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diff --git a/Documentation/admin-guide/ramoops.rst b/Documentation/admin-guide/ramoops.rst new file mode 100644 index 000000000..e9f851421 --- /dev/null +++ b/Documentation/admin-guide/ramoops.rst @@ -0,0 +1,164 @@ +Ramoops oops/panic logger +========================= + +Sergiu Iordache <sergiu@chromium.org> + +Updated: 10 Feb 2021 + +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. + +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 specify 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. +Setting ``mem_type=2`` attempts to treat the memory region as normal memory, +which enables full cache on it. This can improve the performance. + +The memory area is divided into ``record_size`` chunks (also rounded down to +power of two) and each kmesg dump writes a ``record_size`` chunk of +information. + +Limiting which kinds of kmsg dumps are stored can be controlled via +the ``max_reason`` value, as defined in include/linux/kmsg_dump.h's +``enum kmsg_dump_reason``. For example, to store both Oopses and Panics, +``max_reason`` should be set to 2 (KMSG_DUMP_OOPS), to store only Panics +``max_reason`` should be set to 1 (KMSG_DUMP_PANIC). Setting this to 0 +(KMSG_DUMP_UNDEF), means the reason filtering will be controlled by the +``printk.always_kmsg_dump`` boot param: if unset, it'll be KMSG_DUMP_OOPS, +otherwise KMSG_DUMP_MAX. + +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. + +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/devicetree/bindings/reserved-memory/ramoops.yaml``. + 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: + + .. code-block:: c + + #include <linux/pstore_ram.h> + [...] + + static struct ramoops_platform_data ramoops_data = { + .mem_size = <...>, + .mem_address = <...>, + .mem_type = <...>, + .record_size = <...>, + .max_reason = <...>, + .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); + +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. + +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. + +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 |