Date: Tue, 15 Jan 2008 21:04:11 +0100
From: Luotao Fu <l.fu@pengutronix.de>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Luotao Fu <l.fu@pengutronix.de>, 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: <20080115200411.GA12195@pengutronix.de>
Mail-Followup-To: Steven Rostedt <rostedt@goodmis.org>,
	Luotao Fu <l.fu@pengutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	RT <linux-rt-users@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
	Thomas Gleixner <tglx@linutronix.de>
References: <1200336080.318.8.camel@localhost.localdomain> <20080115162729.GF5859@pengutronix.de> <Pine.LNX.4.58.0801151301550.26624@gandalf.stny.rr.com>
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Hi Steven,

On Tue, Jan 15, 2008 at 01:06:25PM -0500, Steven Rostedt wrote:
>=20
>=20
> On Tue, 15 Jan 2008, Luotao Fu wrote:
> >
> > Compiling with tracing function turned seemed still to be broken to me:
>=20
> Could you email me your config (privately)

will send it later in another private mail, actually nothing spectacular. T=
he
early_printk() is simply missing for 32bit powerpc. The 64bit implementation
puts a dummy for early_printk in setup_64.c:
~ grep -A 4 EARLY_PRINTK ./arch/powerpc/kernel/setup_64.c | head -6
#ifdef CONFIG_EARLY_PRINTK
void notrace early_printk(const char *fmt, ...)
{
        BUG();
	}
	#endif /* CONFIG_EARLY_PRINTK */
;-)
that could be why it works for you.

>=20
=2E...
> > After recompiling the kernel started normally and it seems to work. I w=
as even
> > able to make a trace with cyclictest. However there were several crashe=
s (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 an=
d I think
> > that there due to be some better fixes than just throw it out ;-). On t=
he other
> > side, I just can't find in which way mcount_enabled is used in the trac=
er at
> > all. Could you give me some hints on this one?
> >
>=20
> The way we turn on and off mcount function calls at run time is through
> mcount_enable.  I'll look into why this is broken for you. I'm able to run
> with this. My box is a ppc64, and my ppc32 (powerbook) wont come close to
> booting RT (never did).  On boot up it locks up right away and the screen
> looks like it starts to melt. No serial, so I don't have much to debug
> that with.
>=20

ah, thx for the hint. I took a look in the entry_64.S file. The checking of
mcount_enabled is indeed literally the same as the 32bit architecture.
An important point for the failure on 32bit machines is that due to my bdi =
the
machine got stuck still before the mmu is running. Hence I get quite funny =
pc
address like
Current PC        : 0x000135dc
it still lies over on 0x0 as the physical adress of the decompressed kernel=
 in
memory. I think that mcount_enabled is linked somewhere in the virtual adde=
ss
space, so that the cmpwi stucks forever for _mcount is called so early, tha=
t we
still don't work with virtual addresses. I don't understand enough powerpc
assembler to work out how the LOAD_REG_ADDR() (used in entry_64.S to load
mcount_enabled) of 64 bit powerpcs work around this problem. Anyway, If my =
guess
is right, we might need something to get or simply ignore mcount_enabled
checking in the _mcount calls during early initialisation.

> Anyway, I'll take a deeper look into this.
>=20

Thanks


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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