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Based on kernel version 4.9. Page generated on 2016-12-21 14:36 EST.

1	Ramoops oops/panic logger
2	=========================
3	
4	Sergiu Iordache <sergiu@chromium.org>
5	
6	Updated: 17 November 2011
7	
8	0. Introduction
9	
10	Ramoops is an oops/panic logger that writes its logs to RAM before the system
11	crashes. It works by logging oopses and panics in a circular buffer. Ramoops
12	needs a system with persistent RAM so that the content of that area can
13	survive after a restart.
14	
15	1. Ramoops concepts
16	
17	Ramoops uses a predefined memory area to store the dump. The start and size
18	and type of the memory area are set using three variables:
19	  * "mem_address" for the start
20	  * "mem_size" for the size. The memory size will be rounded down to a
21	  power of two.
22	  * "mem_type" to specifiy if the memory type (default is pgprot_writecombine).
23	
24	Typically the default value of mem_type=0 should be used as that sets the pstore
25	mapping to pgprot_writecombine. Setting mem_type=1 attempts to use
26	pgprot_noncached, which only works on some platforms. This is because pstore
27	depends on atomic operations. At least on ARM, pgprot_noncached causes the
28	memory to be mapped strongly ordered, and atomic operations on strongly ordered
29	memory are implementation defined, and won't work on many ARMs such as omaps.
30	
31	The memory area is divided into "record_size" chunks (also rounded down to
32	power of two) and each oops/panic writes a "record_size" chunk of
33	information.
34	
35	Dumping both oopses and panics can be done by setting 1 in the "dump_oops"
36	variable while setting 0 in that variable dumps only the panics.
37	
38	The module uses a counter to record multiple dumps but the counter gets reset
39	on restart (i.e. new dumps after the restart will overwrite old ones).
40	
41	Ramoops also supports software ECC protection of persistent memory regions.
42	This might be useful when a hardware reset was used to bring the machine back
43	to life (i.e. a watchdog triggered). In such cases, RAM may be somewhat
44	corrupt, but usually it is restorable.
45	
46	2. Setting the parameters
47	
48	Setting the ramoops parameters can be done in several different manners:
49	
50	 A. Use the module parameters (which have the names of the variables described
51	 as before). For quick debugging, you can also reserve parts of memory during
52	 boot and then use the reserved memory for ramoops. For example, assuming a
53	 machine with > 128 MB of memory, the following kernel command line will tell
54	 the kernel to use only the first 128 MB of memory, and place ECC-protected
55	 ramoops region at 128 MB boundary:
56	 "mem=128M ramoops.mem_address=0x8000000 ramoops.ecc=1"
57	
58	 B. Use Device Tree bindings, as described in
59	 Documentation/device-tree/bindings/reserved-memory/ramoops.txt.
60	 For example:
61	
62		reserved-memory {
63			#address-cells = <2>;
64			#size-cells = <2>;
65			ranges;
66	
67			ramoops@8f000000 {
68				compatible = "ramoops";
69				reg = <0 0x8f000000 0 0x100000>;
70				record-size = <0x4000>;
71				console-size = <0x4000>;
72			};
73		};
74	
75	 C. Use a platform device and set the platform data. The parameters can then
76	 be set through that platform data. An example of doing that is:
77	
78	#include <linux/pstore_ram.h>
79	[...]
80	
81	static struct ramoops_platform_data ramoops_data = {
82	        .mem_size               = <...>,
83	        .mem_address            = <...>,
84	        .mem_type               = <...>,
85	        .record_size            = <...>,
86	        .dump_oops              = <...>,
87	        .ecc                    = <...>,
88	};
89	
90	static struct platform_device ramoops_dev = {
91	        .name = "ramoops",
92	        .dev = {
93	                .platform_data = &ramoops_data,
94	        },
95	};
96	
97	[... inside a function ...]
98	int ret;
99	
100	ret = platform_device_register(&ramoops_dev);
101	if (ret) {
102		printk(KERN_ERR "unable to register platform device\n");
103		return ret;
104	}
105	
106	You can specify either RAM memory or peripheral devices' memory. However, when
107	specifying RAM, be sure to reserve the memory by issuing memblock_reserve()
108	very early in the architecture code, e.g.:
109	
110	#include <linux/memblock.h>
111	
112	memblock_reserve(ramoops_data.mem_address, ramoops_data.mem_size);
113	
114	3. Dump format
115	
116	The data dump begins with a header, currently defined as "====" followed by a
117	timestamp and a new line. The dump then continues with the actual data.
118	
119	4. Reading the data
120	
121	The dump data can be read from the pstore filesystem. The format for these
122	files is "dmesg-ramoops-N", where N is the record number in memory. To delete
123	a stored record from RAM, simply unlink the respective pstore file.
124	
125	5. Persistent function tracing
126	
127	Persistent function tracing might be useful for debugging software or hardware
128	related hangs. The functions call chain log is stored in a "ftrace-ramoops"
129	file. Here is an example of usage:
130	
131	 # mount -t debugfs debugfs /sys/kernel/debug/
132	 # echo 1 > /sys/kernel/debug/pstore/record_ftrace
133	 # reboot -f
134	 [...]
135	 # mount -t pstore pstore /mnt/
136	 # tail /mnt/ftrace-ramoops
137	 0 ffffffff8101ea64  ffffffff8101bcda  native_apic_mem_read <- disconnect_bsp_APIC+0x6a/0xc0
138	 0 ffffffff8101ea44  ffffffff8101bcf6  native_apic_mem_write <- disconnect_bsp_APIC+0x86/0xc0
139	 0 ffffffff81020084  ffffffff8101a4b5  hpet_disable <- native_machine_shutdown+0x75/0x90
140	 0 ffffffff81005f94  ffffffff8101a4bb  iommu_shutdown_noop <- native_machine_shutdown+0x7b/0x90
141	 0 ffffffff8101a6a1  ffffffff8101a437  native_machine_emergency_restart <- native_machine_restart+0x37/0x40
142	 0 ffffffff811f9876  ffffffff8101a73a  acpi_reboot <- native_machine_emergency_restart+0xaa/0x1e0
143	 0 ffffffff8101a514  ffffffff8101a772  mach_reboot_fixups <- native_machine_emergency_restart+0xe2/0x1e0
144	 0 ffffffff811d9c54  ffffffff8101a7a0  __const_udelay <- native_machine_emergency_restart+0x110/0x1e0
145	 0 ffffffff811d9c34  ffffffff811d9c80  __delay <- __const_udelay+0x30/0x40
146	 0 ffffffff811d9d14  ffffffff811d9c3f  delay_tsc <- __delay+0xf/0x20
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