8619f (size 0x000861a0) barebox
  0x2fc861a0 - 0x2fca35ef (size 0x0001d450) barebox data
  0x2fca35f0 - 0x2fca9007 (size 0x00005a18) bss
  0x2fdd4000 - 0x2fdf3fff (size 0x00020000) ramoops:dump(0/4)
  0x2fdf4000 - 0x2fe13fff (size 0x00020000) ramoops:dump(1/4)
  0x2fe14000 - 0x2fe33fff (size 0x00020000) ramoops:dump(2/4)
  0x2fe34000 - 0x2fe53fff (size 0x00020000) ramoops:dump(3/4)
  0x2fe54000 - 0x2fe73fff (size 0x00020000) ramoops:dump(4/4)
  0x2fe74000 - 0x2fe93fff (size 0x00020000) ramoops:console
  0x2fe94000 - 0x2feb3fff (size 0x00020000) ramoops:ftrace
  0x2feb4000 - 0x2fed3fff (size 0x00020000) ramoops:pmsg
  0x2fee4000 - 0x2fee7fff (size 0x00004000) ttb
  0x2fee8000 - 0x2feeffff (size 0x00008000) stack

If the menu entry FS_PSTORE_CONSOLE is enabled, Barebox itself will add all its own console output to the ramoops:console part, which enables the regular userland later on to have access to the bootloaders output.

All pstore files that could be found are added to the /pstore directory. This is a read-only filesystem with the only supported operation being unlinking: This resets (zaps) the RAMOOPS area and recalculates the ECC.

Zapping is done automatically, if the menu entry FS_PSTORE_RAMOOPS_RO is disabled. In this case, only the barebox log will be available to the kernel and ramoops from previous boots will not survive.

The usual setup is to not zap any buffers, i.e. CONFIG_FS_PSTORE_RAMOOPS_RO=y and no manual unlinking of files in /pstore.

In Linux, the pstore file system is often mounted at /sys/fs/pstore. This will often not contain older output as Linux frequently zaps ramoops buffers to conserve the limited space.

To counteract this, userland needs to archive the pstore contents after booting into Linux and then clear pstore. Systemd has built-in support for this when compiled with -Dpstore=true.

Logs (including barebox log messages if enabled) will then be written to journal by default and are accessible via:

journalctl -b -o verbose -a -t systemd-pstore