Hello everyone,
I have an x86 system with two PCI slots, in which I inserted two

specialized output cards (Dektec DTA-105).
http://www.dektec.com/Products/DTA-105/

(They provide an open source driver.)
My problem is: when I write to the 4 ports (each card has 2 ports) "at

the same time" (not really "at the same time" because I have a

uni-processor system, so "within a short time frame" is more accurate)

the system *completely* locks up.
The manufacturer told me they had seen the problem in their lab. I'm

just trying to provide some helpful debug output to speed up the process

of fixing the problem :-)
I've built a debug 2.6.22.1-rt9 kernel, hoping to get the kernel to dump

something, anything.
+CONFIG_KALLSYMS_ALL=y

+CONFIG_PCI_DEBUG=y

+CONFIG_DEBUG_DRIVER=y

+CONFIG_PRINTK_TIME=y

+CONFIG_MAGIC_SYSRQ=y

+CONFIG_DEBUG_KERNEL=y

+CONFIG_DEBUG_SHIRQ=y

+CONFIG_DETECT_SOFTLOCKUP=y

+CONFIG_DEBUG_SLAB=y

+CONFIG_DEBUG_SLAB_LEAK=y

+CONFIG_DEBUG_PREEMPT=y

+CONFIG_DEBUG_RT_MUTEXES=y

+CONFIG_DEBUG_PI_LIST=y

+CONFIG_RT_MUTEX_TESTER=y

+CONFIG_DEBUG_SPINLOCK=y

+CONFIG_DEBUG_MUTEXES=y

+CONFIG_DEBUG_LOCK_ALLOC=y

+CONFIG_PROVE_LOCKING=y

+CONFIG_LOCKDEP=y

+CONFIG_TRACE_IRQFLAGS=y

+CONFIG_DEBUG_SPINLOCK_SLEEP=y

+CONFIG_DEBUG_LOCKING_API_SELFTESTS=y

+CONFIG_STACKTRACE=y

+CONFIG_PREEMPT_TRACE=y

+CONFIG_DEBUG_BUGVERBOSE=y

+CONFIG_DEBUG_INFO=y

+CONFIG_FRAME_POINTER=y

+CONFIG_FORCED_INLINING=y

+CONFIG_DEBUG_STACKOVERFLOW=y

+CONFIG_DEBUG_RODATA=y

+CONFIG_4KSTACKS=y
I've enabled the serial console, and used SysRq to bump the console

level to 9 (I want everything, even KERN_DEBUG output).
I've enabled the IO-APIC watchdog (nmi_watchdog=1).
Once the system locks up, I get no output, no panic, no oops.

The serial console is frozen, my ssh sessions are frozen.
Suppose the PCI bus "crashes" (whatever that means) or locks up.

Would that make the system completely unresponsive? The I/O does have to

get to/from the south bridge, through the PCI bus AFAIU. I can imagine

...
[ message continues ]

I've tested with a vanilla 2.6.22.10 kernel (no PREEMPT_RT patch).

That system also locks up and remains completely unresponsive (I can't

open new ssh sessions, the system won't answer ICMP echo requests).
How do driver writers deal with complete kernel hangs?
Regards.

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I sent the problematic system to the board manufacturer.

This is what their engineers told me:
"It seems that the PCI clock on this system has a rather large over- and

undershoot and we suspect that the undershoot (of ~1V) is causing a drop

in the core voltage of the on-board FPGA which results in lockup of the

firmware. Both the under- and overshoot are well outside the allowed

ranges (high=VCC+0.5V and low=-0.5V) of the PCI specification and a

premature conclusion might be that the system does not comply to the PCI

spec and that this is the cause of the lockup on this PC."
This is waaay out of my league, as my area is software.
Is it typical for voltage issues to hang hardware?
Is it typical for one PCI board locking up to nail the entire system?
I don't understand why the lockup would only happen when I write to the

4 ports within a small time frame, and not when I only write to 2 ports

(either one port on each card, or 2 ports on the same card). I suspected

some kind of concurrency issue...
I suppose the next logical step is to get the board's engineers

and the system's engineers duke it out? :-)
Regards.

-

This doesn't appear to be a case of the *board* crashing, but rather

the board taking the pci bus and related hardware on-motherboard down

with it. Once that's down, anything that you need that goes through
No, given the hardware guy's description, it's a power issue. Perhaps

when you're writing to a port, you're using more power on the card?

Four ports = 4 * the power draw. When the current load increases,

voltage drops, and if you underpower a chip, it's going to lose its
Yes, all signs point to it being a pure hardware issue. You may be

able to work around it in software by initializing a 'counting

semaphore' to 2 to manage the maximum concurrency, so that you'll

never write more than 2 ports at a time until the hardware guys figure

it out.
Ray

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On Wed, Oct 24, 2007 at 11:17:40AM +0200, John Sigler wrote:
Use different HW. Both IA64 and PARISC gives useful diagnostics

when the machine has a hard crash (MCA or HPMC respectively). I'll bet

PPC does too on the POWER machines.
Maybe a newer x86 machine can provide some MCE data as well?
Otherwise it's what gregkh said...not the "we slowly go crazy"

part. :) Well, sometimes. :)
BTW, getting PCI bus traces would be quite helpful in this case.

It'll give you clear data as to whether the devices are being programmed

as expected (also to rule out chipset/Host bus controller issues) and

whether they are responding as expected (maybe something else dies

when they do).
hth,

grant
hth,

grant

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You need visibility into the actual place where the issue is

happening, and by your description it's the two cards fighting each

other on the PCI bus. (I'm assuming that the problem doesn't occur

when there's only one card.)
I've worked on a platform where only a few things went through the PCI

bus, and via the firmware's debugger (completely outside the kernel),

I was able to do the setup of the PCI chip and mappings, and then

pound on the hardware directly to exhibit a lockup of the bus. That

provided pretty strong proof that it wasn't some other unrelated issue

in the kernel causing us grief.
At that point, it was pretty clear that was it, as there was no kernel

running, and the CPU was still responsive. Renting a logic analyzer
Lots of sleeps in between each and every write to the hardware, with

regular output. Of course, if it's a timing issue (which is what it

sounds like), you're screwed.
Other than that, I look at the hardware guys and say "Your turn," and

walk out of the room to get a cup of coffee.
Ray

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We slowly go crazy :)
Seriously, try to add debugging messages for where you think things

might be dying and slowly start working from there.  It's not a quick

thing to do at times...
Oh, try using kdb, that sometimes will work for people, depending on

your hardware and problem.
good luck,
greg k-h

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