linux-kernel - Re: RFC: Put printk buffer in video ram

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Message-Id: <200911232003.31482.arnd@arndb.de>
Date:	Mon, 23 Nov 2009 20:03:31 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	"Chris Friesen" <cfriesen@...tel.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Dave Jones <davej@...hat.com>,
	"linux-kernel" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: RFC: Put printk buffer in video ram

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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