From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marco <marco.stornelli@gmail.com>
Subject: Re: Saving the kernel log accross boots
Date: Thu, 25 Jun 2009 19:02:43 +0200
Message-ID: <4A43ADB3.20808@gmail.com>
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To: Martin Meuli <Martin.Meuli@gmx.ch>
Cc: linux-embedded@vger.kernel.org

Martin Meuli wrote:
> Hi All
> 
> In case of a crash we would like to save the kernel log and retrieve it
> in the next boot.
> There are already several suggestions on how to do it on this and other
> lists.
> (Kernel crashing and log buffers: Robin Getz)
> Unfortunately we don't have extra SRAM to use for it. We need to do it
> in the 'normal' RAM.
> 
> ARCH=ARM
> 
> What we did:
> - modify kernel/printk.c: use
> __attribute__((__section__(".bss.kringbuf"... to put the log related
> stuf into a separate section
> - modify the linker script to link .bss.kringbuf at the start of the
> .bss section
> - modify arch/arm/kernel/head-common.S to exclude the new section from
> being nulled (if it was a watchdog restart)
> - modify kernel/printk.c to use cach_flush after prink
> - make sure the bootloader does not overwrite that memory
> 
> This seems to work.
> After a crash we get the messages from the new boot and the last crash.
> It seems that the decompress code is using memory at the end of the
> kernel and destroys the log. So we have to use the bootloader to
> decompress the image into RAM.
> 
> Before digging any deeper I would like to know if someone as a better
> (more general) idea.
> 
> Regards, Martin

I submitted for a review a new filesystem for 2.6.30 called pramfs that
it can be useful for your case. You could use a little piece of ram to
store the log.

Marco