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    Subject[PATCH v2 06/11] Documentation: pstore/blk: blkoops: create document for pstore_blk
    Date
    The document, at Documentation/admin-guide/pstore-block.rst, tells us
    how to use pstore/blk and blkoops.

    Signed-off-by: WeiXiong Liao <liaoweixiong@allwinnertech.com>
    ---
    Documentation/admin-guide/pstore-block.rst | 281 +++++++++++++++++++++++++++++
    MAINTAINERS | 1 +
    fs/pstore/Kconfig | 2 +
    3 files changed, 284 insertions(+)
    create mode 100644 Documentation/admin-guide/pstore-block.rst

    diff --git a/Documentation/admin-guide/pstore-block.rst b/Documentation/admin-guide/pstore-block.rst
    new file mode 100644
    index 000000000000..c8a5f68960c3
    --- /dev/null
    +++ b/Documentation/admin-guide/pstore-block.rst
    @@ -0,0 +1,281 @@
    +.. 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 before the system crashes. It also supports non-block devices such
    +as mtd device.
    +
    +There is a trapper named blkoops for pstore/blk, which makes pstore/blk be
    +nicer to device drivers.
    +
    +Pstore block concepts
    +---------------------
    +
    +Pstore/blk works as a zone manager as it cuts the block device or partition
    +into several zones and stores data for different recorders. What device drivers
    +should do is to provide read/write APIs.
    +
    +Pstore/blk begins at function ``blkz_register``. Besides, blkoops, a wrapper of
    +pstore/blk, begins at function ``blkoops_register_blkdev`` for block device and
    +``blkoops_register_device`` for non-block device, which is recommended instead
    +of directly using pstore/blk.
    +
    +Blkoops provides efficient configuration method for pstore/blk, which divides
    +all configurations of pstore/blk into two parts, configurations for user and
    +configurations for driver.
    +
    +Configurations for user determine how pstore/blk works, such as pmsg_size,
    +dmesg_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/non-block device, such as
    +total_size of device and read/write operations. Device driver transfers a
    +structure ``blkoops_device`` defined in *linux/blkoops.h*.
    +
    +All of the following are for blkoops.
    +
    +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::
    +
    + blkoops.blkdev=179:7 blkoops.dmesg_size=64 blkoops.dump_oops=1
    +
    +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 fine to ignore it if you are not using a block device.
    +
    +It accepts the following variants:
    +
    +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.
    +
    +dmesg_size
    +~~~~~~~~~~
    +
    +The chunk size in KB for dmesg(oops/panic). It **MUST** be a multiple of 4.
    +If you don't need it, safely set it to 0 or ignore it.
    +
    +NOTE that, the remaining space, except ``pmsg_size``, ``console_size``` and
    +others, belongs to dmesg. It means that there are multiple chunks for dmesg.
    +
    +Pstore/blk will log to dmesg chunks one by one, and always overwrite the oldest
    +chunk if there is no more free chunks.
    +
    +pmsg_size
    +~~~~~~~~~
    +
    +The chunk size in KB for pmsg. It **MUST** be a multiple of 4. If you do not
    +need it, safely set it to 0 or ignore it.
    +
    +There is only one chunk for pmsg.
    +
    +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. It **MUST** be a multiple of 4. If you
    +do not need it, safely set it to 0 or ignore it.
    +
    +There is only one chunk for console.
    +
    +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. It **MUST** be a multiple of 4. If you
    +do not need it, safely set it to 0 or ignore it.
    +
    +There may be several chunks for ftrace, according to how many processors on
    +your CPU. 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
    +available in /sys/fs/pstore/ftrace-pstore-blk-[N], where N is the processor
    +number.
    +
    +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:109860 c03a4310 c0063ebc cpuidle_select <- cpu_startup_entry+0x1a8/0x1e0
    + CPU:0 ts:109861 c03a5878 c03a4324 menu_select <- cpuidle_select+0x24/0x2c
    + CPU:0 ts:109862 c00670e8 c03a589c pm_qos_request <- menu_select+0x38/0x4cc
    + CPU:0 ts:109863 c0092bbc c03a5960 tick_nohz_get_sleep_length <- menu_select+0xfc/0x4cc
    + CPU:0 ts:109865 c004b2f4 c03a59d4 get_iowait_load <- menu_select+0x170/0x4cc
    + CPU:0 ts:109868 c0063b60 c0063ecc call_cpuidle <- cpu_startup_entry+0x1b8/0x1e0
    + CPU:0 ts:109869 c03a433c c0063b94 cpuidle_enter <- call_cpuidle+0x44/0x48
    + CPU:0 ts:109871 c03a4000 c03a4350 cpuidle_enter_state <- cpuidle_enter+0x24/0x28
    + CPU:0 ts:109873 c0063ba8 c03a4090 sched_idle_set_state <- cpuidle_enter_state+0xa4/0x314
    + CPU:0 ts:109874 c03a605c c03a40b4 arm_enter_idle_state <- cpuidle_enter_state+0xc8/0x314
    +
    +dump_oops
    +~~~~~~~~~
    +
    +Dumping both oopses and panics can be done by setting 1 (not zero) in the
    +``dump_oops`` member while setting 0 in that variable dumps only the panics.
    +
    +Configurations for driver
    +-------------------------
    +
    +Only a device driver cares about these configurations. A block device driver
    +uses ``blkoops_register_blkdev`` while a non-block device driver uses
    +``blkoops_register_device``
    +
    +The parameters of these two APIs may be of interest to you.
    +
    +major
    +~~~~~
    +
    +It is only required by block device which is registered by
    +``blkoops_register_blkdev``. It's the major device number of registered
    +devices, by which blkoops can get the matching driver for @blkdev.
    +
    +total_size
    +~~~~~~~~~~
    +
    +It is only required by non-block device which is registered by
    +``blkoops_register_device``. It tells pstore/blk the total size
    +pstore/blk can use. It is in KB and **MUST** be greater than or equal to 4
    +and a multiple of 4.
    +
    +For block devices, blkoops can get size of block device/partition automatically.
    +
    +read/write
    +~~~~~~~~~~
    +
    +It's generic read/write APIs for pstore/blk, which are required by non-block
    +device. The generic APIs are used for almost all data except panic data,
    +such as pmsg, console, oops and ftrace.
    +
    +The parameter @offset of these interface is the relative position of the device.
    +
    +Normally the number of bytes read/written should be returned, while for error,
    +negative number will be returned. The following return numbers mean more:
    +
    +-EBUSY: pstore/blk should try again later.
    +
    +panic_write (for non-block device)
    +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    +
    +It's a interface for panic recorder and will be used only when panic occurs.
    +Non-block device driver registers it by ``blkoops_register_device``. If panic
    +log is unnecessary, it's fine to ignore it.
    +
    +Note that pstore/blk will recover data from device while mounting pstore
    +filesystem by default. If panic occurs but pstore/blk does not recover yet, the
    +first zone of dmesg will be used.
    +
    +The parameter @offset of this interface is the relative position of the device.
    +
    +Normally the number of bytes written should be returned, while for error,
    +negative number should be returned.
    +
    +panic_write (for block device)
    +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    +
    +It's much similar to panic_write for non-block device, but the position and
    +data size of panic_write for block device must be aligned to SECTOR_SIZE,
    +that's why the parameters are @sects and @start_sect. Block device driver
    +should register it by ``blkoops_register_blkdev``.
    +
    +The parameter @start_sect is the relative position of the block device and
    +partition. If block driver requires absolute position for panic_write,
    +``blkoops_blkdev_info`` will be helpful, which can provide the absolute
    +position of the block device (or partition) on the whole disk/flash.
    +
    +Normally zero should be returned, otherwise it indicates an error.
    +
    +Compression and header
    +----------------------
    +
    +Block device is large enough for uncompressed dmesg data. Actually we do not
    +recommend data compression because pstore/blk will insert some information into
    +the first line of dmesg 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 dmesg(oops|panic), ``pmsg-pstore-blk-0``
    +for pmsg and so on, where N is the record number. To delete a stored
    +record from block device, simply unlink the respective pstore file. The
    +timestamp of the dump file records the trigger time.
    +
    +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.
    +
    +Blkoops supports blkoops_blkdev_info(), which is defined in *linux/blkoops.h*,
    +to get information of block device, such as the device number, sector count and
    +start sector of the whole disk.
    diff --git a/MAINTAINERS b/MAINTAINERS
    index e4ba97130560..a5122e3aaf76 100644
    --- a/MAINTAINERS
    +++ b/MAINTAINERS
    @@ -13380,6 +13380,7 @@ F: include/linux/pstore*
    F: drivers/firmware/efi/efi-pstore.c
    F: drivers/acpi/apei/erst.c
    F: Documentation/admin-guide/ramoops.rst
    +F: Documentation/admin-guide/pstore-block.rst
    F: Documentation/devicetree/bindings/reserved-memory/ramoops.txt
    K: \b(pstore|ramoops|blkoops)

    diff --git a/fs/pstore/Kconfig b/fs/pstore/Kconfig
    index 308a0a4c5ee5..466908a242aa 100644
    --- a/fs/pstore/Kconfig
    +++ b/fs/pstore/Kconfig
    @@ -162,6 +162,8 @@ config PSTORE_BLK
    This enables panic and oops message to be logged to a block dev
    where it can be read back at some later point.

    + For more information, see Documentation/admin-guide/pstore-block.rst.
    +
    If unsure, say N.

    config PSTORE_BLKOOPS
    --
    1.9.1
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