Date:	Thu, 17 Jan 2008 15:36:44 -0700
From:	"Jordan Crouse" <jordan.crouse@....com>
To:	"Arnd Hannemann" <hannemann@...informatik.rwth-aachen.de>
cc:	"Andres Salomon" <dilinger@...ued.net>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.24-rc8 hangs at mfgpt-timer

On 17/01/08 22:50 +0100, Arnd Hannemann wrote:
> Jordan Crouse schrieb:
> > 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.
> Yes I can confirm this, changed MFGPT_MAX_TIMERS from 7 to 8 in the old
> kernel and it still works.
> 
> >> 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.
> Great analysis! I think I can confirm this too. I tried the following:
> 
> First in mfgpt_timer_setup I commented out "clockevents_register_device"
> result: the system still hangs with "registering the MFGT timer as a
> clock event" !
> 
> Then I also commented out "ret = setup_irq(irq, &mfgptirq)".
> result: system boots, voila!

Hmmm - not sure whats happening here.  I wonder if we're stuck in an
interrupt storm of some sort as soon as you register the interrupt handler.
But I would think that whatever was causing the interrupt storm would be
running well before we hit setup_irq(), and you would be recording "nobody
cared" interrupts left and right.

The thing that scares me is that the TinyBIOS seems to know that we want
to use the MFGPT timers, and I wonder if they did anything behind the scenes
to "help us out" even though we didn't ask for it.

I don't know how easy it would be for you - but can you try reading 
MSRs 0x51400020 - 0x51400023?   If you need a command line app to do it,
you can use rdmsr from here:

http://wiki.laptop.org/go/Flashing_LinuxBIOS_on_A-Test_Boards

> Watchdog for the new API would be great :-)

Coming soon.

Jordan
-- 
Jordan Crouse
Systems Software Development Engineer 
Advanced Micro Devices, Inc.


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