Messages in this thread | | | Subject | Re: [RFC v9 5/5] Documentation: pstore/blk: create document for pstore_blk | From | Randy Dunlap <> | Date | Wed, 27 Feb 2019 21:15:35 -0800 |
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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@allwinnertech.com> > --- > 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
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