linux-kernel - Re: 2.6.24-rc8 hangs at mfgpt-timer

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Message-ID: <20080117211917.GF8244@cosmic.amd.com>
Date:	Thu, 17 Jan 2008 14:19:17 -0700
From:	"Jordan Crouse" <jordan.crouse@....com>
To:	"Arnd Hannemann" <hannemann@...informatik.rwth-aachen.de>
cc:	"Andres Salomon" <dilinger@...ued.net>,
	linux-kernel@...r.kernel.org
Subject: Re: 2.6.24-rc8 hangs at mfgpt-timer

On 17/01/08 20:53 +0100, Arnd Hannemann wrote:
> Andres Salomon schrieb:
> > On Thu, 17 Jan 2008 10:54:30 +0100
> > Arnd Hannemann <hannemann@...informatik.rwth-aachen.de> wrote:
> > 
> >> Andres Salomon schrieb:
> >>> On Wed, 16 Jan 2008 16:19:12 -0500
> >>> Andres Salomon <dilinger@...ued.net> wrote:
> >>>
> >>>> On Wed, 16 Jan 2008 18:44:07 +0100
> >>>> Arnd Hannemann <hannemann@...informatik.rwth-aachen.de> wrote:
> >>>>
> >>>>> Hi,
> >>>>>
> >>>>> I'm trying to boot 2.6.24-rc8 on a GEODE LX board (ALIX.3),
> >>>>> and it hangs during boot:
> >>>>>
> >>>>> [   12.689971] NET: Registered protocol family 16
> >>>>> [   12.703329] geode-mfgpt:  Registered timer 0
> >>>>> [   12.716149] mfgpt-timer:  registering the MFGT timer as a clock event...
> >>>>>
> >>>> What BIOS are you using?  It's possible that our detection code is
> >>>> failing to detect in-use timers.
> >> I'm using v0.99 (latest available).
> > 
> > 
> > v0.99 of what?  Jordan seems to think it's an Award BIOS, but I'd like
> > to make sure.
> 
> Its an ALIX board from PCEngines, they have their own BIOS
> implementation (tinyBios).
> http://www.pcengines.ch/alix.htm
> 
> > 
> >> Also note when I do enable the mysterios "MFGPT workaround" option in
> >> the bios the machine hangs directly after:
> >> [   36.780990] NET: Registered protocol family 16
> > 
> > 
> > "MFGPT workaround"?  That sounds a bit frightening.
> > 
> > Presumably, the BIOS is using the MFGPTs, but we're not detecting them as
> > being in use.
> Yes I think so too, for the fun of it I compiled a 2.6.16.29 kernel with
> the attached patch from fi4l.

Okay - thats an MFPGT patch from pre-OLPC days. I am the guilty and
dubious party.  We changed the API to work better with the timer tick,
and thats the version that ended up in the kernel.

I really wish I could take back this patch, because it keeps coming back
to torment me.  We must, as a people, put it behind us and forgot it. :)

> relevant output is this:
> [   31.015425] geode-mfgpt:  7 timers available.
> ...
> [   31.245875] geode-mfgpt:  Registered timer 0

> So the above kernel detects only 7 timers not 8, and it works. But note
> that timer 0 is not used as a clock event source but as a watchdog,
> which btw actually works fine :-)

It detects 7 timers because of a bug in the code -  there really are 8 
timers, which the current code correctly identifies.

> The funny thing is the #define workaround part of this dubious patch and
> its interaction with the bios:
> 
> #ifdef WORKAROUND:
>  I have to turn the "MFPGT workaround" option in the bios ON, to boot
> the kernel probably.
>
> #ifndef WORKAROUND:
>  I have to turn the "MFPGT workaround" option in the bios OFF, to boot
> the kernel probably.

So the workaround works around the workaround.   Fun.  I think that Mitch
Bradley verified that if you write the magic MSR when all the clocks are
already clear that bad things happen.  The workaround probably adds a 
dummy clock in.  Notice that the "magic MSR" no longer is in the vanilla
code, and thats the way it should be.  If the BIOS doesn't allow use of
the clocks, then we have to live with that.

So, based on everything you are saying, I think its clear that our
problem isn't in the MFGPT, but rather in the timer tick (because, as
you said, the watchdog works).  We try to use IRQ 7 for the tick, which
Andres and I totally plucked out of thin air based on what we had to work
with on OLPC.  Its totally possible that the TinyBIOS had other ideas.
Please try to boot with nomfgpt, and see which interrupts are free, and
use mfgpt_irq= to change it to something else if 7 is in use.  Based on
your findings above, you'll probably need to leave the MFGPT workaround
off from now on.

I'll port the watchdog timer to the new API, and we can use that instead
of the timer tick to just make sure that it isn't the timer that is broken.
Also, hopefully that will cease the stream of angry emails asking me why
the ancient patch doesn't work on a current kernel... :)

Jordan

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