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 For quick debugging, you can also reserve parts of memory during boot
  44 and then use the reserved memory for ramoops. For example, assuming a machine
  45 with > 128 MB of memory, the following kernel command line will tell the
  46 kernel to use only the first 128 MB of memory, and place ECC-protected ramoops
  47 region at 128 MB boundary:
  48 "mem=128M ramoops.mem_address=0x8000000 ramoops.ecc=1"
  49 2. Use a platform device and set the platform data. The parameters can then
  50 be set through that platform data. An example of doing that is:
  51
  52#include <linux/pstore_ram.h>
  53[...]
  54
  55static struct ramoops_platform_data ramoops_data = {
  56        .mem_size               = <...>,
  57        .mem_address            = <...>,
  58        .record_size            = <...>,
  59        .dump_oops              = <...>,
  60        .ecc                    = <...>,
  61};
  62
  63static struct platform_device ramoops_dev = {
  64        .name = "ramoops",
  65        .dev = {
  66                .platform_data = &ramoops_data,
  67        },
  68};
  69
  70[... inside a function ...]
  71int ret;
  72
  73ret = platform_device_register(&ramoops_dev);
  74if (ret) {
  75        printk(KERN_ERR "unable to register platform device\n");
  76        return ret;
  77}
  78
  79You can specify either RAM memory or peripheral devices' memory. However, when
  80specifying RAM, be sure to reserve the memory by issuing memblock_reserve()
  81very early in the architecture code, e.g.:
  82
  83#include <linux/memblock.h>
  84
  85memblock_reserve(ramoops_data.mem_address, ramoops_data.mem_size);
  86
  873. Dump format
  88
  89The data dump begins with a header, currently defined as "====" followed by a
  90timestamp and a new line. The dump then continues with the actual data.
  91
  924. Reading the data
  93
  94The dump data can be read from the pstore filesystem. The format for these
  95files is "dmesg-ramoops-N", where N is the record number in memory. To delete
  96a stored record from RAM, simply unlink the respective pstore file.
  97
  985. Persistent function tracing
  99
 100Persistent function tracing might be useful for debugging software or hardware
 101related hangs. The functions call chain log is stored in a "ftrace-ramoops"
 102file. Here is an example of usage:
 103
 104 # mount -t debugfs debugfs /sys/kernel/debug/
 105 # echo 1 > /sys/kernel/debug/pstore/record_ftrace
 106 # reboot -f
 107 [...]
 108 # mount -t pstore pstore /mnt/
 109 # tail /mnt/ftrace-ramoops
 110 0 ffffffff8101ea64  ffffffff8101bcda  native_apic_mem_read <- disconnect_bsp_APIC+0x6a/0xc0
 111 0 ffffffff8101ea44  ffffffff8101bcf6  native_apic_mem_write <- disconnect_bsp_APIC+0x86/0xc0
 112 0 ffffffff81020084  ffffffff8101a4b5  hpet_disable <- native_machine_shutdown+0x75/0x90
 113 0 ffffffff81005f94  ffffffff8101a4bb  iommu_shutdown_noop <- native_machine_shutdown+0x7b/0x90
 114 0 ffffffff8101a6a1  ffffffff8101a437  native_machine_emergency_restart <- native_machine_restart+0x37/0x40
 115 0 ffffffff811f9876  ffffffff8101a73a  acpi_reboot <- native_machine_emergency_restart+0xaa/0x1e0
 116 0 ffffffff8101a514  ffffffff8101a772  mach_reboot_fixups <- native_machine_emergency_restart+0xe2/0x1e0
 117 0 ffffffff811d9c54  ffffffff8101a7a0  __const_udelay <- native_machine_emergency_restart+0x110/0x1e0
 118 0 ffffffff811d9c34  ffffffff811d9c80  __delay <- __const_udelay+0x30/0x40
 119 0 ffffffff811d9d14  ffffffff811d9c3f  delay_tsc <- __delay+0xf/0x20
 120
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