Re: [PATCH 0/5] Persist printk buffer across reboots.

[Stephen Boyd]

On 3/12/2012 10:36 PM, Avery Pennarun wrote:
> The last patch in this series implements a new CONFIG_PRINTK_PERSIST option
> that, when enabled, puts the printk buffer in a well-defined memory location
> so that we can keep appending to it after a reboot.  The upshot is that,
> even after a kernel panic or non-panic hard lockup, on the next boot
> userspace will be able to grab the kernel messages leading up to it.  It
> could then upload the messages to a server (for example) to keep crash
> statistics.
>
> The preceding patches in the series are mostly just things I fixed up while
> working on that patch.
>
> Some notes:
>
> - I'm not totally sure of the locking or portability issues when calling
>    memblock or bootmem.  This all happens really early, and I *think*
>    interrupts are still disabled at that time, so it's probably okay.
>
> - Tested this version on x86 (kvm) and it works with soft reboot (ie. reboot
>    -f).  Since some BIOSes wipe the memory during boot, you might not have
>    any luck.  It should be great on many embedded systems, though, including
>    the MIPS system I've tested a variant of this patch on.  (Our MIPS build
>    is based on a slightly older kernel so it's not 100% the same, but I think
>    this should behave identically.)
>
> - The way we choose a well-defined memory location is slightly suspicious
>    (we just count down from the top of the address space) but I've tested it
>    pretty carefully, and it seems to be okay.
>
> - In printk.c with CONFIG_PRINTK_PERSIST set, we're #defining words like
>    log_end.  It might be cleaner to replace all instances of log_end with
>    LOG_END to make this more clear.  This is also the reason the struct
>    logbits members start with _: because otherwise they conflict with the
>    macro.  Suggestions welcome.

Android has something similar called ram_console (see 
staging/android/ram_console.c). The console is dumped to a ram buffer 
that is reserved very early in platform setup code. Then when the phone 
reboots you can cat /proc/last_kmsg to get the previous kernel message 
for debugging. Can you use that code?

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Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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