.. SPDX-License-Identifier: GPL-2.0

pstore block oops/panic logger
==============================

Introduction
------------

pstore block (pstore/blk) is an oops/panic logger that writes its logs to a
block device and non-block device before the system crashes. You can get
these log files by mounting pstore filesystem like::

    mount -t pstore pstore /sys/fs/pstore


pstore block concepts
---------------------

pstore/blk provides efficient configuration method for pstore/blk, which
divides all configurations into two parts, configurations for user and
configurations for driver.

Configurations for user determine how pstore/blk works, such as pmsg_size,
kmsg_size and so on. All of them support both Kconfig and module parameters,
but module parameters have priority over Kconfig.

Configurations for driver are all about block device and non-block device,
such as total_size of block device and read/write operations.

Configurations for user
-----------------------

All of these configurations support both Kconfig and module parameters, but
module parameters have priority over Kconfig.

Here is an example for module parameters::

        pstore_blk.blkdev=/dev/mmcblk0p7 pstore_blk.kmsg_size=64 best_effort=y

The detail of each configurations may be of interest to you.

blkdev
~~~~~~

The block device to use. Most of the time, it is a partition of block device.
It's required for pstore/blk. It is also used for MTD device.

It accepts the following variants for block device:

1. <hex_major><hex_minor> device number in hexadecimal represents itself; no
   leading 0x, for example b302.
#. /dev/<disk_name> represents the device number of disk
#. /dev/<disk_name><decimal> represents the device number of partition - device
   number of disk plus the partition number
#. /dev/<disk_name>p<decimal> - same as the above; this form is used when disk
   name of partitioned disk ends with a digit.
#. PARTUUID=00112233-4455-6677-8899-AABBCCDDEEFF represents the unique id of
   a partition if the partition table provides it. The UUID may be either an
   EFI/GPT UUID, or refer to an MSDOS partition using the format SSSSSSSS-PP,
   where SSSSSSSS is a zero-filled hex representation of the 32-bit
   "NT disk signature", and PP is a zero-filled hex representation of the
   1-based partition number.
#. PARTUUID=<UUID>/PARTNROFF=<int> to select a partition in relation to a
   partition with a known unique id.
#. <major>:<minor> major and minor number of the device separated by a colon.

It accepts the following variants for MTD device:

1. <device name> MTD device name. "pstore" is recommended.
#. <device number> MTD device number.

kmsg_size
~~~~~~~~~

The chunk size in KB for oops/panic front-end. It **MUST** be a multiple of 4.
It's optional if you do not care oops/panic log.

There are multiple chunks for oops/panic front-end depending on the remaining
space except other pstore front-ends.

pstore/blk will log to oops/panic chunks one by one, and always overwrite the
oldest chunk if there is no more free chunk.

pmsg_size
~~~~~~~~~

The chunk size in KB for pmsg front-end. It **MUST** be a multiple of 4.
It's optional if you do not care pmsg log.

Unlike oops/panic front-end, there is only one chunk for pmsg front-end.

Pmsg is a user space accessible pstore object. Writes to */dev/pmsg0* are
appended to the chunk. On reboot the contents are available in
*/sys/fs/pstore/pmsg-pstore-blk-0*.

console_size
~~~~~~~~~~~~

The chunk size in KB for console front-end.  It **MUST** be a multiple of 4.
It's optional if you do not care console log.

Similar to pmsg front-end, there is only one chunk for console front-end.

All log of console will be appended to the chunk. On reboot the contents are
available in */sys/fs/pstore/console-pstore-blk-0*.

ftrace_size
~~~~~~~~~~~

The chunk size in KB for ftrace front-end. It **MUST** be a multiple of 4.
It's optional if you do not care console log.

Similar to oops front-end, there are multiple chunks for ftrace front-end
depending on the count of cpu processors. Each chunk size is equal to
ftrace_size / processors_count.

All log of ftrace will be appended to the chunk. On reboot the contents are
combined and available in */sys/fs/pstore/ftrace-pstore-blk-0*.

Persistent function tracing might be useful for debugging software or hardware
related hangs. Here is an example of usage::

 # mount -t pstore pstore /sys/fs/pstore
 # mount -t debugfs debugfs /sys/kernel/debug/
 # echo 1 > /sys/kernel/debug/pstore/record_ftrace
 # reboot -f
 [...]
 # mount -t pstore pstore /sys/fs/pstore
 # tail /sys/fs/pstore/ftrace-pstore-blk-0
 CPU:0 ts:5914676 c0063828  c0063b94  call_cpuidle <- cpu_startup_entry+0x1b8/0x1e0
 CPU:0 ts:5914678 c039ecdc  c006385c  cpuidle_enter_state <- call_cpuidle+0x44/0x48
 CPU:0 ts:5914680 c039e9a0  c039ecf0  cpuidle_enter_freeze <- cpuidle_enter_state+0x304/0x314
 CPU:0 ts:5914681 c0063870  c039ea30  sched_idle_set_state <- cpuidle_enter_state+0x44/0x314
 CPU:1 ts:5916720 c0160f59  c015ee04  kernfs_unmap_bin_file <- __kernfs_remove+0x140/0x204
 CPU:1 ts:5916721 c05ca625  c015ee0c  __mutex_lock_slowpath <- __kernfs_remove+0x148/0x204
 CPU:1 ts:5916723 c05c813d  c05ca630  yield_to <- __mutex_lock_slowpath+0x314/0x358
 CPU:1 ts:5916724 c05ca2d1  c05ca638  __ww_mutex_lock <- __mutex_lock_slowpath+0x31c/0x358

max_reason
~~~~~~~~~~

Limiting which kinds of kmsg dumps are stored can be controlled via
the ``max_reason`` value, as defined in include/linux/kmsg_dump.h's
``enum kmsg_dump_reason``. For example, to store both Oopses and Panics,
``max_reason`` should be set to 2 (KMSG_DUMP_OOPS), to store only Panics
``max_reason`` should be set to 1 (KMSG_DUMP_PANIC). Setting this to 0
(KMSG_DUMP_UNDEF), means the reason filtering will be controlled by the
``printk.always_kmsg_dump`` boot param: if unset, it'll be KMSG_DUMP_OOPS,
otherwise KMSG_DUMP_MAX.

Configurations for driver
-------------------------

A device driver uses ``register_pstore_device`` with
``struct pstore_device_info`` to register to pstore/blk.

.. kernel-doc:: fs/pstore/blk.c
   :export:

Compression and header
----------------------

Block device is large enough for uncompressed oops data. Actually we do not
recommend data compression because pstore/blk will insert some information into
the first line of oops/panic data. For example::

        Panic: Total 16 times

It means that it's OOPS|Panic for the 16th time since the first booting.
Sometimes the number of occurrences of oops|panic since the first booting is
important to judge whether the system is stable.

The following line is inserted by pstore filesystem. For example::

        Oops#2 Part1

It means that it's OOPS for the 2nd time on the last boot.

Reading the data
----------------

The dump data can be read from the pstore filesystem. The format for these
files is ``dmesg-pstore-blk-[N]`` for oops/panic front-end,
``pmsg-pstore-blk-0`` for pmsg front-end and so on.  The timestamp of the
dump file records the trigger time. To delete a stored record from block
device, simply unlink the respective pstore file.

Attentions in panic read/write APIs
-----------------------------------

If on panic, the kernel is not going to run for much longer, the tasks will not
be scheduled and most kernel resources will be out of service. It
looks like a single-threaded program running on a single-core computer.

The following points require special attention for panic read/write APIs:

1. Can **NOT** allocate any memory.
   If you need memory, just allocate while the block driver is initializing
   rather than waiting until the panic.
#. Must be polled, **NOT** interrupt driven.
   No task schedule any more. The block driver should delay to ensure the write
   succeeds, but NOT sleep.
#. Can **NOT** take any lock.
   There is no other task, nor any shared resource; you are safe to break all
   locks.
#. Just use CPU to transfer.
   Do not use DMA to transfer unless you are sure that DMA will not keep lock.
#. Control registers directly.
   Please control registers directly rather than use Linux kernel resources.
   Do I/O map while initializing rather than wait until a panic occurs.
#. Reset your block device and controller if necessary.
   If you are not sure of the state of your block device and controller when
   a panic occurs, you are safe to stop and reset them.

pstore/blk supports psblk_blkdev_info(), which is defined in
*linux/pstore_blk.h*, to get information of using block device, such as the
device number, sector count and start sector of the whole disk.

pstore block internals
----------------------

For developer reference, here are all the important structures and APIs:

.. kernel-doc:: fs/pstore/zone.c
   :internal:

.. kernel-doc:: include/linux/pstore_zone.h
   :internal:

.. kernel-doc:: fs/pstore/blk.c
   :internal:

.. kernel-doc:: include/linux/pstore_blk.h
   :internal:
