linux/Documentation/ramoops.txt
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   1Ramoops oops/panic logger
   2=========================
   3
   4Sergiu Iordache <sergiu@chromium.org>
   5
   6Updated: 17 November 2011
   7
   80. Introduction
   9
  10Ramoops is an oops/panic logger that writes its logs to RAM before the system
  11crashes. It works by logging oopses and panics in a circular buffer. Ramoops
  12needs a system with persistent RAM so that the content of that area can
  13survive after a restart.
  14
  151. Ramoops concepts
  16
  17Ramoops uses a predefined memory area to store the dump. The start and size of
  18the 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
  23The memory area is divided into "record_size" chunks (also rounded down to
  24power of two) and each oops/panic writes a "record_size" chunk of
  25information.
  26
  27Dumping both oopses and panics can be done by setting 1 in the "dump_oops"
  28variable while setting 0 in that variable dumps only the panics.
  29
  30The module uses a counter to record multiple dumps but the counter gets reset
  31on restart (i.e. new dumps after the restart will overwrite old ones).
  32
  33Ramoops also supports software ECC protection of persistent memory regions.
  34This might be useful when a hardware reset was used to bring the machine back
  35to life (i.e. a watchdog triggered). In such cases, RAM may be somewhat
  36corrupt, but usually it is restorable.
  37
  382. Setting the parameters
  39
  40Setting 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
  46#include <linux/pstore_ram.h>
  47[...]
  48
  49static struct ramoops_platform_data ramoops_data = {
  50        .mem_size               = <...>,
  51        .mem_address            = <...>,
  52        .record_size            = <...>,
  53        .dump_oops              = <...>,
  54        .ecc                    = <...>,
  55};
  56
  57static struct platform_device ramoops_dev = {
  58        .name = "ramoops",
  59        .dev = {
  60                .platform_data = &ramoops_data,
  61        },
  62};
  63
  64[... inside a function ...]
  65int ret;
  66
  67ret = platform_device_register(&ramoops_dev);
  68if (ret) {
  69        printk(KERN_ERR "unable to register platform device\n");
  70        return ret;
  71}
  72
  733. Dump format
  74
  75The data dump begins with a header, currently defined as "====" followed by a
  76timestamp and a new line. The dump then continues with the actual data.
  77
  784. Reading the data
  79
  80The dump data can be read from the pstore filesystem. The format for these
  81files is "dmesg-ramoops-N", where N is the record number in memory. To delete
  82a stored record from RAM, simply unlink the respective pstore file.
  83
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