--- /dev/null
+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 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 = <...>,
+ .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\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/
+ # 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
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