linux/Documentation/ramoops.txt

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 of
the memory area are set using two variables:
  * "mem_address" for the start
  * "mem_size" for the size. The memory size will be rounded down to a
  power of two.

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 2 different manners:
 1. 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"
 2. 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            = <...>,
        .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\n");
	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/
 # cd /sys/kernel/debug/tracing
 # echo function > current_tracer
 # echo 1 > options/func_pstore
 # 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