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 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
  81static 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
  90static struct platform_device ramoops_dev = {
  91        .name = "ramoops",
  92        .dev = {
  93                .platform_data = &ramoops_data,
  94        },
  95};
  96
  97[... inside a function ...]
  98int ret;
  99
 100ret = platform_device_register(&ramoops_dev);
 101if (ret) {
 102        printk(KERN_ERR "unable to register platform device\n");
 103        return ret;
 104}
 105
 106You can specify either RAM memory or peripheral devices' memory. However, when
 107specifying RAM, be sure to reserve the memory by issuing memblock_reserve()
 108very early in the architecture code, e.g.:
 109
 110#include <linux/memblock.h>
 111
 112memblock_reserve(ramoops_data.mem_address, ramoops_data.mem_size);
 113
 1143. Dump format
 115
 116The data dump begins with a header, currently defined as "====" followed by a
 117timestamp and a new line. The dump then continues with the actual data.
 118
 1194. Reading the data
 120
 121The dump data can be read from the pstore filesystem. The format for these
 122files is "dmesg-ramoops-N", where N is the record number in memory. To delete
 123a stored record from RAM, simply unlink the respective pstore file.
 124
 1255. Persistent function tracing
 126
 127Persistent function tracing might be useful for debugging software or hardware
 128related hangs. The functions call chain log is stored in a "ftrace-ramoops"
 129file. 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
 147