linux-kernel - Re: [regression bisected] HR-timers bug >=2.6.25

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Date:	Fri, 16 May 2008 09:15:54 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Justin Madru <jdm64@...ab.com>
cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	David Airlie <airlied@...ux.ie>,
	Sylvain Meyer <sylvain.meyer@...ldonline.fr>,
	Antonino Daplas <adaplas@...il.com>,
	Roman Zippel <zippel@...ux-m68k.org>, Greg KH <gregkh@...e.de>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [regression bisected] HR-timers bug >=2.6.25

On Thu, 15 May 2008, Justin Madru wrote:
> I've reported this problem previously:
> http://lkml.org/lkml/2008/3/12/290
> http://bugzilla.kernel.org/show_bug.cgi?id=10235
> https://bugs.freedesktop.org/show_bug.cgi?id=15602
> 
> This bug is still in mainline as of 2.6.26-rc1. (affected versions 2.6.25-rc0
> - 2.6.26-rc1)
> 
> Ever since 2.6.25-rc3 (the first rc I tested), my screen would blank out when
> mode switching from the Ubuntu uplash to the gdm login. Normally after the
> boot splash, the screen goes black and "refreshes" to select the correct
> resolution. But when using a kernel newer than 2.6.24 the screen doesn't come
> back on - it stays blank. This doesn't happen every time, but a good majority
> of the time. The workaround is to disable the splash, and that fixes the
> problem almost completely.
> 
> I did a git bisect. Rebooting 3 times if the problem didn't occur just to make
> sure.
> The git bisect reports that it's timing related - which is what Jesse Barnes
> said it could be.

Well, it's a timing problem related to splash which is exposed by the
scheduler timing changes. This is neither a hrtimer nor a scheduler
bug. Something in the splash / video driver switchover relies on some
obscure timing which is nowhere guaranteed.

Thanks,
	tglx


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