Re: 2.6.24-rc8 hangs at mfgpt-timer

[Jordan Crouse]

On 22/01/08 21:15 +0100, Willy Tarreau wrote:
> Good news! I read the mfgpt patch for 2.6.22 and saw what the workaround
> consisted in (writing 0xff at MSR 0x5140002B). So I tried adding the
> following on top of 2.6.24-rc8 :
> 
> static int __init mfgpt_fix(char *s)
> {
> 	u32 val, dummy;
> 
>         /* The following udocumented bit resets the MFGPT timers */
>         val = 0xFF;
>         wrmsr(0x5140002B, val, dummy);
> 
> 	return 1;
> }
> __setup("mfgptfix", mfgpt_fix);
> 
> and booted with the newly added option (mfgptfix). It worked like a charm :
> 
<snip>

> So it seems like applying the workaround on top of TinyBIOS 0.98 undoes
> this BIOS's workaround. I'm now wondering how we could detect whether
> the workaround was applied or not :-/

Actually, the TinyBIOS "workaround" is the same thing.

Like I may have said before, there is a reason why this MSR is undocumented.
It works, but the behavior is unvalidated, and obviously erratic.  When we
first developed this code, we were using the Geode GX on the OLPC with VSA,
so using the MSR was nessesary if we wanted to get our hands on any 
timers at all.  Mitch Bradley (author of OpenFirmware) determined through
testing that the MSR was erratic, especially when you ran it when all the
timers were already clear.  I suspect thats the problem that we see here -
writing the MSR in TinyBIOS unstablizied the registers, and writing the
MSR again kicked it back.  Of course, the unfortunate corollary is that
when the workaround isn't in v0.99, if you run the MSR in the kernel, then
you will end up destabilizing everything.

Furthermore, using the MSR is okay with TinyBIOS, but not okay with the
other Geode BIOSen (Insyde, General Software, and for the moment LinuxBIOS)
because VSA (the SMM handler) _does_ use some of the timers.  So needless
to say, I'm concerned.

> Interestingly, during the boot, I got thousands of the following line :
> geode_mfgpt: reading 0x6200 + 0x6 + (0x0 * 8) = 0x6206                          
> 
> It's a debug line I added in geode_mfgpt_read() which writes the timer and
> reg being read. It slowed the boot down due to being written to a serial
> console, but fortunately stopped when syslogd had changed the console log
> level. Just checking...

We control the timer and the status bits for the timer in the setup 
register, which is what you are showing above.  Thats fine.

> # grep mfgpt /proc/interrupts;sleep 10;grep mfgpt /proc/interrupts
>   7:      82320    XT-PIC-XT        mfgpt-timer
>   7:      84882    XT-PIC-XT        mfgpt-timer
> 
> It delivers 256 IRQs/s. That makes me think about this comment in mfgpt_32.c :

Hmm - are you running with nohz?    I ran the same thing on the OLPC
and I'm getting 81 IRQ/s which is okay, considering that sugar was running
in the background. 

>  * We are using the 32Khz input clock - its the only one that has the
>  * ranges we find desirable.  The following table lists the suitable
>  * divisors and the associated hz, minimum interval
>  * and the maximum interval:
>  *
>  *  Divisor   Hz      Min Delta (S) Max Delta (S)
>  *   1        32000     .0005          2.048
>  *   2        16000      .001          4.096
>  *   4         8000      .002          8.192
> ...
> 
> When you say a 32kHz clock, you mean 32000 Hz. Are you really sure ? Most
> 32 kHz clocks everywhere are really 32768 Hz (the watch quartz). BTW, I'm
> seeing a 32.768 kHz xtal close to the CS5536, and the numbers above seem
> to support this suggestion too.

Hmm - yeah, my math is off there - it is a 32768 Hz clock.  That
shouldn't affect things too negatively - if it does we can adjust the
divisor we use - currently we use 16.

> So right now that I've found what caused old kernel to unexpectedly work,
> I'm planning a BIOS upgrade. I'm still just wondering what we can do to
> detect that the workaround should be needed. I suspect nothing, of course,
> but just in case... Maybe we can detect the effects of the workaround ?

I'm not sure if we can - all we can tell is if the registers are zero or
not.  Like I said, running the MSR is probably dangerous in 9 out of 10
situations, the one good use being the one you determined.  I would
support adding the mfgptfix bit though - just as long as it isn't 
automagic.

Thank you very much for your help!
Jordan

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


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