xref: /linux/Documentation/admin-guide/pstore-blk.rst (revision 26fbb4c8c7c3ee9a4c3b4de555a8587b5a19154e)
1.. SPDX-License-Identifier: GPL-2.0
2
3pstore block oops/panic logger
4==============================
5
6Introduction
7------------
8
9pstore block (pstore/blk) is an oops/panic logger that writes its logs to a
10block device and non-block device before the system crashes. You can get
11these log files by mounting pstore filesystem like::
12
13    mount -t pstore pstore /sys/fs/pstore
14
15
16pstore block concepts
17---------------------
18
19pstore/blk provides efficient configuration method for pstore/blk, which
20divides all configurations into two parts, configurations for user and
21configurations for driver.
22
23Configurations for user determine how pstore/blk works, such as pmsg_size,
24kmsg_size and so on. All of them support both Kconfig and module parameters,
25but module parameters have priority over Kconfig.
26
27Configurations for driver are all about block device and non-block device,
28such as total_size of block device and read/write operations.
29
30Configurations for user
31-----------------------
32
33All of these configurations support both Kconfig and module parameters, but
34module parameters have priority over Kconfig.
35
36Here is an example for module parameters::
37
38        pstore_blk.blkdev=/dev/mmcblk0p7 pstore_blk.kmsg_size=64 best_effort=y
39
40The detail of each configurations may be of interest to you.
41
42blkdev
43~~~~~~
44
45The block device to use. Most of the time, it is a partition of block device.
46It's required for pstore/blk. It is also used for MTD device.
47
48It accepts the following variants for block device:
49
501. <hex_major><hex_minor> device number in hexadecimal represents itself; no
51   leading 0x, for example b302.
52#. /dev/<disk_name> represents the device number of disk
53#. /dev/<disk_name><decimal> represents the device number of partition - device
54   number of disk plus the partition number
55#. /dev/<disk_name>p<decimal> - same as the above; this form is used when disk
56   name of partitioned disk ends with a digit.
57#. PARTUUID=00112233-4455-6677-8899-AABBCCDDEEFF represents the unique id of
58   a partition if the partition table provides it. The UUID may be either an
59   EFI/GPT UUID, or refer to an MSDOS partition using the format SSSSSSSS-PP,
60   where SSSSSSSS is a zero-filled hex representation of the 32-bit
61   "NT disk signature", and PP is a zero-filled hex representation of the
62   1-based partition number.
63#. PARTUUID=<UUID>/PARTNROFF=<int> to select a partition in relation to a
64   partition with a known unique id.
65#. <major>:<minor> major and minor number of the device separated by a colon.
66
67It accepts the following variants for MTD device:
68
691. <device name> MTD device name. "pstore" is recommended.
70#. <device number> MTD device number.
71
72kmsg_size
73~~~~~~~~~
74
75The chunk size in KB for oops/panic front-end. It **MUST** be a multiple of 4.
76It's optional if you do not care oops/panic log.
77
78There are multiple chunks for oops/panic front-end depending on the remaining
79space except other pstore front-ends.
80
81pstore/blk will log to oops/panic chunks one by one, and always overwrite the
82oldest chunk if there is no more free chunk.
83
84pmsg_size
85~~~~~~~~~
86
87The chunk size in KB for pmsg front-end. It **MUST** be a multiple of 4.
88It's optional if you do not care pmsg log.
89
90Unlike oops/panic front-end, there is only one chunk for pmsg front-end.
91
92Pmsg is a user space accessible pstore object. Writes to */dev/pmsg0* are
93appended to the chunk. On reboot the contents are available in
94*/sys/fs/pstore/pmsg-pstore-blk-0*.
95
96console_size
97~~~~~~~~~~~~
98
99The chunk size in KB for console front-end.  It **MUST** be a multiple of 4.
100It's optional if you do not care console log.
101
102Similar to pmsg front-end, there is only one chunk for console front-end.
103
104All log of console will be appended to the chunk. On reboot the contents are
105available in */sys/fs/pstore/console-pstore-blk-0*.
106
107ftrace_size
108~~~~~~~~~~~
109
110The chunk size in KB for ftrace front-end. It **MUST** be a multiple of 4.
111It's optional if you do not care console log.
112
113Similar to oops front-end, there are multiple chunks for ftrace front-end
114depending on the count of cpu processors. Each chunk size is equal to
115ftrace_size / processors_count.
116
117All log of ftrace will be appended to the chunk. On reboot the contents are
118combined and available in */sys/fs/pstore/ftrace-pstore-blk-0*.
119
120Persistent function tracing might be useful for debugging software or hardware
121related hangs. Here is an example of usage::
122
123 # mount -t pstore pstore /sys/fs/pstore
124 # mount -t debugfs debugfs /sys/kernel/debug/
125 # echo 1 > /sys/kernel/debug/pstore/record_ftrace
126 # reboot -f
127 [...]
128 # mount -t pstore pstore /sys/fs/pstore
129 # tail /sys/fs/pstore/ftrace-pstore-blk-0
130 CPU:0 ts:5914676 c0063828  c0063b94  call_cpuidle <- cpu_startup_entry+0x1b8/0x1e0
131 CPU:0 ts:5914678 c039ecdc  c006385c  cpuidle_enter_state <- call_cpuidle+0x44/0x48
132 CPU:0 ts:5914680 c039e9a0  c039ecf0  cpuidle_enter_freeze <- cpuidle_enter_state+0x304/0x314
133 CPU:0 ts:5914681 c0063870  c039ea30  sched_idle_set_state <- cpuidle_enter_state+0x44/0x314
134 CPU:1 ts:5916720 c0160f59  c015ee04  kernfs_unmap_bin_file <- __kernfs_remove+0x140/0x204
135 CPU:1 ts:5916721 c05ca625  c015ee0c  __mutex_lock_slowpath <- __kernfs_remove+0x148/0x204
136 CPU:1 ts:5916723 c05c813d  c05ca630  yield_to <- __mutex_lock_slowpath+0x314/0x358
137 CPU:1 ts:5916724 c05ca2d1  c05ca638  __ww_mutex_lock <- __mutex_lock_slowpath+0x31c/0x358
138
139max_reason
140~~~~~~~~~~
141
142Limiting which kinds of kmsg dumps are stored can be controlled via
143the ``max_reason`` value, as defined in include/linux/kmsg_dump.h's
144``enum kmsg_dump_reason``. For example, to store both Oopses and Panics,
145``max_reason`` should be set to 2 (KMSG_DUMP_OOPS), to store only Panics
146``max_reason`` should be set to 1 (KMSG_DUMP_PANIC). Setting this to 0
147(KMSG_DUMP_UNDEF), means the reason filtering will be controlled by the
148``printk.always_kmsg_dump`` boot param: if unset, it'll be KMSG_DUMP_OOPS,
149otherwise KMSG_DUMP_MAX.
150
151Configurations for driver
152-------------------------
153
154A device driver uses ``register_pstore_device`` with
155``struct pstore_device_info`` to register to pstore/blk.
156
157.. kernel-doc:: fs/pstore/blk.c
158   :export:
159
160Compression and header
161----------------------
162
163Block device is large enough for uncompressed oops data. Actually we do not
164recommend data compression because pstore/blk will insert some information into
165the first line of oops/panic data. For example::
166
167        Panic: Total 16 times
168
169It means that it's OOPS|Panic for the 16th time since the first booting.
170Sometimes the number of occurrences of oops|panic since the first booting is
171important to judge whether the system is stable.
172
173The following line is inserted by pstore filesystem. For example::
174
175        Oops#2 Part1
176
177It means that it's OOPS for the 2nd time on the last boot.
178
179Reading the data
180----------------
181
182The dump data can be read from the pstore filesystem. The format for these
183files is ``dmesg-pstore-blk-[N]`` for oops/panic front-end,
184``pmsg-pstore-blk-0`` for pmsg front-end and so on.  The timestamp of the
185dump file records the trigger time. To delete a stored record from block
186device, simply unlink the respective pstore file.
187
188Attentions in panic read/write APIs
189-----------------------------------
190
191If on panic, the kernel is not going to run for much longer, the tasks will not
192be scheduled and most kernel resources will be out of service. It
193looks like a single-threaded program running on a single-core computer.
194
195The following points require special attention for panic read/write APIs:
196
1971. Can **NOT** allocate any memory.
198   If you need memory, just allocate while the block driver is initializing
199   rather than waiting until the panic.
200#. Must be polled, **NOT** interrupt driven.
201   No task schedule any more. The block driver should delay to ensure the write
202   succeeds, but NOT sleep.
203#. Can **NOT** take any lock.
204   There is no other task, nor any shared resource; you are safe to break all
205   locks.
206#. Just use CPU to transfer.
207   Do not use DMA to transfer unless you are sure that DMA will not keep lock.
208#. Control registers directly.
209   Please control registers directly rather than use Linux kernel resources.
210   Do I/O map while initializing rather than wait until a panic occurs.
211#. Reset your block device and controller if necessary.
212   If you are not sure of the state of your block device and controller when
213   a panic occurs, you are safe to stop and reset them.
214
215pstore/blk supports psblk_blkdev_info(), which is defined in
216*linux/pstore_blk.h*, to get information of using block device, such as the
217device number, sector count and start sector of the whole disk.
218
219pstore block internals
220----------------------
221
222For developer reference, here are all the important structures and APIs:
223
224.. kernel-doc:: fs/pstore/zone.c
225   :internal:
226
227.. kernel-doc:: include/linux/pstore_zone.h
228   :internal:
229
230.. kernel-doc:: fs/pstore/blk.c
231   :internal:
232
233.. kernel-doc:: include/linux/pstore_blk.h
234   :internal:
235