tree 405abccfbdcf2d00a31dfa801b883100475f043e
parent b7d8da79fcf1660c55591414e66e3a157faff1fd
author Avery Pennarun <apenwarr@gmail.com> 1331279691 -0500
committer Ke Dong <kedong@google.com> 1366254975 -0700

CONFIG_PRINTK_PERSIST: persist printk buffer across reboots.

Instead of using alloc_bootmem() when log_buf_len= is provided on the
command line, use reserve_bootmem() instead to try to reserve the memory at
a predictable physical address.  If we manage to get such an address, check
whether it has a valid header from last time, and if so, keep the data in
the existing buffer as if it had been printk'd as part of the current
session.  You can then retrieve or clear it with dmesg.  Note: you must
supply log_buf_len= on the kernel command line to activate this feature.

If reserve_bootmem() doesn't work out, we fall back to the old
alloc_bootmem() method.

The nice thing about this feature is it allows us to capture and upload
printk results after a crash and reboot, even if the system had hard crashed
so there was no chance to do something like panic or kexec.  The last few
messages before the crash might give a clue as to the crash.

Note: None of this is any use if your bootloader or BIOS wipes memory
between reboots.  On embedded systems, you have somewhat more control over
this.

Change-Id: Ia62774b435ab889f8e0285a0d30212bc47dd5d32