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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.

1	Ramoops oops/panic logger
2	=========================
3	
4	Sergiu Iordache <sergiu@chromium.org>
5	
6	Updated: 17 November 2011
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 specifiy if the memory type (default is pgprot_writecombine).
26	
27	Typically the default value of ``mem_type=0`` should be used as that sets the pstore
28	mapping to pgprot_writecombine. Setting ``mem_type=1`` attempts to use
29	``pgprot_noncached``, which only works on some platforms. This is because pstore
30	depends on atomic operations. At least on ARM, pgprot_noncached causes the
31	memory to be mapped strongly ordered, and atomic operations on strongly ordered
32	memory are implementation defined, and won't work on many ARMs such as omaps.
33	
34	The memory area is divided into ``record_size`` chunks (also rounded down to
35	power of two) and each oops/panic writes a ``record_size`` chunk of
36	information.
37	
38	Dumping both oopses and panics can be done by setting 1 in the ``dump_oops``
39	variable while setting 0 in that variable dumps only the panics.
40	
41	The module uses a counter to record multiple dumps but the counter gets reset
42	on restart (i.e. new dumps after the restart will overwrite old ones).
43	
44	Ramoops also supports software ECC protection of persistent memory regions.
45	This might be useful when a hardware reset was used to bring the machine back
46	to life (i.e. a watchdog triggered). In such cases, RAM may be somewhat
47	corrupt, but usually it is restorable.
48	
49	Setting the parameters
50	----------------------
51	
52	Setting the ramoops parameters can be done in several different manners:
53	
54	 A. Use the module parameters (which have the names of the variables described
55	 as before). For quick debugging, you can also reserve parts of memory during
56	 boot and then use the reserved memory for ramoops. For example, assuming a
57	 machine with > 128 MB of memory, the following kernel command line will tell
58	 the kernel to use only the first 128 MB of memory, and place ECC-protected
59	 ramoops region at 128 MB boundary::
60	
61		mem=128M ramoops.mem_address=0x8000000 ramoops.ecc=1
62	
63	 B. Use Device Tree bindings, as described in
64	 ``Documentation/device-tree/bindings/reserved-memory/admin-guide/ramoops.rst``.
65	 For example::
66	
67		reserved-memory {
68			#address-cells = <2>;
69			#size-cells = <2>;
70			ranges;
71	
72			ramoops@8f000000 {
73				compatible = "ramoops";
74				reg = <0 0x8f000000 0 0x100000>;
75				record-size = <0x4000>;
76				console-size = <0x4000>;
77			};
78		};
79	
80	 C. Use a platform device and set the platform data. The parameters can then
81	 be set through that platform data. An example of doing that is:
82	
83	 .. code-block:: c
84	
85	  #include <linux/pstore_ram.h>
86	  [...]
87	
88	  static struct ramoops_platform_data ramoops_data = {
89	        .mem_size               = <...>,
90	        .mem_address            = <...>,
91	        .mem_type               = <...>,
92	        .record_size            = <...>,
93	        .dump_oops              = <...>,
94	        .ecc                    = <...>,
95	  };
96	
97	  static struct platform_device ramoops_dev = {
98	        .name = "ramoops",
99	        .dev = {
100	                .platform_data = &ramoops_data,
101	        },
102	  };
103	
104	  [... inside a function ...]
105	  int ret;
106	
107	  ret = platform_device_register(&ramoops_dev);
108	  if (ret) {
109		printk(KERN_ERR "unable to register platform device\n");
110		return ret;
111	  }
112	
113	You can specify either RAM memory or peripheral devices' memory. However, when
114	specifying RAM, be sure to reserve the memory by issuing memblock_reserve()
115	very early in the architecture code, e.g.::
116	
117		#include <linux/memblock.h>
118	
119		memblock_reserve(ramoops_data.mem_address, ramoops_data.mem_size);
120	
121	Dump format
122	-----------
123	
124	The data dump begins with a header, currently defined as ``====`` followed by a
125	timestamp and a new line. The dump then continues with the actual data.
126	
127	Reading the data
128	----------------
129	
130	The dump data can be read from the pstore filesystem. The format for these
131	files is ``dmesg-ramoops-N``, where N is the record number in memory. To delete
132	a stored record from RAM, simply unlink the respective pstore file.
133	
134	Persistent function tracing
135	---------------------------
136	
137	Persistent function tracing might be useful for debugging software or hardware
138	related hangs. The functions call chain log is stored in a ``ftrace-ramoops``
139	file. Here is an example of usage::
140	
141	 # mount -t debugfs debugfs /sys/kernel/debug/
142	 # echo 1 > /sys/kernel/debug/pstore/record_ftrace
143	 # reboot -f
144	 [...]
145	 # mount -t pstore pstore /mnt/
146	 # tail /mnt/ftrace-ramoops
147	 0 ffffffff8101ea64  ffffffff8101bcda  native_apic_mem_read <- disconnect_bsp_APIC+0x6a/0xc0
148	 0 ffffffff8101ea44  ffffffff8101bcf6  native_apic_mem_write <- disconnect_bsp_APIC+0x86/0xc0
149	 0 ffffffff81020084  ffffffff8101a4b5  hpet_disable <- native_machine_shutdown+0x75/0x90
150	 0 ffffffff81005f94  ffffffff8101a4bb  iommu_shutdown_noop <- native_machine_shutdown+0x7b/0x90
151	 0 ffffffff8101a6a1  ffffffff8101a437  native_machine_emergency_restart <- native_machine_restart+0x37/0x40
152	 0 ffffffff811f9876  ffffffff8101a73a  acpi_reboot <- native_machine_emergency_restart+0xaa/0x1e0
153	 0 ffffffff8101a514  ffffffff8101a772  mach_reboot_fixups <- native_machine_emergency_restart+0xe2/0x1e0
154	 0 ffffffff811d9c54  ffffffff8101a7a0  __const_udelay <- native_machine_emergency_restart+0x110/0x1e0
155	 0 ffffffff811d9c34  ffffffff811d9c80  __delay <- __const_udelay+0x30/0x40
156	 0 ffffffff811d9d14  ffffffff811d9c3f  delay_tsc <- __delay+0xf/0x20
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