Re: [RFC v9 5/5] Documentation: pstore/blk: create document for pstore_blk

[Randy Dunlap]

On 2/19/19 3:52 AM, liaoweixiong wrote:
> The document, at Documentation/admin-guide/pstore-block.rst,
> tells user how to use pstore_blk and the attentions about panic
> read/write
> 
> Signed-off-by: liaoweixiong <liaoweixiong@xxxxxxxxxxxxxxxxx>
> ---
>  Documentation/admin-guide/pstore-block.rst | 233 +++++++++++++++++++++++++++++
>  MAINTAINERS                                |   1 +
>  fs/pstore/Kconfig                          |   4 +
>  3 files changed, 238 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 0000000..a828274
> --- /dev/null
> +++ b/Documentation/admin-guide/pstore-block.rst
> @@ -0,0 +1,233 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +Pstore block oops/panic logger
> +==============================
> +
> +Introduction
> +------------
> +
> +Pstore block (pstore_blk) is an oops/panic logger that write its logs to block

                                                                         to a block

> +device before the system crashes. Pstore_blk needs block device driver

                                                needs the block

> +registering a partition path of the block device, like /dev/mmcblk0p7 for mmc

   to register                                                           for MMC

> +driver, and read/write APIs for this partition when on panic.
> +
> +Pstore block concepts
> +---------------------
> +
> +Pstore block begins at function ``blkz_register``, by which block driver

                                                      by which a block driver

> +registers to pstore_blk. Note that, block driver should register to pstore_blk

                            Note that the block driver should

> +after block device has registered. Block driver transfers a structure

                                      The block driver

> +``blkz_info`` which is defined in *linux/pstore_blk.h*.
> +
> +The following key members of ``struct blkz_info`` 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 ok to keep it as NULL if you passing ``read`` and ``write`` in blkz_info as

                              if you are passing

> +``blkdev`` is used by blkz_default_general_read/write. If both of ``blkdev``,
> +``read`` and ``write`` are NULL, no block device is effective and the data will
> +be saved in ddr buffer.

what is ddr buffer?

> +
> +It accept the following variants:
> +
> +1. <hex_major><hex_minor> device number in hexadecimal represents itself no

                                                                     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, that form is used when disk

                                               above; this form

> +   name of partitioned disk ends on a digit.

                               ends with a digit.

> +#. PARTUUID=00112233-4455-6677-8899-AABBCCDDEEFF representing 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.
> +
> +See more on section **read/write**.

            in section

> +
> +total_size
> +~~~~~~~~~~
> +
> +The total size in bytes of block device used for pstore_blk. It **MUST** be less
> +than or equal to size of block device if ``blkdev`` valid. It **MUST** be a
> +multiple of 4096. If ``total_size`` is zero with ``blkdev``, ``total_size`` will be
> +set to equal to size of ``blkdev``.
> +
> +The block device area is divided into many chunks, and each event writes a chunk
> +of information.
> +
> +dmesg_size
> +~~~~~~~~~~
> +
> +The chunk size in bytes for dmesg(oops/panic). It **MUST** be a multiple of
> +SECTOR_SIZE (Most of the time, the SECTOR_SIZE is 512). If you don't need dmesg,
> +you are safely to set it to 0.

   you can safely

> +
> +NOTE that, the remaining space, except ``pmsg_size`` and others, belongs to
> +dmesg. It means that there are multiple chunks for dmesg.
> +
> +Psotre_blk will log to dmesg chunks one by one, and always overwrite the oldest

   Pstore_blk

> +chunk if no free chunk.
> +
> +pmsg_size
> +~~~~~~~~~
> +
> +The chunk size in bytes for pmsg. It **MUST** be a multiple of SECTOR_SIZE (Most
> +of the time, the SECTOR_SIZE is 512). If you don't need pmsg, you are safely to

                                                                 you can safely {drop "to"}

> +set it to 0.
> +
> +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.
> +
> +dump_oops
> +~~~~~~~~~
> +
> +Dumping both oopses and panics can be done by setting 1 in the ``dump_oops``
> +member while setting 0 in that variable dumps only the panics.
> +
> +read/write
> +~~~~~~~~~~
> +
> +They are general ``read/write`` APIs. It is safely and recommended to ignore it,

                                         It is safe and recommended

> +but set ``blkdev``.
> +
> +These general APIs are used all the time expect panic. The ``read`` API is
> +usually used to recover data from block device, and the ``write`` API is usually
> +to flush new data and erase to block device.
> +
> +Pstore_blk will temporarily hold all new data before block device is ready. If
> +you ignore both of ``read/write`` and ``blkdev``, the old data will be lost.
> +
> +NOTE that, the general APIs must check whether the block device is ready if

   NOTE that the general

> +self-defined.
> +
> +panic_read/panic_write
> +~~~~~~~~~~~~~~~~~~~~~~
> +
> +They are ``read/write`` APIs for panic. They are likely to general

                                           They are like the general


> +``read/write`` but will be used only when on panic.
> +
> +The attentions for panic read/write see section
> +**Attentions in panic read/write APIs**.
> +
> +Register to pstore block
> +------------------------
> +
> +Block device driver call ``blkz_register`` to register to Psotre_blk.

                                                             Pstore_blk.

> +For example:
> +
> +.. code-block:: c
> +
> + #include <linux/pstore_blk.h>
> + [...]
> +
> + static ssize_t XXXX_panic_read(char *buf, size bytes, loff_t pos)
> + {
> +    [...]
> + }
> +
> + static ssize_t XXXX_panic_write(const char *buf, size_t bytes, loff_t pos)
> + {
> +        [...]
> + }
> +
> + struct blkz_info XXXX_info = {
> +        .onwer = THIS_MODULE,
> +        .name = <...>,
> +        .dmesg_size = <...>,
> +        .pmsg_size = <...>,
> +        .dump_oops = true,
> +        .panic_read = XXXX_panic_read,
> +        .panic_write = XXXX_panic_write,
> + };
> +
> + static int __init XXXX_init(void)
> + {
> +        [... get block device information ...]
> +        XXXX_info.blkdev = <...>;
> +        XXXX_info.total_size = <...>;
> +
> +        [...]
> +        return blkz_register(&XXXX_info);
> + }
> +
> +There are multiple ways by which you can get block device information.
> +
> +A. Use the module parameters and kernel cmdline.
> +B. Use Device Tree bindings.
> +C. Use Kconfig.
> +D. Use Driver Feature.
> +   For example, traverse all MTD device by ``register_mtd_user``, and get the

                                    devices

> +   matching name MTD partition.
> +
> +NOTE that, all of above are done by block driver rather then pstore_blk. You can

   NOTE that all of the above are done by the block driver

> +get sample on blkoops.
> +
> +The attentions for panic read/write see section
> +**Attentions in panic read/write APIs**.
> +
> +Compression and header
> +----------------------
> +
> +Block device is large enough, it is not necessary to compress dmesg data.
> +Actually, we recommend not compress. Because pstore_blk will insert some

                              compressing because

> +information into the first line of dmesg data if no compression.
> +For example::
> +
> +        Panic: Total 16 times
> +
> +It means that it's the 16th times panic log since burning.

what is "burning"?

> +Sometimes, the oops|panic counter since burning is very important for embedded
> +device to judge whether the system is stable.
> +
> +The follow line is insert by pstore filesystem.

       following line is inserted

> +For example::
> +
> +        Oops#2 Part1
> +
> +It means that it's the 2nd times oops log on last booting.
> +
> +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) and
> +``pmsg-pstore-blk-0`` for pmsg, 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 be running for much longer. The tasks
> +will not be scheduled and the most kernel resources will be out of service. It
> +looks like a single-threaded program running on a single-core computer.
> +
> +The following points need special attention for panic read/write APIs:
> +
> +1. Can **NOT** allocate any memory.
> +   If you need memory, just allocate while the block driver is initialing rather

                                                                  initializing

> +   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, no any share resource, you are safely to break all

                              nor any shared resource; you are safe to break all

> +   locks.
> +#. Just use cpu to transfer.

               CPU

> +   Do not use DMA to transfer unless you are sure that DMA will not keep lock.
> +#. Operate register directly.
> +   Try not to use linux kernel resources. Do io map while initialing rather than

                     Linux                      I/O          initializing

> +   waiting until the panic.
> +#. Reset your block device and controller if necessary.
> +   If you are not sure the state of you block device and controller when panic,
> +   you are safely to stop and reset them.

      you are safe to



cheers.
-- 
~Randy