reason for delay in arch/x86/kernel/traps.c::io_check_error()?

reason for delay in arch/x86/kernel/traps.c::io_check_error()?

[Chris Friesen]

Hi all,

I was just wondering about the basis for the delay in io_check_error(). 
  The ICH7 manual doesn't have any mention of a delay being required 
here--is it necessary for other hardware, something not mentioned in the 
manual, or just an accident?

Thanks,

Chris

Re: reason for delay in arch/x86/kernel/traps.c::io_check_error()?

[Andi Kleen]

On Mon, Mar 09, 2009 at 01:33:44PM -0600, Chris Friesen wrote:
> 
> Hi all,
> 
> I was just wondering about the basis for the delay in io_check_error(). 
>  The ICH7 manual doesn't have any mention of a delay being required 
> here--is it necessary for other hardware, something not mentioned in the 
> manual, or just an accident?

The complete NMI error logging handling does not really apply to any modern 
chipset; it's for ancient hardware (286, 386 generation). Those often
needed strange delays too.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.

Re: reason for delay in arch/x86/kernel/traps.c::io_check_error()?

[Ingo Molnar]

* Chris Friesen <cfriesen@nortel.com> wrote:

> Hi all,
>
> I was just wondering about the basis for the delay in 
> io_check_error().  The ICH7 manual doesn't have any mention of 
> a delay being required here--is it necessary for other 
> hardware, something not mentioned in the manual, or just an 
> accident?

That code has seriously bitrotten along the years. All those 
port 61H accesses:

arch/x86/kernel/traps.c:                reason = get_nmi_reason();
arch/x86/kernel/traps.c:        outb(reason, 0x61);
arch/x86/kernel/traps.c:        outb(reason, 0x61);
arch/x86/kernel/traps.c:        outb(reason, 0x61);

... are often wrong on modern chipsets - including the logic in 
io_check_error(). But we dont really have lowlevel chipset 
drivers on this level in Linux, so there's nothing suitable to 
replace it with and it never got fixed.

Can you see this trigger on a box perhaps? Or are you worried 
about the potential unbound execution time of this function 
which can be up to 2 seconds in NMI context?

	Ingo

Re: reason for delay in arch/x86/kernel/traps.c::io_check_error()?

[Chris Friesen]

Ingo Molnar wrote:
> * Chris Friesen <cfriesen@nortel.com> wrote:
> 
>> Hi all,
>>
>> I was just wondering about the basis for the delay in 
>> io_check_error().  The ICH7 manual doesn't have any mention of 
>> a delay being required here--is it necessary for other 
>> hardware, something not mentioned in the manual, or just an 
>> accident?
> 
> That code has seriously bitrotten along the years. All those 
> port 61H accesses:
> 
> arch/x86/kernel/traps.c:                reason = get_nmi_reason();
> arch/x86/kernel/traps.c:        outb(reason, 0x61);
> arch/x86/kernel/traps.c:        outb(reason, 0x61);
> arch/x86/kernel/traps.c:        outb(reason, 0x61);
> 
> ... are often wrong on modern chipsets - including the logic in 
> io_check_error(). But we dont really have lowlevel chipset 
> drivers on this level in Linux, so there's nothing suitable to 
> replace it with and it never got fixed.
> 
> Can you see this trigger on a box perhaps? Or are you worried 
> about the potential unbound execution time of this function 
> which can be up to 2 seconds in NMI context?

This is in the context of an embedded highly available compute blade. 
As part of our enhanced error handling we've modified the memory parity 
error code to reenable rather than disable the error line.

Given that the memory and IO code paths are just different bits in the 
same register we originally added the delay to the memory parity path as 
well.  However, we subsequently hit the memory parity error path, and 
the 2sec delay triggered our hardware watchdog causing the board to reboot.

As you can imagine this is undesirable, so we were hoping to remove the 
delay from both paths.  From what you've said and the fact that no delay 
is mentioned in the chip manual, it seems like this should be fairly safe.

Thanks,
Chris