1 Ramoops oops/panic logger
2 =========================
3
4 Sergiu Iordache <sergiu@chromium.org>
5
6 Updated: 10 Feb 2021
7
8 Introduction
9 ------------
10
11 Ramoops is an oops/panic logger that writes its logs to RAM before the system
12 crashes. It works by logging oopses and panics in a circular buffer. Ramoops
13 needs a system with persistent RAM so that the content of that area can
14 survive after a restart.
15
16 Ramoops concepts
17 ----------------
18
19 Ramoops uses a predefined memory area to store the dump. The start and size
20 and type of the memory area are set using three variables:
21
22 * ``mem_address`` for the start
23 * ``mem_size`` for the size. The memory size will be rounded down to a
24 power of two.
25 * ``mem_type`` to specify if the memory type (default is pgprot_writecombine).
26 * ``mem_name`` to specify a memory region defined by ``reserve_mem`` command
27 line parameter.
28
29 Typically the default value of ``mem_type=0`` should be used as that sets the pstore
30 mapping to pgprot_writecombine. Setting ``mem_type=1`` attempts to use
31 ``pgprot_noncached``, which only works on some platforms. This is because pstore
32 depends on atomic operations. At least on ARM, pgprot_noncached causes the
33 memory to be mapped strongly ordered, and atomic operations on strongly ordered
34 memory are implementation defined, and won't work on many ARMs such as omaps.
35 Setting ``mem_type=2`` attempts to treat the memory region as normal memory,
36 which enables full cache on it. This can improve the performance.
37
38 The memory area is divided into ``record_size`` chunks (also rounded down to
39 power of two) and each kmesg dump writes a ``record_size`` chunk of
40 information.
41
42 Limiting which kinds of kmsg dumps are stored can be controlled via
43 the ``max_reason`` value, as defined in include/linux/kmsg_dump.h's
44 ``enum kmsg_dump_reason``. For example, to store both Oopses and Panics,
45 ``max_reason`` should be set to 2 (KMSG_DUMP_OOPS), to store only Panics
46 ``max_reason`` should be set to 1 (KMSG_DUMP_PANIC). Setting this to 0
47 (KMSG_DUMP_UNDEF), means the reason filtering will be controlled by the
48 ``printk.always_kmsg_dump`` boot param: if unset, it'll be KMSG_DUMP_OOPS,
49 otherwise KMSG_DUMP_MAX.
50
51 The module uses a counter to record multiple dumps but the counter gets reset
52 on restart (i.e. new dumps after the restart will overwrite old ones).
53
54 Ramoops also supports software ECC protection of persistent memory regions.
55 This might be useful when a hardware reset was used to bring the machine back
56 to life (i.e. a watchdog triggered). In such cases, RAM may be somewhat
57 corrupt, but usually it is restorable.
58
59 Setting the parameters
60 ----------------------
61
62 Setting the ramoops parameters can be done in several different manners:
63
64 A. Use the module parameters (which have the names of the variables described
65 as before). For quick debugging, you can also reserve parts of memory during
66 boot and then use the reserved memory for ramoops. For example, assuming a
67 machine with > 128 MB of memory, the following kernel command line will tell
68 the kernel to use only the first 128 MB of memory, and place ECC-protected
69 ramoops region at 128 MB boundary::
70
71 mem=128M ramoops.mem_address=0x8000000 ramoops.ecc=1
72
73 B. Use Device Tree bindings, as described in
74 ``Documentation/devicetree/bindings/reserved-memory/ramoops.yaml``.
75 For example::
76
77 reserved-memory {
78 #address-cells = <2>;
79 #size-cells = <2>;
80 ranges;
81
82 ramoops@8f000000 {
83 compatible = "ramoops";
84 reg = <0 0x8f000000 0 0x100000>;
85 record-size = <0x4000>;
86 console-size = <0x4000>;
87 };
88 };
89
90 C. Use a platform device and set the platform data. The parameters can then
91 be set through that platform data. An example of doing that is:
92
93 .. code-block:: c
94
95 #include <linux/pstore_ram.h>
96 [...]
97
98 static struct ramoops_platform_data ramoops_data = {
99 .mem_size = <...>,
100 .mem_address = <...>,
101 .mem_type = <...>,
102 .record_size = <...>,
103 .max_reason = <...>,
104 .ecc = <...>,
105 };
106
107 static struct platform_device ramoops_dev = {
108 .name = "ramoops",
109 .dev = {
110 .platform_data = &ramoops_data,
111 },
112 };
113
114 [... inside a function ...]
115 int ret;
116
117 ret = platform_device_register(&ramoops_dev);
118 if (ret) {
119 printk(KERN_ERR "unable to register platform device\n");
120 return ret;
121 }
122
123 D. Using a region of memory reserved via ``reserve_mem`` command line
124 parameter. The address and size will be defined by the ``reserve_mem``
125 parameter. Note, that ``reserve_mem`` may not always allocate memory
126 in the same location, and cannot be relied upon. Testing will need
127 to be done, and it may not work on every machine, nor every kernel.
128 Consider this a "best effort" approach. The ``reserve_mem`` option
129 takes a size, alignment and name as arguments. The name is used
130 to map the memory to a label that can be retrieved by ramoops.
131
132 reserve_mem=2M:4096:oops ramoops.mem_name=oops
133
134 You can specify either RAM memory or peripheral devices' memory. However, when
135 specifying RAM, be sure to reserve the memory by issuing memblock_reserve()
136 very early in the architecture code, e.g.::
137
138 #include <linux/memblock.h>
139
140 memblock_reserve(ramoops_data.mem_address, ramoops_data.mem_size);
141
142 Dump format
143 -----------
144
145 The data dump begins with a header, currently defined as ``====`` followed by a
146 timestamp and a new line. The dump then continues with the actual data.
147
148 Reading the data
149 ----------------
150
151 The dump data can be read from the pstore filesystem. The format for these
152 files is ``dmesg-ramoops-N``, where N is the record number in memory. To delete
153 a stored record from RAM, simply unlink the respective pstore file.
154
155 Persistent function tracing
156 ---------------------------
157
158 Persistent function tracing might be useful for debugging software or hardware
159 related hangs. The functions call chain log is stored in a ``ftrace-ramoops``
160 file. Here is an example of usage::
161
162 # mount -t debugfs debugfs /sys/kernel/debug/
163 # echo 1 > /sys/kernel/debug/pstore/record_ftrace
164 # reboot -f
165 [...]
166 # mount -t pstore pstore /mnt/
167 # tail /mnt/ftrace-ramoops
168 0 ffffffff8101ea64 ffffffff8101bcda native_apic_mem_read <- disconnect_bsp_APIC+0x6a/0xc0
169 0 ffffffff8101ea44 ffffffff8101bcf6 native_apic_mem_write <- disconnect_bsp_APIC+0x86/0xc0
170 0 ffffffff81020084 ffffffff8101a4b5 hpet_disable <- native_machine_shutdown+0x75/0x90
171 0 ffffffff81005f94 ffffffff8101a4bb iommu_shutdown_noop <- native_machine_shutdown+0x7b/0x90
172 0 ffffffff8101a6a1 ffffffff8101a437 native_machine_emergency_restart <- native_machine_restart+0x37/0x40
173 0 ffffffff811f9876 ffffffff8101a73a acpi_reboot <- native_machine_emergency_restart+0xaa/0x1e0
174 0 ffffffff8101a514 ffffffff8101a772 mach_reboot_fixups <- native_machine_emergency_restart+0xe2/0x1e0
175 0 ffffffff811d9c54 ffffffff8101a7a0 __const_udelay <- native_machine_emergency_restart+0x110/0x1e0
176 0 ffffffff811d9c34 ffffffff811d9c80 __delay <- __const_udelay+0x30/0x40
177 0 ffffffff811d9d14 ffffffff811d9c3f delay_tsc <- __delay+0xf/0x20
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