Date: Tue, 15 Jan 2008 17:27:29 +0100
From: Luotao Fu <l.fu@pengutronix.de>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: LKML <linux-kernel@vger.kernel.org>, RT <linux-rt-users@vger.kernel.org>,
       Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: 2.6.24-rc7-rt2
Message-ID: <20080115162729.GF5859@pengutronix.de>
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	LKML <linux-kernel@vger.kernel.org>,
	RT <linux-rt-users@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
	Thomas Gleixner <tglx@linutronix.de>
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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:
>=20
>   http://rt.et.redhat.com/download/
>=20
=2E.........=20
> Changes since 2.6.24-rc7-rt1
>=20
=20
=2E.........
>   - PPC compile fix (reported by: Robert Schwebel)
>=20

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. Howeve=
r 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 var=
iable
mcount_enabled is loaded and checked. Obviously the variable is still not v=
alid
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 e=
ven
able to make a trace with cyclictest. However there were several crashes (w=
hich
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 o=
ther
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
--=20
   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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