1Ramoops oops/panic logger 2========================= 3 4Sergiu Iordache <sergiu@chromium.org> 5 6Updated: 10 Feb 2021 7 8Introduction 9------------ 10 11Ramoops is an oops/panic logger that writes its logs to RAM before the system 12crashes. It works by logging oopses and panics in a circular buffer. Ramoops 13needs a system with persistent RAM so that the content of that area can 14survive after a restart. 15 16Ramoops concepts 17---------------- 18 19Ramoops uses a predefined memory area to store the dump. The start and size 20and type of the memory area are set using three variables: 21 22 * ``mem_address`` for the start 23 * ``mem_size`` for the size. The memory size will be rounded down to a 24 power of two. 25 * ``mem_type`` to specify if the memory type (default is pgprot_writecombine). 26 * ``mem_name`` to specify a memory region defined by ``reserve_mem`` command 27 line parameter. 28 29Typically the default value of ``mem_type=0`` should be used as that sets the pstore 30mapping to pgprot_writecombine. Setting ``mem_type=1`` attempts to use 31``pgprot_noncached``, which only works on some platforms. This is because pstore 32depends on atomic operations. At least on ARM, pgprot_noncached causes the 33memory to be mapped strongly ordered, and atomic operations on strongly ordered 34memory are implementation defined, and won't work on many ARMs such as omaps. 35Setting ``mem_type=2`` attempts to treat the memory region as normal memory, 36which enables full cache on it. This can improve the performance. 37 38The memory area is divided into ``record_size`` chunks (also rounded down to 39power of two) and each kmesg dump writes a ``record_size`` chunk of 40information. 41 42Limiting which kinds of kmsg dumps are stored can be controlled via 43the ``max_reason`` value, as defined in include/linux/kmsg_dump.h's 44``enum kmsg_dump_reason``. For example, to store both Oopses and Panics, 45``max_reason`` should be set to 2 (KMSG_DUMP_OOPS), to store only Panics 46``max_reason`` should be set to 1 (KMSG_DUMP_PANIC). Setting this to 0 47(KMSG_DUMP_UNDEF), means the reason filtering will be controlled by the 48``printk.always_kmsg_dump`` boot param: if unset, it'll be KMSG_DUMP_OOPS, 49otherwise KMSG_DUMP_MAX. 50 51The module uses a counter to record multiple dumps but the counter gets reset 52on restart (i.e. new dumps after the restart will overwrite old ones). 53 54Ramoops also supports software ECC protection of persistent memory regions. 55This might be useful when a hardware reset was used to bring the machine back 56to life (i.e. a watchdog triggered). In such cases, RAM may be somewhat 57corrupt, but usually it is restorable. 58 59Setting the parameters 60---------------------- 61 62Setting the ramoops parameters can be done in several different manners: 63 64 A. Use the module parameters (which have the names of the variables described 65 as before). For quick debugging, you can also reserve parts of memory during 66 boot and then use the reserved memory for ramoops. For example, assuming a 67 machine with > 128 MB of memory, the following kernel command line will tell 68 the kernel to use only the first 128 MB of memory, and place ECC-protected 69 ramoops region at 128 MB boundary:: 70 71 mem=128M ramoops.mem_address=0x8000000 ramoops.ecc=1 72 73 B. Use Device Tree bindings, as described in 74 ``Documentation/devicetree/bindings/reserved-memory/ramoops.yaml``. 75 For example:: 76 77 reserved-memory { 78 #address-cells = <2>; 79 #size-cells = <2>; 80 ranges; 81 82 ramoops@8f000000 { 83 compatible = "ramoops"; 84 reg = <0 0x8f000000 0 0x100000>; 85 record-size = <0x4000>; 86 console-size = <0x4000>; 87 }; 88 }; 89 90 C. Use a platform device and set the platform data. The parameters can then 91 be set through that platform data. An example of doing that is: 92 93 .. code-block:: c 94 95 #include <linux/pstore_ram.h> 96 [...] 97 98 static struct ramoops_platform_data ramoops_data = { 99 .mem_size = <...>, 100 .mem_address = <...>, 101 .mem_type = <...>, 102 .record_size = <...>, 103 .max_reason = <...>, 104 .ecc = <...>, 105 }; 106 107 static struct platform_device ramoops_dev = { 108 .name = "ramoops", 109 .dev = { 110 .platform_data = &ramoops_data, 111 }, 112 }; 113 114 [... inside a function ...] 115 int ret; 116 117 ret = platform_device_register(&ramoops_dev); 118 if (ret) { 119 printk(KERN_ERR "unable to register platform device\n"); 120 return ret; 121 } 122 123 D. Using a region of memory reserved via ``reserve_mem`` command line 124 parameter. The address and size will be defined by the ``reserve_mem`` 125 parameter. Note, that ``reserve_mem`` may not always allocate memory 126 in the same location, and cannot be relied upon. Testing will need 127 to be done, and it may not work on every machine, nor every kernel. 128 Consider this a "best effort" approach. The ``reserve_mem`` option 129 takes a size, alignment and name as arguments. The name is used 130 to map the memory to a label that can be retrieved by ramoops. 131 132 reserve_mem=2M:4096:oops ramoops.mem_name=oops 133 134You can specify either RAM memory or peripheral devices' memory. However, when 135specifying RAM, be sure to reserve the memory by issuing memblock_reserve() 136very early in the architecture code, e.g.:: 137 138 #include <linux/memblock.h> 139 140 memblock_reserve(ramoops_data.mem_address, ramoops_data.mem_size); 141 142Dump format 143----------- 144 145The data dump begins with a header, currently defined as ``====`` followed by a 146timestamp and a new line. The dump then continues with the actual data. 147 148Reading the data 149---------------- 150 151The dump data can be read from the pstore filesystem. The format for these 152files is ``dmesg-ramoops-N``, where N is the record number in memory. To delete 153a stored record from RAM, simply unlink the respective pstore file. 154 155Persistent function tracing 156--------------------------- 157 158Persistent function tracing might be useful for debugging software or hardware 159related hangs. The functions call chain log is stored in a ``ftrace-ramoops`` 160file. Here is an example of usage:: 161 162 # mount -t debugfs debugfs /sys/kernel/debug/ 163 # echo 1 > /sys/kernel/debug/pstore/record_ftrace 164 # reboot -f 165 [...] 166 # mount -t pstore pstore /mnt/ 167 # tail /mnt/ftrace-ramoops 168 0 ffffffff8101ea64 ffffffff8101bcda native_apic_mem_read <- disconnect_bsp_APIC+0x6a/0xc0 169 0 ffffffff8101ea44 ffffffff8101bcf6 native_apic_mem_write <- disconnect_bsp_APIC+0x86/0xc0 170 0 ffffffff81020084 ffffffff8101a4b5 hpet_disable <- native_machine_shutdown+0x75/0x90 171 0 ffffffff81005f94 ffffffff8101a4bb iommu_shutdown_noop <- native_machine_shutdown+0x7b/0x90 172 0 ffffffff8101a6a1 ffffffff8101a437 native_machine_emergency_restart <- native_machine_restart+0x37/0x40 173 0 ffffffff811f9876 ffffffff8101a73a acpi_reboot <- native_machine_emergency_restart+0xaa/0x1e0 174 0 ffffffff8101a514 ffffffff8101a772 mach_reboot_fixups <- native_machine_emergency_restart+0xe2/0x1e0 175 0 ffffffff811d9c54 ffffffff8101a7a0 __const_udelay <- native_machine_emergency_restart+0x110/0x1e0 176 0 ffffffff811d9c34 ffffffff811d9c80 __delay <- __const_udelay+0x30/0x40 177 0 ffffffff811d9d14 ffffffff811d9c3f delay_tsc <- __delay+0xf/0x20 178