linux-kernel - Re: 2.6.24-rc7-rt2

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Date:	Tue, 15 Jan 2008 17:27:29 +0100
From:	Luotao Fu <l.fu@...gutronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: 2.6.24-rc7-rt2

Hi,

On Mon, Jan 14, 2008 at 01:41:20PM -0500, Steven Rostedt wrote:
> We are pleased to announce the 2.6.24-rc7-rt2 tree, which can be
> downloaded from the location:
> 
>   http://rt.et.redhat.com/download/
> 
.......... 
> Changes since 2.6.24-rc7-rt1
> 

..........
>   - PPC compile fix (reported by: Robert Schwebel)
> 

Compiling with tracing function turned seemed still to be broken to me:
kernel/built-in.o: In function `early_printk_name':
latency_trace.c:(.text+0x3cd18): undefined reference to `early_printk'
latency_trace.c:(.text+0x3cd40): undefined reference to `early_printk'
kernel/built-in.o: In function `early_print_entry':
latency_trace.c:(.text+0x3cd80): undefined reference to `early_printk'
latency_trace.c:(.text+0x3cdd8): undefined reference to `early_printk'
latency_trace.c:(.text+0x3cdfc): undefined reference to `early_printk'

We made a early_printk patch based on the 8250_early for powerpc 32bit here.
(still absolutely untested and incomplete, we will post it asap). I added
early_printk() and exported _mcount() and could compile succesfully. However the
kernel will not boot. I attached a bdi to it and found out that it stucks at
_mcount in arch/powerpc/kernel/entry_32.S at about line 1080, where the variable
mcount_enabled is loaded and checked. Obviously the variable is still not valid
at the time of check. To check this I took out the lines like above

--- arch/powerpc/kernel/entry_32.S.orig 2008-01-15 17:01:25.000000000 +0100
+++ arch/powerpc/kernel/entry_32.S      2008-01-15 17:17:18.000000000 +0100
@@ -1075,9 +1075,6 @@
        stw     r6, 24(r1)
        mflr    r3      /* will use as first arg to __trace() */
        mfcr    r4
-       lis     r5,mcount_enabled@ha
-       lwz     r5,mcount_enabled@l(r5)
-       cmpwi   r5,0
        stw     r3, 44(r1)      /* lr */
        stw     r4,  8(r1)      /* cr */
        stw     r7, 28(r1)

After recompiling the kernel started normally and it seems to work. I was even
able to make a trace with cyclictest. However there were several crashes (which
might be caused by other problems). I still have to take a closer look.

I am just wondering why the check for mcount_enabled is there at all and I think
that there due to be some better fixes than just throw it out ;-). On the other
side, I just can't find in which way mcount_enabled is used in the tracer at
all. Could you give me some hints on this one?

cheers
Luotao Fu
-- 
   Dipl.-Ing. Luotao Fu | Phone: +49-5121-206917-3
Pengutronix - Linux Solutions for Science and Industry
Entwicklungszentrum Nord     http://www.pengutronix.de

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