linux-kernel - Re: Fwd: Commit 76a2a6ee8a0660a29127f05989ac59ae1ce865fa breaks PXA270 (at least)?

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Message-Id: <1217401810.6364.3.camel@twins>
Date:	Wed, 30 Jul 2008 09:10:10 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	vatsa@...ux.vnet.ibm.com, efault@....de, mingo@...e.hu,
	linux-kernel@...r.kernel.org
Subject: Re: Fwd: Commit 76a2a6ee8a0660a29127f05989ac59ae1ce865fa breaks
	PXA270 (at least)?

On Tue, 2008-07-29 at 23:56 +0100, Russell King - ARM Linux wrote:
> On Tue, Jul 29, 2008 at 03:46:05PM -0700, Andrew Morton wrote:
> > On Tue, 29 Jul 2008 23:31:05 +0100
> > Russell King - ARM Linux <linux@....linux.org.uk> wrote:
> > 
> > > But then some bright spark thought it would be a good idea to get
> > > rid of printk_clock().
> > 
> > <does git-log, searches for printk_clock>
> > 
> >   commit 86faf39d0fc04272b05fab1db6d683f3ac7199d1
> >   Author: Ingo Molnar <mingo@...e.hu>
> >   Date:   Fri Jan 25 21:07:59 2008 +0100
> >   
> >       sched: remove printk_clock references from ia64
> >       
> >       remove remaining printk_clock references from ia64.
> >       
> >       Signed-off-by: Ingo Molnar <mingo@...e.hu>
> >   
> >   commit b842271fbb9c8b5fd0e1c3e1895a3b67ba5bcc54
> >   Author: Ingo Molnar <mingo@...e.hu>
> >   Date:   Fri Jan 25 21:07:59 2008 +0100
> >   
> >       sched: remove printk_clock()
> >       
> >       printk_clock() is obsolete - it has been replaced with cpu_clock().
> >       
> >       Signed-off-by: Ingo Molnar <mingo@...e.hu>
> >   
> > 
> > That's it?  It rates a 0.5/10 for changelogging :(
> > 
> > Looking further on...
> > 
> >   commit e97126cd9056b3b42cdc862ace2ed66f8026f55b
> >   Author: Russell King <rmk@...-67.arm.linux.org.uk>
> >   Date:   Mon Jan 8 19:49:12 2007 +0000
> >   
> >       [ARM] Provide basic printk_clock() implementation
> >       
> >       Current sched_clock() implementations on ARM cause unbootable kernels
> >       with PRINTK_TIME support enabled.  To avoid this, provide a basic
> >       printk_clock() implementation which avoids sched_clock() being called
> >       before the page tables have been set up.
> >   
> > 
> > which I assume is why arm is crashing again?
> 
> Precisely - Ingo has effectively progressively backed out my bug fix
> in a round-about manner by making printk require a working sched_clock
> again.
> 
> > Really, I think arch-overrideable printk_clock() was a good idea. 
> > printk is just _special_.  It's called wildly early and it is called in
> > all conceivable contexts and it just must work no matter what.  It's
> > totally understandable that an architecture would need to override
> > printk's timestamp generator.
> 
> That's one solution, the other is to have that commit reverted.  I
> don't really mind which, only that we have a solution ASAP so that
> the current popular ARM SoCs have a bootable kernel again.

Can't you make sched_clock() return 0 when its called before its up and
running? Otherwise - perhaps you can play with the location of
sched_clock_init(). cpu_clock() will return 0 before that.

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