1Ramoops oops/panic logger 2========================= 3 4Sergiu Iordache <sergiu@chromium.org> 5 6Updated: 17 November 2011 7 80. Introduction 9 10Ramoops is an oops/panic logger that writes its logs to RAM before the system 11crashes. It works by logging oopses and panics in a circular buffer. Ramoops 12needs a system with persistent RAM so that the content of that area can 13survive after a restart. 14 151. Ramoops concepts 16 17Ramoops uses a predefined memory area to store the dump. The start and size of 18the memory area are set using two variables: 19 * "mem_address" for the start 20 * "mem_size" for the size. The memory size will be rounded down to a 21 power of two. 22 23The memory area is divided into "record_size" chunks (also rounded down to 24power of two) and each oops/panic writes a "record_size" chunk of 25information. 26 27Dumping both oopses and panics can be done by setting 1 in the "dump_oops" 28variable while setting 0 in that variable dumps only the panics. 29 30The module uses a counter to record multiple dumps but the counter gets reset 31on restart (i.e. new dumps after the restart will overwrite old ones). 32 33Ramoops also supports software ECC protection of persistent memory regions. 34This might be useful when a hardware reset was used to bring the machine back 35to life (i.e. a watchdog triggered). In such cases, RAM may be somewhat 36corrupt, but usually it is restorable. 37 382. Setting the parameters 39 40Setting the ramoops parameters can be done in 2 different manners: 41 1. Use the module parameters (which have the names of the variables described 42 as before). 43 For quick debugging, you can also reserve parts of memory during boot 44 and then use the reserved memory for ramoops. For example, assuming a machine 45 with > 128 MB of memory, the following kernel command line will tell the 46 kernel to use only the first 128 MB of memory, and place ECC-protected ramoops 47 region at 128 MB boundary: 48 "mem=128M ramoops.mem_address=0x8000000 ramoops.ecc=1" 49 2. Use a platform device and set the platform data. The parameters can then 50 be set through that platform data. An example of doing that is: 51 52#include <linux/pstore_ram.h> 53[...] 54 55static struct ramoops_platform_data ramoops_data = { 56 .mem_size = <...>, 57 .mem_address = <...>, 58 .record_size = <...>, 59 .dump_oops = <...>, 60 .ecc = <...>, 61}; 62 63static struct platform_device ramoops_dev = { 64 .name = "ramoops", 65 .dev = { 66 .platform_data = &ramoops_data, 67 }, 68}; 69 70[... inside a function ...] 71int ret; 72 73ret = platform_device_register(&ramoops_dev); 74if (ret) { 75 printk(KERN_ERR "unable to register platform device\n"); 76 return ret; 77} 78 79You can specify either RAM memory or peripheral devices' memory. However, when 80specifying RAM, be sure to reserve the memory by issuing memblock_reserve() 81very early in the architecture code, e.g.: 82 83#include <linux/memblock.h> 84 85memblock_reserve(ramoops_data.mem_address, ramoops_data.mem_size); 86 873. Dump format 88 89The data dump begins with a header, currently defined as "====" followed by a 90timestamp and a new line. The dump then continues with the actual data. 91 924. Reading the data 93 94The dump data can be read from the pstore filesystem. The format for these 95files is "dmesg-ramoops-N", where N is the record number in memory. To delete 96a stored record from RAM, simply unlink the respective pstore file. 97 985. Persistent function tracing 99 100Persistent function tracing might be useful for debugging software or hardware 101related hangs. The functions call chain log is stored in a "ftrace-ramoops" 102file. Here is an example of usage: 103 104 # mount -t debugfs debugfs /sys/kernel/debug/ 105 # echo 1 > /sys/kernel/debug/pstore/record_ftrace 106 # reboot -f 107 [...] 108 # mount -t pstore pstore /mnt/ 109 # tail /mnt/ftrace-ramoops 110 0 ffffffff8101ea64 ffffffff8101bcda native_apic_mem_read <- disconnect_bsp_APIC+0x6a/0xc0 111 0 ffffffff8101ea44 ffffffff8101bcf6 native_apic_mem_write <- disconnect_bsp_APIC+0x86/0xc0 112 0 ffffffff81020084 ffffffff8101a4b5 hpet_disable <- native_machine_shutdown+0x75/0x90 113 0 ffffffff81005f94 ffffffff8101a4bb iommu_shutdown_noop <- native_machine_shutdown+0x7b/0x90 114 0 ffffffff8101a6a1 ffffffff8101a437 native_machine_emergency_restart <- native_machine_restart+0x37/0x40 115 0 ffffffff811f9876 ffffffff8101a73a acpi_reboot <- native_machine_emergency_restart+0xaa/0x1e0 116 0 ffffffff8101a514 ffffffff8101a772 mach_reboot_fixups <- native_machine_emergency_restart+0xe2/0x1e0 117 0 ffffffff811d9c54 ffffffff8101a7a0 __const_udelay <- native_machine_emergency_restart+0x110/0x1e0 118 0 ffffffff811d9c34 ffffffff811d9c80 __delay <- __const_udelay+0x30/0x40 119 0 ffffffff811d9d14 ffffffff811d9c3f delay_tsc <- __delay+0xf/0x20 120