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Documentation/ramoops.txt
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Ramoops oops/panic logger ========================= Sergiu Iordache <sergiu@chromium.org> |
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Updated: 17 November 2011 |
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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). |
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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. |
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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). |
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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" |
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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: |
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#include <linux/pstore_ram.h> |
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[...] static struct ramoops_platform_data ramoops_data = { .mem_size = <...>, .mem_address = <...>, .record_size = <...>, .dump_oops = <...>, |
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.ecc = <...>, |
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}; 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 "); return ret; } |
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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); |
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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 |
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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. |
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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/ |
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# echo 1 > /sys/kernel/debug/pstore/record_ftrace |
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# 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 |