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
  18and 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
  24Typically the default value of mem_type=0 should be used as that sets the pstore
  25mapping to pgprot_writecombine. Setting mem_type=1 attempts to use
  26pgprot_noncached, which only works on some platforms. This is because pstore
  27depends on atomic operations. At least on ARM, pgprot_noncached causes the
  28memory to be mapped strongly ordered, and atomic operations on strongly ordered
  29memory are implementation defined, and won't work on many ARMs such as omaps.
  30
  31The memory area is divided into "record_size" chunks (also rounded down to
  32power of two) and each oops/panic writes a "record_size" chunk of
  33information.
  34
  35Dumping both oopses and panics can be done by setting 1 in the "dump_oops"
  36variable while setting 0 in that variable dumps only the panics.
  37
  38The module uses a counter to record multiple dumps but the counter gets reset
  39on restart (i.e. new dumps after the restart will overwrite old ones).
  40
  41Ramoops also supports software ECC protection of persistent memory regions.
  42This might be useful when a hardware reset was used to bring the machine back
  43to life (i.e. a watchdog triggered). In such cases, RAM may be somewhat
  44corrupt, but usually it is restorable.
  45
  462. Setting the parameters
  47
  48Setting the ramoops parameters can be done in 2 different manners:
  49 1. Use the module parameters (which have the names of the variables described
  50 as before).
  51 For quick debugging, you can also reserve parts of memory during boot
  52 and then use the reserved memory for ramoops. For example, assuming a machine
  53 with > 128 MB of memory, the following kernel command line will tell the
  54 kernel to use only the first 128 MB of memory, and place ECC-protected ramoops
  55 region at 128 MB boundary:
  56 "mem=128M ramoops.mem_address=0x8000000 ramoops.ecc=1"
  57 2. Use a platform device and set the platform data. The parameters can then
  58 be set through that platform data. An example of doing that is:
  59
  60#include <linux/pstore_ram.h>
  61[...]
  62
  63static struct ramoops_platform_data ramoops_data = {
  64        .mem_size               = <...>,
  65        .mem_address            = <...>,
  66        .mem_type               = <...>,
  67        .record_size            = <...>,
  68        .dump_oops              = <...>,
  69        .ecc                    = <...>,
  70};
  71
  72static struct platform_device ramoops_dev = {
  73        .name = "ramoops",
  74        .dev = {
  75                .platform_data = &ramoops_data,
  76        },
  77};
  78
  79[... inside a function ...]
  80int ret;
  81
  82ret = platform_device_register(&ramoops_dev);
  83if (ret) {
  84        printk(KERN_ERR "unable to register platform device\n");
  85        return ret;
  86}
  87
  88You can specify either RAM memory or peripheral devices' memory. However, when
  89specifying RAM, be sure to reserve the memory by issuing memblock_reserve()
  90very early in the architecture code, e.g.:
  91
  92#include <linux/memblock.h>
  93
  94memblock_reserve(ramoops_data.mem_address, ramoops_data.mem_size);
  95
  963. Dump format
  97
  98The data dump begins with a header, currently defined as "====" followed by a
  99timestamp and a new line. The dump then continues with the actual data.
 100
 1014. Reading the data
 102
 103The dump data can be read from the pstore filesystem. The format for these
 104files is "dmesg-ramoops-N", where N is the record number in memory. To delete
 105a stored record from RAM, simply unlink the respective pstore file.
 106
 1075. Persistent function tracing
 108
 109Persistent function tracing might be useful for debugging software or hardware
 110related hangs. The functions call chain log is stored in a "ftrace-ramoops"
 111file. Here is an example of usage:
 112
 113 # mount -t debugfs debugfs /sys/kernel/debug/
 114 # echo 1 > /sys/kernel/debug/pstore/record_ftrace
 115 # reboot -f
 116 [...]
 117 # mount -t pstore pstore /mnt/
 118 # tail /mnt/ftrace-ramoops
 119 0 ffffffff8101ea64  ffffffff8101bcda  native_apic_mem_read <- disconnect_bsp_APIC+0x6a/0xc0
 120 0 ffffffff8101ea44  ffffffff8101bcf6  native_apic_mem_write <- disconnect_bsp_APIC+0x86/0xc0
 121 0 ffffffff81020084  ffffffff8101a4b5  hpet_disable <- native_machine_shutdown+0x75/0x90
 122 0 ffffffff81005f94  ffffffff8101a4bb  iommu_shutdown_noop <- native_machine_shutdown+0x7b/0x90
 123 0 ffffffff8101a6a1  ffffffff8101a437  native_machine_emergency_restart <- native_machine_restart+0x37/0x40
 124 0 ffffffff811f9876  ffffffff8101a73a  acpi_reboot <- native_machine_emergency_restart+0xaa/0x1e0
 125 0 ffffffff8101a514  ffffffff8101a772  mach_reboot_fixups <- native_machine_emergency_restart+0xe2/0x1e0
 126 0 ffffffff811d9c54  ffffffff8101a7a0  __const_udelay <- native_machine_emergency_restart+0x110/0x1e0
 127 0 ffffffff811d9c34  ffffffff811d9c80  __delay <- __const_udelay+0x30/0x40
 128 0 ffffffff811d9d14  ffffffff811d9c3f  delay_tsc <- __delay+0xf/0x20
 129
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