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Date: Tue, 29 Jul 2008 23:56:23 +0100
From: Russell King - ARM Linux <linux@arm.linux.org.uk>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: a.p.zijlstra@chello.nl, vatsa@linux.vnet.ibm.com, efault@gmx.de,
       mingo@elte.hu, linux-kernel@vger.kernel.org
Subject: Re: Fwd: Commit 76a2a6ee8a0660a29127f05989ac59ae1ce865fa breaks PXA270 (at least)?
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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@arm.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@elte.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@elte.hu>
>   
>   commit b842271fbb9c8b5fd0e1c3e1895a3b67ba5bcc54
>   Author: Ingo Molnar <mingo@elte.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       printk_clock() is obsolete - it has been replaced with cpu_clock().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@elte.hu>
>   
> 
> That's it?  It rates a 0.5/10 for changelogging :(
> 
> Looking further on...
> 
>   commit e97126cd9056b3b42cdc862ace2ed66f8026f55b
>   Author: Russell King <rmk@dyn-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.
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