From: Arnd Bergmann <arnd@arndb.de>
To: "Chris Friesen" <cfriesen@nortel.com>
Subject: Re: RFC: Put printk buffer in video ram
Date: Mon, 23 Nov 2009 20:03:31 +0100
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Cc: Ingo Molnar <mingo@elte.hu>, Maxim Levitsky <maximlevitsky@gmail.com>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       Arjan van de Ven <arjan@infradead.org>, Dave Jones <davej@redhat.com>,
       "linux-kernel" <linux-kernel@vger.kernel.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Steven Rostedt <rostedt@goodmis.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       linuxppc-dev@lists.ozlabs.org
References: <1258855506.3153.1.camel@maxim-laptop> <20091123183220.GB24482@elte.hu> <4B0ADA84.5050300@nortel.com>
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On Monday 23 November 2009, Chris Friesen wrote:
> We've had a mechanism sort of like this for quite a while.  Hasn't been
> pushed to mainline because it used board-specific hardware and we're
> usually multiple kernel versions behind mainline.
> 
> Anyways, a couple things that we've found to be useful are:
> 1) The ability to allocate a chunk of this persistent memory area for a
> special purpose.  This allows things like memory-mapped circular buffers
> for per-cpu binary data.
> 2) An API to log just to this persistent area and bypass the normal
> console completely.  This can be useful when debugging issues where the
> normal logging paths result in a hang.

Some powerpc machines have a memory-mapped nvram, in which the kernel
can install persistant 'partitions'. Not all of them are memory mapped,
but for those that are (e.g. IBM QS22), your approach sounds perfect.

	Arnd <><
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