[deliverable/linux.git] / Documentation / ramoops.txt

Ramoops oops/panic logger
=========================

Sergiu Iordache <sergiu@chromium.org>

Updated: 17 November 2011

0. Introduction

Ramoops is an oops/panic logger that writes its logs to RAM before the system
crashes. It works by logging oopses and panics in a circular buffer. Ramoops
needs a system with persistent RAM so that the content of that area can
survive after a restart.

1. Ramoops concepts

Ramoops uses a predefined memory area to store the dump. The start and size of
the memory area are set using two variables:
  * "mem_address" for the start
  * "mem_size" for the size. The memory size will be rounded down to a
  power of two.

The memory area is divided into "record_size" chunks (also rounded down to
power of two) and each oops/panic writes a "record_size" chunk of
information.

Dumping both oopses and panics can be done by setting 1 in the "dump_oops"
variable while setting 0 in that variable dumps only the panics.

The module uses a counter to record multiple dumps but the counter gets reset
on restart (i.e. new dumps after the restart will overwrite old ones).

Ramoops also supports software ECC protection of persistent memory regions.
This might be useful when a hardware reset was used to bring the machine back
to life (i.e. a watchdog triggered). In such cases, RAM may be somewhat
corrupt, but usually it is restorable.

2. Setting the parameters

Setting the ramoops parameters can be done in 2 different manners:
 1. Use the module parameters (which have the names of the variables described
 as before).
 2. Use a platform device and set the platform data. The parameters can then
 be set through that platform data. An example of doing that is:

#include <linux/pstore_ram.h>
[...]

static struct ramoops_platform_data ramoops_data = {
        .mem_size               = <...>,
        .mem_address            = <...>,
        .record_size            = <...>,
        .dump_oops              = <...>,
        .ecc                    = <...>,
};

static struct platform_device ramoops_dev = {
        .name = "ramoops",
        .dev = {
                .platform_data = &ramoops_data,
        },
};

[... inside a function ...]
int ret;

ret = platform_device_register(&ramoops_dev);
if (ret) {
	printk(KERN_ERR "unable to register platform device\n");
	return ret;
}

3. Dump format

The data dump begins with a header, currently defined as "====" followed by a
timestamp and a new line. The dump then continues with the actual data.

4. Reading the data

The dump data can be read from the pstore filesystem. The format for these
files is "dmesg-ramoops-N", where N is the record number in memory. To delete
a stored record from RAM, simply unlink the respective pstore file.
Commit	Line	Data
4126dacb SI	1	Ramoops oops/panic logger
	2	=========================
	3
	4	Sergiu Iordache <sergiu@chromium.org>
	5
9ba80d99	6	Updated: 17 November 2011
4126dacb SI	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 of
	18	the memory area are set using two 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
	23	The memory area is divided into "record_size" chunks (also rounded down to
	24	power of two) and each oops/panic writes a "record_size" chunk of
	25	information.
	26
	27	Dumping both oopses and panics can be done by setting 1 in the "dump_oops"
	28	variable while setting 0 in that variable dumps only the panics.
	29
	30	The module uses a counter to record multiple dumps but the counter gets reset
	31	on restart (i.e. new dumps after the restart will overwrite old ones).
	32
39eb7e97 AV	33	Ramoops also supports software ECC protection of persistent memory regions.
	34	This might be useful when a hardware reset was used to bring the machine back
	35	to life (i.e. a watchdog triggered). In such cases, RAM may be somewhat
	36	corrupt, but usually it is restorable.
	37
4126dacb SI	38	2. Setting the parameters
	39
	40	Setting the ramoops parameters can be done in 2 different manners:
	41	1. Use the module parameters (which have the names of the variables described
	42	as before).
	43	2. Use a platform device and set the platform data. The parameters can then
	44	be set through that platform data. An example of doing that is:
	45
1894a253	46	#include <linux/pstore_ram.h>
4126dacb SI	47	[...]
	48
	49	static struct ramoops_platform_data ramoops_data = {
	50	.mem_size = <...>,
	51	.mem_address = <...>,
	52	.record_size = <...>,
	53	.dump_oops = <...>,
39eb7e97	54	.ecc = <...>,
4126dacb SI	55	};
	56
	57	static struct platform_device ramoops_dev = {
	58	.name = "ramoops",
	59	.dev = {
	60	.platform_data = &ramoops_data,
	61	},
	62	};
	63
	64	[... inside a function ...]
	65	int ret;
	66
	67	ret = platform_device_register(&ramoops_dev);
	68	if (ret) {
	69	printk(KERN_ERR "unable to register platform device\n");
	70	return ret;
	71	}
	72
	73	3. Dump format
	74
	75	The data dump begins with a header, currently defined as "====" followed by a
	76	timestamp and a new line. The dump then continues with the actual data.
	77
	78	4. Reading the data
	79
9ba80d99 KC	80	The dump data can be read from the pstore filesystem. The format for these
	81	files is "dmesg-ramoops-N", where N is the record number in memory. To delete
	82	a stored record from RAM, simply unlink the respective pstore file.