[RFC v10 4/4] Documentation: pstore/blk: create document for pstore_blk

From: liaoweixiong
Date: Tue Feb 26 2019 - 01:34:05 EST

The document, at Documentation/admin-guide/pstore-block.rst,
tells user how to use pstore_blk and the attentions about panic

Signed-off-by: liaoweixiong <liaoweixiong@xxxxxxxxxxxxxxxxx>
Documentation/admin-guide/pstore-block.rst | 233 +++++++++++++++++++++++++++++
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
+Pstore block (pstore_blk) is an oops/panic logger that write its logs to block
+device before the system crashes. Pstore_blk needs block device driver
+registering a partition path of the block device, like /dev/mmcblk0p7 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
+registers to pstore_blk. Note that, block driver should register to pstore_blk
+after block device has registered. Block driver transfers a structure
+``blkz_info`` which is defined in *linux/pstore_blk.h*.
+The following key members of ``struct blkz_info`` may be of interest to you.
+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
+``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.
+It accept 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, that form is used when disk
+ name of partitioned disk ends on 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**.
+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.
+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.
+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
+chunk if no free chunk.
+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
+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
+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.
+They are general ``read/write`` APIs. It is safely and recommended to ignore it,
+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
+They are ``read/write`` APIs for panic. They are likely to 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.
+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
+ matching name MTD partition.
+NOTE that, all of above are done by block driver rather then pstore_blk. You can
+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
+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.
+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.
+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
+ 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
+ locks.
+#. Just use cpu to transfer.
+ 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
+ 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.
index 4e9242a..9ddca0e 100644
@@ -12317,6 +12317,7 @@ 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 b417bf5..3921a63 100644
--- a/fs/pstore/Kconfig
+++ b/fs/pstore/Kconfig
@@ -161,6 +161,10 @@ 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.
tristate "pstore block with oops logger"
depends on PSTORE_BLK