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
  and type of the memory area are set using three variables:

    * "mem_address" for the start
    * "mem_size" for the size. The memory size will be rounded down to a
    power of two.

    * "mem_type" to specifiy if the memory type (default is pgprot_writecombine).
  
  Typically the default value of mem_type=0 should be used as that sets the pstore
  mapping to pgprot_writecombine. Setting mem_type=1 attempts to use
  pgprot_noncached, which only works on some platforms. This is because pstore
  depends on atomic operations. At least on ARM, pgprot_noncached causes the
  memory to be mapped strongly ordered, and atomic operations on strongly ordered
  memory are implementation defined, and won't work on many ARMs such as omaps.

  
  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 several different manners:
  
   A. Use the module parameters (which have the names of the variables described
   as before). For quick debugging, you can also reserve parts of memory during
   boot and then use the reserved memory for ramoops. For example, assuming a
   machine with > 128 MB of memory, the following kernel command line will tell
   the kernel to use only the first 128 MB of memory, and place ECC-protected
   ramoops region at 128 MB boundary:

   "mem=128M ramoops.mem_address=0x8000000 ramoops.ecc=1"

  
   B. Use Device Tree bindings, as described in
   Documentation/device-tree/bindings/reserved-memory/ramoops.txt.
   For example:
  
  	reserved-memory {
  		#address-cells = <2>;
  		#size-cells = <2>;
  		ranges;
  
  		ramoops@8f000000 {
  			compatible = "ramoops";
  			reg = <0 0x8f000000 0 0x100000>;
  			record-size = <0x4000>;
  			console-size = <0x4000>;
  		};
  	};
  
   C. 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            = <...>,

          .mem_type               = <...>,

          .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
  ");
  	return ret;
  }

  You can specify either RAM memory or peripheral devices' memory. However, when
  specifying RAM, be sure to reserve the memory by issuing memblock_reserve()
  very early in the architecture code, e.g.:
  
  #include <linux/memblock.h>
  
  memblock_reserve(ramoops_data.mem_address, ramoops_data.mem_size);

  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.

  
  5. Persistent function tracing
  
  Persistent function tracing might be useful for debugging software or hardware
  related hangs. The functions call chain log is stored in a "ftrace-ramoops"
  file. Here is an example of usage:
  
   # mount -t debugfs debugfs /sys/kernel/debug/

   # echo 1 > /sys/kernel/debug/pstore/record_ftrace

   # reboot -f
   [...]
   # mount -t pstore pstore /mnt/
   # tail /mnt/ftrace-ramoops
   0 ffffffff8101ea64  ffffffff8101bcda  native_apic_mem_read <- disconnect_bsp_APIC+0x6a/0xc0
   0 ffffffff8101ea44  ffffffff8101bcf6  native_apic_mem_write <- disconnect_bsp_APIC+0x86/0xc0
   0 ffffffff81020084  ffffffff8101a4b5  hpet_disable <- native_machine_shutdown+0x75/0x90
   0 ffffffff81005f94  ffffffff8101a4bb  iommu_shutdown_noop <- native_machine_shutdown+0x7b/0x90
   0 ffffffff8101a6a1  ffffffff8101a437  native_machine_emergency_restart <- native_machine_restart+0x37/0x40
   0 ffffffff811f9876  ffffffff8101a73a  acpi_reboot <- native_machine_emergency_restart+0xaa/0x1e0
   0 ffffffff8101a514  ffffffff8101a772  mach_reboot_fixups <- native_machine_emergency_restart+0xe2/0x1e0
   0 ffffffff811d9c54  ffffffff8101a7a0  __const_udelay <- native_machine_emergency_restart+0x110/0x1e0
   0 ffffffff811d9c34  ffffffff811d9c80  __delay <- __const_udelay+0x30/0x40
   0 ffffffff811d9d14  ffffffff811d9c3f  delay_tsc <- __delay+0xf/0x20