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

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Date:	Mon, 23 Nov 2009 19:32:20 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Maxim Levitsky <maximlevitsky@...il.com>
Cc:	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>
Subject: Re: RFC: Put printk buffer in video ram


* Maxim Levitsky <maximlevitsky@...il.com> wrote:

> On Sun, 2009-11-22 at 14:15 +0200, Maxim Levitsky wrote: 
> > On Sun, 2009-11-22 at 09:25 +0100, Ingo Molnar wrote: 
> > > * Frederic Weisbecker <fweisbec@...il.com> wrote:
> > > 
> > > > On Sun, Nov 22, 2009 at 04:05:06AM +0200, Maxim Levitsky wrote:
> > > > > After doing some successful debugging by placing printk buffer in video
> > > > > ram, here I publish cleaned version of it.
> > > > > 
> > > > > I discovered that on my system video ram isn't cleared on reboot, and I
> > > > > took advantage of that by placing printk buffer directly there.
> > > > > This allows to capture oopses/panicks almost from everywhere.
> > > > > It is also very simple to setup.
> 
> Few more thoughts:
> 
> First of all, if I implement this as a console driver, I won't be able
> to capture all kernel log, but only from the point I register the
> console.

Adding an early console driver would solve this, right?

There's an earlyprintk=..,keep option that also allows such a console 
driver to be active during normal system operation - that way you could 
get this feature with a single console driver.

> My implementation, also isn't set up very early, but I copy already
> written log to the memory buffer.
> 
> Problem of automatic unregistration isn't a problem, its just a feature,
> I could skip using.
> 
> Secondary, I found out that system ram isn't cleared ether on my
> notebook, however I found out that I can't reserve small memory range
> with 'memmap' kernel parameter. System just panics.
> I used for reference 'memmap=2M$0x60000000'
> My system has 2GB of memory.
> 
> I could reserve the memory same way using the code, but it would
> probably fail in same way. Also I could try early reservations, but I am
> not sure I can reserve _any_ address range. I test this.
> 
> I think that patches to put printk buffer at predefined area in system
> ram were once posted on LKML.

The way to early-reserve memory on x86 is to use find_e820_area() + 
reserve_early().

	Ingo
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